Chapter 2. Types, Values, and Variables

Computer programs work by manipulating values, such as the number 3.14 or the text “Hello World.” The kinds of values that can be represented and manipulated in a programming language are known as types, and one of the most fundamental characteristics of a programming language is the set of types it supports. When a program needs to retain a value for future use, it assigns the value to (or “stores” the value in) a variable. A variable defines a symbolic name for a value and allows the value to be referred to by name. The way that variables work is another fundamental characteristic of any programming language. This chapter explains types, values, and variables in JavaScript.

2.0 Introduction

JavaScript types can be divided into two categories: primitive types and object types. JavaScript’s primitive types include numbers, strings of text (known as strings), and Boolean truth values (known as booleans). A significant portion of this chapter is dedicated to a detailed explanation of the numeric (§2.1) and string (§2.2) types in JavaScript. Booleans are covered in §2.3.

The special JavaScript values null and undefined are primitive values, but they are not numbers, strings, or booleans. Each value is typically considered to be the sole member of its own special type. §2.4 has more about null and undefined. ECMAScript 6 adds a new special-purpose type, known as Symbol, that enables the definition of language extensions without harming backward compatibility. Symbols are covered briefly in §2.5.

Any JavaScript value that is not a number, a string, a boolean, or null or undefined is an object. An object (that is, a member of the type object) is a collection of properties where each property has a name and a value (either a primitive value, such as a number or string, or an object). One very special object, the global object, is covered in §2.6, but more general and more detailed coverage of objects is in Chapter 5.

An ordinary JavaScript object is an unordered collection of named values. The language also defines a special kind of object, known as an array, that represents an ordered collection of numbered values. The JavaScript language includes special syntax for working with arrays, and arrays have some special behavior that distinguishes them from ordinary objects. Arrays are the subject of Chapter 6.

In addition to basic objects and arrays, JavaScript defines a number of other useful object types. Set represents a set of values. Map represents a mapping from keys to values. Various “typed array” types facilitate operations on arrays of bytes and other binary data. The RegExp type represents textual patterns and enables sophisticated matching, searching and replacing operations on strings. The Date type represents dates and times and supports rudimentary string arithmetic. Error and its sub-types represent errors that can arise when executing JavaScript code. All of these types are covered in Chapter 9.

JavaScript differs from more static languages in that functions and classes are not just part of the language syntax: they are themselves values that can be manipulated by JavaScript programs. Like any JavaScript values that is not a primitive value, functions and classes are a specialized kind of object. They are covered in detail in Chapter 7 and Chapter 8.

The JavaScript interpreter performs automatic garbage collection for memory management. This means that a JavaScript programmer never needs to worry about destruction or deallocation of objects or other values. When a value is no longer reachable—when a program no longer has any way to refer to it—the interpreter knows it can never be used again and automatically reclaims the memory it was occupying.

JavaScript is an object-oriented language. Loosely, this means that rather than having globally defined functions to operate on values of various types, the types themselves define methods for working with values. To sort the elements of an array a, for example, we don’t pass a to a sort() function. Instead, we invoke the sort() method of a:

a.sort();       // The object-oriented version of sort(a).

Method definition is covered in Chapter 8. Technically, it is only JavaScript objects that have methods. But numbers, strings, boolean and symbol values behave as if they have methods. In JavaScript, null and undefined are the only values that methods cannot be invoked on.

JavaScript’s object types are mutable and its primitive types are immutable. A value of a mutable type can change: a JavaScript program can change the values of object properties and array elements. Numbers, booleans, null, and undefined are immutable—it doesn’t even make sense to talk about changing the value of a number, for example. Strings can be thought of as arrays of characters, and you might expect them to be mutable. In JavaScript, however, strings are immutable: you can access the text at any index of a string, but JavaScript provides no way to alter the text of an existing string. The differences between mutable and immutable values are explored further in §2.7.

JavaScript converts values liberally from one type to another. If a program expects a string, for example, and you give it a number, it will automatically convert the number to a string for you. And if you use a non-boolean value where a boolean is expected, JavaScript will convert accordingly. The rules for value conversion are explained in §2.8. JavaScript’s liberal value conversion rules affect its definition of equality, and the == equality operator performs type conversions as described in §2.9.1.

Constants and variables allow you to use names to refer to values in your programs. Constants are declared with const and variables are declared with let (or with var in older JavaScript code). JavaScript constants and variable are untyped: declarations do not specify what kind of values will be assigned. Variable declaration and assignment are covered in §2.9.

As you can see from this long introduction, this is a wide-ranging chapter that explains a lot of fundamental details about how data is represented and manipulated in JavaScript. We’ll begin by diving right in to the details of JavaScript numbers and text.

2.1 Numbers

JavaScript’s primary numeric type, Number, is used to represent integers and to approximate real numbers. JavaScript represents numbers using the 64-bit floating-point format defined by the IEEE 754 standard,1 which means it can represent numbers as large as ±1.7976931348623157 × 10308 and as small as ±5 × 10−324.

The JavaScript number format allows you to exactly represent all integers between −9007199254740992 (−253) and 9007199254740992 (253), inclusive. If you use integer values larger than this, you may lose precision in the trailing digits. Note, however, that certain operations in JavaScript (such as array indexing and the bitwise operators described in Chapter 3) are performed with 32-bit integers. If you need to exactly represent larger integers, see §2.2.5.

When a number appears directly in a JavaScript program, it’s called a numeric literal. JavaScript supports numeric literals in several formats, as described in the following sections. Note that any numeric literal can be preceded by a minus sign (-) to make the number negative.

2.2.1 Integer Literals

In a JavaScript program, a base-10 integer is written as a sequence of digits. For example:


In addition to base-10 integer literals, JavaScript recognizes hexadecimal (base-16) values. A hexadecimal literal begins with 0x or 0X, followed by a string of hexadecimal digits. A hexadecimal digit is one of the digits 0 through 9 or the letters a (or A) through f (or F), which represent values 10 through 15. Here are examples of hexadecimal integer literals:

0xff       // => 255: (15*16 + 15)
0xBADCAFE  // => 195939070

In ECMAScript 6 and later you can also express integers in binary (base 2) or octal (base 8) using the prefixes 0b and 0o (or 0B and 0O) instead of 0x:

0b10101  // => 21:  (1*16 + 0*8 + 1*4 + 0*2 + 1*1)
0o377    // => 255: (3*64 + 7*8 + 7*1)

2.2.2 Floating-Point Literals

Floating-point literals can have a decimal point; they use the traditional syntax for real numbers. A real value is represented as the integral part of the number, followed by a decimal point and the fractional part of the number.

Floating-point literals may also be represented using exponential notation: a real number followed by the letter e (or E), followed by an optional plus or minus sign, followed by an integer exponent. This notation represents the real number multiplied by 10 to the power of the exponent.

More succinctly, the syntax is:


For example:

6.02e23        // 6.02 × 10²³
1.4738223E-32  // 1.4738223 × 10⁻³²

2.2.3 Arithmetic in JavaScript

JavaScript programs work with numbers using the arithmetic operators that the language provides. These include + for addition, - for subtraction, * for multiplication, / for division, and % for modulo (remainder after division). ECMAScript 2016 adds ** for exponentiation. Full details on these and other operators can be found in Chapter 3.

In addition to these basic arithmetic operators, JavaScript supports more complex mathematical operations through a set of functions and constants defined as properties of the Math object:

Math.pow(2,53)           // => 9007199254740992: 2 to the power 53
Math.round(.6)           // => 1.0: round to the nearest integer
Math.ceil(.6)            // => 1.0: round up to an integer
Math.floor(.6)           // => 0.0: round down to an integer
Math.abs(-5)             // => 5: absolute value
Math.max(x,y,z)          // Return the largest argument
Math.min(x,y,z)          // Return the smallest argument
Math.random()            // Pseudo-random number x where 0 <= x < 1.0
Math.PI                  // π: circumference of a circle / diameter
Math.E                   // e: The base of the natural logarithm
Math.sqrt(3)             // => 3**0.5: the square root of 3
Math.pow(3, 1/3)         // => 3**(1/3): the cube root of 3
Math.sin(0)              // Trigonometry: also Math.cos, Math.atan, etc.
Math.log(10)             // Natural logarithm of 10
Math.log(100)/Math.LN10  // Base 10 logarithm of 100
Math.log(512)/Math.LN2   // Base 2 logarithm of 512
Math.exp(3)              // Math.E cubed

ECMAScript 6 defines more functions on the Math object:

Math.cbrt(27)    // => 3: cube root
Math.hypot(3, 4) // => 5: square root of sum of squares of all arguments
Math.log10(100)  // => 2: Base-10 logarithm
Math.log2(1024)  // => 10: Base-2 logarithm
Math.log1p(x)    // Natural log of (1+x); accurate for very small x
Math.expm1(x)    // Math.exp(x)-1; the inverse of Math.log1p()
Math.sign(x)     // -1, 0, or 1 for arguments <, ==, or > 0
Math.imul(2,3)   // => 6: optimized multiplication of 32-bit integers
Math.clz32(0xf)  // => 28: number of leading zero bits in a 32-bit integer
Math.trunc(3.9)  // => 3: convert to an integer by truncating fractional part
Math.fround(x)   // Round to nearest 32-bit float number
Math.sinh(x)     // Hyperbolic sine. Also Math.cosh(), Math.tanh()
Math.asinh(x)    // Hyperbolic arcsine. Also Math.acosh(), Math.atanh()

Arithmetic in JavaScript does not raise errors in cases of overflow, underflow, or division by zero. When the result of a numeric operation is larger than the largest representable number (overflow), the result is a special infinity value, which JavaScript prints as Infinity. Similarly, when the absolute value of a negative value becomes larger than the absolute value of the largest representable negative number, the result is negative infinity, printed as -Infinity. The infinite values behave as you would expect: adding, subtracting, multiplying, or dividing them by anything results in an infinite value (possibly with the sign reversed).

Underflow occurs when the result of a numeric operation is closer to zero than the smallest representable number. In this case, JavaScript returns 0. If underflow occurs from a negative number, JavaScript returns a special value known as “negative zero.” This value is almost completely indistinguishable from regular zero and JavaScript programmers rarely need to detect it.

Division by zero is not an error in JavaScript: it simply returns infinity or negative infinity. There is one exception, however: zero divided by zero does not have a well-defined value, and the result of this operation is the special not-a-number value, printed as NaN. NaN also arises if you attempt to divide infinity by infinity, or take the square root of a negative number or use arithmetic operators with non-numeric operands that cannot be converted to numbers.

JavaScript pre-defines global constants Infinity and NaN to hold the positive infinity and not-a-number value, and these values are also available as properties of the Number object:

Infinity                    // A positive number too big to represent
Number.POSITIVE_INFINITY    // Same value
1/0                         // => Infinity
Number.MAX_VALUE * 2        // => Infinity; overflow!

-Infinity                   // A negative number too big to represent
Number.NEGATIVE_INFINITY    // The same value
-1/0                        // => -Infinity
-Number.MAX_VALUE * 2       // => -Infinity

NaN                         // The not-a-number value
Number.NaN                  // The same value, written another way
0/0                         // => NaN

Number.MIN_VALUE/2          // => 0: underflow!
-Number.MIN_VALUE/2         // => -0: negative zero
-1/Infinity                 // -> -0: also negative 0

// The following Number properties are defined in ECMAScript 6
Number.parseInt()       // Same as the global parseInt() function
Number.parseFloat()     // Same as the global parseFloat() function
Number.isNaN(x)         // Is x the NaN value? Works like x !== x
Number.isFinite(x)      // Is x a number and finite?
Number.isInteger(x)     // Is x an integer?
Number.isSafeInteger(x) // Is x an integer -(2**53) < x < 2**53?
Number.MIN_SAFE_INTEGER // => -(2**53 - 1)
Number.MAX_SAFE_INTEGER // => 2**53 - 1
Number.EPSILON          // => 2**-52: smallest difference between numbers

The not-a-number value has one unusual feature in JavaScript: it does not compare equal to any other value, including itself. This means that you can’t write x === NaN to determine whether the value of a variable x is NaN. Instead, you should write x != x or Number.isNaN(x). That expression will be true if, and only if, x is NaN. The global function isNaN() is similar. It returns true if its argument is NaN, or if that argument is a non-numeric value that can not be converted to a number. The related function Number.isFinite() returns true if its argument is a number other than NaN, Infinity, or -Infinity. The global isFinite() function returns true if its argument is, or can be converted to a finite number.

The negative zero value is also somewhat unusual. It compares equal (even using JavaScript’s strict equality test) to positive zero, which means that the two values are almost indistinguishable, except when used as a divisor:

let zero = 0;         // Regular zero
let negz = -0;        // Negative zero
zero === negz         // => true: zero and negative zero are equal
1/zero === 1/negz     // => false: Infinity and -Infinity are not equal

2.2.4 Binary Floating-Point and Rounding Errors

There are infinitely many real numbers, but only a finite number of them (18437736874454810627, to be exact) can be represented exactly by the JavaScript floating-point format. This means that when you’re working with real numbers in JavaScript, the representation of the number will often be an approximation of the actual number.

The IEEE-754 floating-point representation used by JavaScript (and just about every other modern programming language) is a binary representation, which can exactly represent fractions like 1/2, 1/8, and 1/1024. Unfortunately, the fractions we use most commonly (especially when performing financial calculations) are decimal fractions 1/10, 1/100, and so on. Binary floating-point representations cannot exactly represent numbers as simple as 0.1.

JavaScript numbers have plenty of precision and can approximate 0.1 very closely. But the fact that this number cannot be represented exactly can lead to problems. Consider this code:

let x = .3 - .2;    // thirty cents minus 20 cents
let y = .2 - .1;    // twenty cents minus 10 cents
x === y             // => false: the two values are not the same!
x === .1            // => false: .3-.2 is not equal to .1
y === .1            // => true: .2-.1 is equal to .1

Because of rounding error, the difference between the approximations of .3 and .2 is not exactly the same as the difference between the approximations of .2 and .1. It is important to understand that this problem is not specific to JavaScript: it affects any programming language that uses binary floating-point numbers. Also, note that the values x and y in the code above are very close to each other and to the correct value. The computed values are adequate for almost any purpose: the problem arises when we attempt to compare values for equality.

If these floating-point approximations are problematic for your programs, consider using scaled integers. For example, you might manipulate monetary values as integer cents rather than fractional dollars.

2.2.5 Arbitrary Precision Integers with BigInt

As of mid-2019, a new JavaScript numeric type, known as BigInt, is nearing standardization. Although it has not yet been formally standardized, it has been implemented in Chrome, Firefox and Node, and there is an implementation in progress in Safari. As the name implies, BigInt is a numeric type whose values are integers. The type was added to JavaScript mainly to allow the representation of 64-bit integers which are required for compatibility with many other programming languages and APIs. But BigInt values can have thousands or even millions of digits, should you have need to work with numbers that large. (Note, however, that BigInt implementations are not suitable for cryptography because they do not attempt to prevent timing attacks.)

BigInt literals are written as a string of digits followed by a lowercase letter n. By default, the are in base 10, but you can use the 0b, 0o and 0x prefixes for binary, octal and hexadecimal BigInts:

1234n                // A not-so-big BigInt literal
0b111111n            // A binary BigInt
0o7777n              // An octal BigInt
0x8000000000000000n  // => 2n**63n: A 64-bit integer

You can use BigInt() as a function for converting regular JavaScript numbers or strings to BigInt values:

BigInt(Number.MAX_SAFE_INTEGER)     // => 9007199254740991n
let string = '1' + '0'.repeat(100); // 1 followed by 100 zeros.
BigInt(string)                      // => 10n**100n: one googol

Arithmetic with BigInt values works like arithmetic with regular JavaScript numbers, except that division drops any remainder and rounds down (toward zero):

1000n + 2000n  // => 3000n
3000n - 2000n  // => 1000n
2000n * 3000n  // => 6000000n
3000n / 997n   // => 3n: the quotient is 3
3000n % 997n   // => 9n: and the remainder is 9
(2n ** 131071n) - 1n  // A Mersenne prime with 39457 decimal digits

Although the standard +, -, *, /, %, and ** operators work with BigInt, it is important to understand that you may not mix operands of type BigInt with regular number operands. This may seem confusing at first, but there is a good reason for it. If one numeric type was more general than the other, it would be easy to define arithmetic on mixed operands to simply return a value of the more general type. But neither type is more general than the other: BigInt can represent extraordinarily large values, making it more general than regular numbers. But BigInt can only represent integers, making the regular JavaScript number type more general. There is no way around this problem, so JavaScript sidesteps it by simply not allowing mixed operands to the arithmetic operators.

Comparison operators, by contrast, do work with mixed numeric types (but see §2.9.1 for more about the difference between == and ===):

1 < 2n     // => true
2 > 1n     // => true
0 == 0n    // => true
0 === 0n   // => false: the === checks for type equality as well

The bitwise operators (described in §3.8.3) generally work with BigInt operands. None of the functions of the Math object accept BigInt operands, however.

2.2.6 Dates and Times

JavaScript defines a simple Date class for representing and manipulating the numbers that represent dates and times. Because JavaScript Dates are objects rather than primitive types, however, they are not covered in this chapter. See §9.4 for details.

2.2 Text

The JavaScript type for representing text is the string. A string is an immutable ordered sequence of 16-bit values, each of which typically represents a Unicode character. The length of a string is the number of 16-bit values it contains. JavaScript’s strings (and its arrays) use zero-based indexing: the first 16-bit value is at position 0, the second at position 1 and so on. The empty string is the string of length 0. JavaScript does not have a special type that represents a single element of a string. To represent a single 16-bit value, simply use a string that has a length of 1.

2.3.1 String Literals

To include a string literally in a JavaScript program, simply enclose the characters of the string within a matched pair of single or double quotes (' or "). Double-quote characters may be contained within strings delimited by single-quote characters, and single-quote characters may be contained within strings delimited by double quotes. Here are examples of string literals:

""  // The empty string: it has zero characters
"Wouldn't you prefer O'Reilly's book?"
"This string\nhas two lines"
"π is the ratio of a circle's circumference to its diameter"

In ECMAScript 6 you can also delimit strings with backticks (\`), and strings delimited this way have a special syntax that allows JavaScript expressions to be embedded within the strings. This new kind of string literal is covered in §2.3.4.

The original versions of JavaScript required string literals to be written on a single line, and it is common to see JavaScript code that creates long strings by concatenating single-line strings with the + operator. As of ECMAScript 5, however, you can break a string literal across multiple lines by ending each line but the last with a backslash (\). Neither the backslash nor the line terminator that follow it are part of the string literal. If you need to include a newline character in a string literal, use the character sequence \n (documented below):

// A string representing 2 lines written on one line:
// A one-line string written on 3 lines:

Note that when you use single quotes to delimit your strings, you must be careful with English contractions and possessives, such as can’t and O’Reilly’s. Since the apostrophe is the same as the single-quote character, you must use the backslash character (\) to “escape” any apostrophes that appear in single-quoted strings (escapes are explained in the next section).

In client-side JavaScript programming, JavaScript code may contain strings of HTML code, and HTML code may contain strings of JavaScript code. Like JavaScript, HTML uses either single or double quotes to delimit its strings. Thus, when combining JavaScript and HTML, it is a good idea to use one style of quotes for JavaScript and the other style for HTML. In the following example, the string “Thank you” is single-quoted within a JavaScript expression, which is then double-quoted within an HTML event-handler attribute:

<button onclick="alert('Thank you')">Click Me</button>

2.3.2 Escape Sequences in String Literals

The backslash character (\) has a special purpose in JavaScript strings. Combined with the character that follows it, it represents a character that is not otherwise representable within the string. For example, \n is an escape sequence that represents a newline character.

Another example, mentioned above, is the \' escape, which represents the single quote (or apostrophe) character. This escape sequence is useful when you need to include an apostrophe in a string literal that is contained within single quotes. You can see why these are called escape sequences: the backslash allows you to escape from the usual interpretation of the single-quote character. Instead of using it to mark the end of the string, you use it as an apostrophe:

'You\'re right, it can\'t be a quote'

Table 2-1 lists the JavaScript escape sequences and the characters they represent. Two escape sequences are generic and can be used to represent any character by specifying its Latin-1 or Unicode character code as a hexadecimal number. For example, the sequence \xA9 represents the copyright symbol, which has the Latin-1 encoding given by the hexadecimal number A9. Similarly, the \u escape represents an arbitrary Unicode character specified by four hexadecimal digits; \u03c0 represents the character π, for example.

Table 2-1. JavaScript escape sequences
Sequence Character represented


The NUL character (\u0000)


Backspace (\u0008)


Horizontal tab (\u0009)


Newline (\u000A)


Vertical tab (\u000B)


Form feed (\u000C)


Carriage return (\u000D)


Double quote (\u0022)


Apostrophe or single quote (\u0027)


Backslash (\u005C)


The Latin-1 character specified by the two hexadecimal digits nn


The Unicode character specified by the four hexadecimal digits nnnn


The Unicode character specified by the codepoint n, where n is one to six hexadecimal digits between 0 and 10FFFF.

If the \ character precedes any character other than those shown in Table 2-1, the backslash is simply ignored (although future versions of the language may, of course, define new escape sequences). For example, \# is the same as #. Finally, as noted above, ECMAScript 5 allows a backslash before a line break to break a string literal across multiple lines.

2.3.3 Working with Strings

One of the built-in features of JavaScript is the ability to concatenate strings. If you use the + operator with numbers, it adds them. But if you use this operator on strings, it joins them by appending the second to the first. For example:

let msg = "Hello, " + "world";   // Produces the string "Hello, world"
let greeting = "Welcome to my blog," + " " + name;

Strings can be compared with the standard === equality and !== inequality operators: two strings are equal if and only if they consist of exactly the same sequence of 16-bit values. Strings can also be compared with the <, <=, > and >= operators. String comparison is done simply by comparing the 16-bit values. (For more robust locale-aware string comparison and sorting, see §9.7.3.)

To determine the length of a string—the number of 16-bit values it contains—use the length property of the string. Determine the length of a string s like this:


In addition to this length property, JavaScript provides a rich API for working with strings:

let s = "Hello, world"; // Start with some text.

// Obtaining portions of a string
s.substring(1,4)        // => "ell": the 2nd, 3rd and 4th characters.
s.slice(1,4)            // => "ell": same thing
s.slice(-3)             // => "rld": last 3 characters
s.split(", ")           // => ["Hello", "world"]: split at delimiter string

// Searching a string
s.indexOf("l")          // => 2: position of first letter l.
s.indexOf("l", 3)       // => 3: position of first "l" at or after 3
s.indexOf("zz")         // => -1: s does not include the substring "zz"
s.lastIndexOf("l")      // => 10: position of last letter l.

// Boolean searching functions in ES6 and later
s.startsWith("Hell")    // => true: the string starts with these
s.endsWith("!")         // => false: s does not end with that
s.includes("or")        // => true: s includes substring "or"

// Creating modified versions of a string
s.replace("llo", "ya")  // => "Heya, world"
s.toLowerCase()         // => "hello, world"
s.toUpperCase()         // => "HELLO, WORLD"
s.normalize()           // Unicode NFC normalization: ES6
s.normalize("NFD")      // NFD normalization. Also "NFKC", "NFKD"

// Inspecting individual characters of a string
s.charAt(0)             // => "H": the first character.
s.charAt(s.length-1)    // => "d": the last character.
s.charCodeAt(0)         // => 72: 16-bit number at the specified position
s.codePointAt(0)        // => 72: ES6, works for codepoints > 16 bits

// String padding functions in ES2017
"x".padStart(3)         // => "  x": add spaces on the left to a length of 3
"x".padEnd(3)           // => "x  ": add spaces on the right to a length of 3
"x".padStart(3, "*")    // => "**x": add stars on the left to a length of 3
"x".padEnd(3, "-")      // => "x--": add dashes on the right to a length of 3

// Space trimming functions. trim() is ES5; others ES2019
" test ".trim()         // => "test": remove spaces at start and end
" test ".trimStart()    // => "test ": remove spaces on left. Also trimLeft
" test ".trimEnd()      // => " test": remove spaces at right. Also trimRight

// Miscellaneous string methods
s.concat("!")           // => "Hello, world!": just use + operator instead
"<>".repeat(5)          // => "<><><><><>": concatenate n copies. ES6

Remember that strings are immutable in JavaScript. Methods like replace() and toUpperCase() return new strings: they do not modify the string on which they are invoked.

In ECMAScript 5 and later, strings can be treated like read-only arrays, and you can access individual characters (16-bit values) from a string using square brackets instead of the charAt() method:

let s = "hello, world";
s[0]                  // => "h"
s[s.length-1]         // => "d"

2.3.4 Template Literals

In ECMAScript 6 and later, you can also use backticks to delimit a string:

let s = `hello world`;

This is more than just another string literal syntax, however, because these template literals can include arbitrary JavaScript expressions, whose values are converted to strings and embedded within the template:

let name = "Bill";
let greeting = `Hello ${ name }.`;  // greeting == "Hello Bill."

Everything between the ${ character sequence and the matching } is interpreted as a JavaScript expression. Everything outside the curly braces is normal string literal text. The value inside the braces is converted to a string and inserted into the template, replacing the dollar sign, the curly braces and everything in between them.

A template literal may include any number of expressions. It can use any of the escape characters that normal strings can. And it can span any number of lines, with no special escaping required. The following template literal includes four JavaScript expressions, a Unicode escape sequence and at least four newlines (the expression values may include newlines as well, of course):

let errorMessage = `\
\u2718 Test failure at ${filename}:${linenumber}:
Stack trace:

The backslash at the end of the first line above escapes the initial newline so that the resulting string begins with the Unicode ✘ character rather than a newline.

Tagged Template Literals

A powerful, but less-commonly used, feature of template literals is that if a function name (or “tag”) comes right before the opening backtick, then the text and the values within the template literal are passed to the function and value of the complete “tagged template literal” is the return value of the function. This could be used, for example, to apply HTML or SQL escaping to the values before substituting them into the text, for example.

ECMAScript 6 has one built-in tag function: String.raw(): it returns the text within backticks raw, without any processing of backslash escapes:

String.raw`\n`  // => "\\n": this is a backslash character and the letter n

Note that even though the tag portion of a tagged template literal is a function, there are no parentheses used in its invocation. In this very specific case the backtick characters replace the open and close parentheses.

The ability to define your own template tag functions is a powerful feature of JavaScript. These functions do not even need to return strings and can be used as a kind of constructor, as if defining a new literal syntax for the language. We’ll see an example in [Link to Come].

2.3.5 Pattern Matching

JavaScript defines a datatype known as a regular expression (or RegExp) for describing and matching patterns in strings of text. RegExps are not one of the fundamental data types in JavaScript, but they have a literal syntax like numbers and strings do, so the sometimes seem like they are a fundamental data type. The grammar of regular expression literals is complex and the API they define is nontrivial. They are documented in detail in §9.3. Because RegExps are powerful and commonly used for text processing, however, this section provides a brief overview.

Text between a pair of slashes constitutes a regular expression literal. The second slash in the pair can also be followed by one or more letters, which modify the meaning of the pattern. For example:

/^HTML/;             // Match the letters H T M L at the start of a string
/[1-9][0-9]*/;       // Match a non-zero digit, followed by any # of digits
/\bjavascript\b/i;   // Match "javascript" as a word, case-insensitive

RegExp objects define a number of useful methods, and strings also have methods that accept RegExp arguments. For example:

let text = "testing: 1, 2, 3";   // Sample text
let pattern = /\d+/g;            // Matches all instances of one or more digits
pattern.test(text)               // => true: a match exists             // => 9: position of first match
text.match(pattern)              // => ["1", "2", "3"]: array of all matches
text.replace(pattern, "#")       // => "testing: #, #, #"
text.split(/\D+/)                // => ["","1","2","3"]: split on non-digits

2.3 Boolean Values

A boolean value represents truth or falsehood, on or off, yes or no. There are only two possible values of this type. The reserved words true and false evaluate to these two values.

Boolean values are generally the result of comparisons you make in your JavaScript programs. For example:

a === 4

This code tests to see whether the value of the variable a is equal to the number 4. If it is, the result of this comparison is the boolean value true. If a is not equal to 4, the result of the comparison is false.

Boolean values are commonly used in JavaScript control structures. For example, the if/else statement in JavaScript performs one action if a boolean value is true and another action if the value is false. You usually combine a comparison that creates a boolean value directly with a statement that uses it. The result looks like this:

if (a === 4) {
    b = b + 1;
} else {
    a = a + 1;

This code checks whether a equals 4. If so, it adds 1 to b; otherwise, it adds 1 to a.

As we’ll discuss in §2.8, any JavaScript value can be converted to a boolean value. The following values convert to, and therefore work like, false:

""  // the empty string

All other values, including all objects (and arrays) convert to, and work like, true. false, and the six values that convert to it, are sometimes called falsy values, and all other values are called truthy. Any time JavaScript expects a boolean value, a falsy value works like false and a truthy value works like true.

As an example, suppose that the variable o either holds an object or the value null. You can test explicitly to see if o is non-null with an if statement like this:

if (o !== null) ...

The not-equal operator !== compares o to null and evaluates to either true or false. But you can omit the comparison and instead rely on the fact that null is falsy and objects are truthy:

if (o) ...

In the first case, the body of the if will be executed only if o is not null. The second case is less strict: it will execute the body of the if only if o is not false or any falsy value (such as null or undefined). Which if statement is appropriate for your program really depends on what values you expect to be assigned to o. If you need to distinguish null from 0 and "", then you should use an explicit comparison.

Boolean values have a toString() method that you can use to convert them to the strings “true” or “false”, but they do not have any other useful methods. Despite the trivial API, there are three important boolean operators.

The &amp;&amp; operator performs the Boolean AND operation. It evaluates to a truthy value if and only if both of its operands are truthy; it evaluates to a falsy value otherwise. The || operator is the Boolean OR operation: it evaluates to a truthy value if either one (or both) of its operands is truthy and evaluates to a falsy value if both operands are falsy. Finally, the unary ! operator performs the Boolean NOT operation: it evaluates to true if its operand is falsy and evaluates to false if its operand is truthy. For example:

if ((x === 0 && y === 0) || !(z === 0)) {
    // x and y are both zero or z is non-zero

Full details on these operators are in §3.10.

2.4 null and undefined

null is a language keyword that evaluates to a special value that is usually used to indicate the absence of a value. Using the typeof operator on null returns the string “object”, indicating that null can be thought of as a special object value that indicates “no object”. In practice, however, null is typically regarded as the sole member of its own type, and it can be used to indicate “no value” for numbers and strings as well as objects. Most programming languages have an equivalent to JavaScript’s null: you may be familiar with it as NULL, nil or None.

JavaScript also has a second value that indicates absence of value. The undefined value represents a deeper kind of absence. It is the value of variables that have not been initialized and the value you get when you query the value of an object property or array element that does not exist. The undefined value is also the return value of functions that do not explicitly return a value, and the value of function parameters for which no argument is passed. undefined is a predefined global constant (not a language keyword like null, though this is not an important distinction in practice) that is initialized to the undefined value. If you apply the typeof operator to the undefined value, it returns “undefined”, indicating that this value is the sole member of a special type.

Despite these differences, null and undefined both indicate an absence of value and can often be used interchangeably. The equality operator == considers them to be equal. (Use the strict equality operator === to distinguish them.) Both are falsy values—they behave like false when a boolean value is required. Neither null nor undefined have any properties or methods. In fact, using . or [] to access a property or method of these values causes a TypeError.

I consider undefined to represent a system-level, unexpected, or error-like absence of value and null to represent program-level, normal, or expected absence of value. I avoid using null and undefined when I can, but if I need to assign one of these values to a variable or property or pass or return one of these values to or from a function, I usually use null. Some programmers strive to avoid null entirely and use undefined in its place whereever they can.

2.5 Symbols

Symbol is a specialized type introduced in ECMAScript 6 to serve as non-string property names. To understand symbols, you need to know that JavaScript’s fundamental Object type is an unordered collection of properties, where each property has a name and a value. Property names are typically (and until ES6, were exclusively) strings. But in ECMAScript 6 and later, Symbols can also serve this purpose:

let strname = "string name";      // A string to use as a property name
let symname = Symbol("propname"); // A Symbol to use as a property name
typeof strname                    // => "string": strname is a string
typeof symname                    // => "symbol": symname is a symbol
let o = {};                       // Create a new object
o[strname] = 1;                   // Define a property with a string name
o[symname] = 2;                   // Define a property with a Symbol name
o[strname]                        // => 1: access the string-named property
o[symname]                        // => 2: access the symbol-named property

In order to understand why you might want to use a Symbol as a property name, you need to know that Symbol values are obtained with the Symbol() function and that this function never returns the same value twice, even when called with the same argument. If you call Symbol() to obtain a Symbol value, you can safely use that value as a property name to add a new property to an object and do not need to worry that you might be overwriting an existing property with the same name. Similarly, if you use symbolic property names and do not share those symbols, you can be confident that other modules of code in your program will not accidentally overwrite your properties.

In practice, Symbols serve as a language extension mechanism. When ES6 introduced the for/of loop and iterable objects, it needed to define standard method that classes could implement to make themselves iterable. But standardizing any particular string name for this iterator method would have broken existing code, so a symbolic name was used instead. As we’ll see in Chapter 10, Symbol.iterator is a Symbol value that can be used as a method name to make an object iterable.

The Symbol() function takes an optional string argument and returns a unique Symbol value. If you supply a string argument, that string will be included in the output of the Symbol’s toString() method. Note, however, that calling Symbol() twice with the same string produces two completely different Symbol values.

let s = Symbol("sym_x");
s.toString()             // => "Symbol(sym_x)"

toString() is the only interesting method of Symbol instances. There are two other Symbol-related functions you should know about, however. Sometimes when using symbols, you want to keep them private to your own code so you have a guarantee that your properties will never conflict with properties used by other code. Other times, however, you might want to define a Symbol value and share it widely with other code. This would be the case, for example, if you were defining some kind of extension that you wanted other code to be able to participate in, as with the Symbol.iterator mechanism described above.

To serve this latter use-case, JavaScript defines a global symbol registry. The Symbol.for() function takes a string argument and returns a Symbol value that is associated with the string you pass. If no symbol is already associated with that string, then a new one is created and returned, otherwise the already existing symbol is returned. That is, the Symbol.for() function is completely different than the Symbol() function: Symbol() never returns the same value twice, but Symbol.for() always returns the same value when called with the same string. The string passed to Symbol.for() appears in the output of toString() for the returned Symbol, and it can also be retrieved by calling Symbol.keyFor() on the returned Symbol.

let s = Symbol.for("shared");
let t = Symbol.for("shared");
s === t          // => true
s.toString()     // => "Symbol(shared)"
Symbol.keyFor(t) // => "shared"

2.6 The Global Object

The sections above have explained JavaScript’s primitive types and values. Object types—objects, arrays, and functions—are covered in chapters of their own later in this book. But there is one very important object value that we must cover now. The global object is a regular JavaScript object that serves a very important purpose: the properties of this object are the globally defined symbols that are available to a JavaScript program. When the JavaScript interpreter starts (or whenever a web browser loads a new page), it creates a new global object and gives it an initial set of properties that define:

  • global constants like undefined, Infinity, and NaN

  • global functions like isNaN(), parseInt() (§2.9.2), and eval() (§3.12).

  • constructor functions like Date(), RegExp(), String(), Object(), and Array() (§2.9.2)

  • global objects like Math and JSON (§5.8)

The initial properties of the global object are not reserved words, but they deserve to be treated as if they are. This chapter has already described some of these global properties. Most of the others will be covered elsewhere in this book.

In Node, the global object has a property named global whose value is the global object itself, so you can always refer to the global object with the name global.

In client-side JavaScript, the Window object serves as the global object for all JavaScript code contained in the browser window it represents. This global Window object has a self-referential window property that can be used instead of this to refer to the global object. The Window object defines the core global properties, but it also defines quite a few other globals that are specific to web browsers and client-side JavaScript.

2.7 Immutable Primitive Values and Mutable Object References

There is a fundamental difference in JavaScript between primitive values (undefined, null, booleans, numbers, and strings) and objects (including arrays and functions). Primitives are immutable: there is no way to change (or “mutate”) a primitive value. This is obvious for numbers and booleans—it doesn’t even make sense to change the value of a number. It is not so obvious for strings, however. Since strings are like arrays of characters, you might expect to be able to alter the character at any specified index. In fact, JavaScript does not allow this, and all string methods that appear to return a modified string are, in fact, returning a new string value. For example:

let s = "hello";   // Start with some lowercase text
s.toUpperCase();   // Returns "HELLO", but doesn't alter s
s                  // => "hello": the original string has not changed

Primitives are also compared by value: two values are the same only if they have the same value. This sounds circular for numbers, booleans, null, and undefined: there is no other way that they could be compared. Again, however, it is not so obvious for strings. If two distinct string values are compared, JavaScript treats them as equal if, and only if, they have the same length and if the character at each index is the same.

Objects are different than primitives. First, they are mutable—their values can change:

let o = { x: 1 };  // Start with an object
o.x = 2;           // Mutate it by changing the value of a property
o.y = 3;           // Mutate it again by adding a new property

let a = [1,2,3];   // Arrays are also mutable
a[0] = 0;          // Change the value of an array element
a[3] = 4;          // Add a new array element

Objects are not compared by value: two objects are not equal even if they have the same properties and values. And two arrays are not equal even if they have the same elements in the same order:

let o = {x: 1}, p = {x: 1};  // Two objects with the same properties
o === p                      // => false: distinct objects are never equal
let a = [], b = [];          // Two distinct, empty arrays
a === b                      // => false: distinct arrays are never equal

Objects are sometimes called reference types to distinguish them from JavaScript’s primitive types. Using this terminology, object values are references, and we say that objects are compared by reference: two object values are the same if and only if they refer to the same underlying object.

let a = [];   // The variable a refers to an empty array.
let b = a;    // Now b refers to the same array.
b[0] = 1;     // Mutate the array referred to by variable b.
a[0]          // => 1: the change is also visible through variable a.
a === b       // => true: a and b refer to the same object, so they are equal.

As you can see from the code above, assigning an object (or array) to a variable simply assigns the reference: it does not create a new copy of the object. If you want to make a new copy of an object or array, you must explicitly copy the properties of the object or the elements of the array. This example demonstrates using a for loop (§4.4.3):

let a = ["a","b","c"];              // An array we want to copy
let b = [];                         // A distinct array we'll copy into
for(let i = 0; i < a.length; i++) { // For each index of a[]
    b[i] = a[i];                    // Copy an element of a into b

Similarly, if we want to compare two distinct objects or arrays, we must compare their properties or elements. This code defines a function to compare two arrays:

function equalArrays(a,b) {
    if (a.length !== b.length) return false; // Different-size arrays not equal
    for(let i = 0; i < a.length; i++) {      // Loop through all elements
        if (a[i] !== b[i]) return false;     // If any differ, arrays not equal
    return true;                             // Otherwise they are equal

2.8 Type Conversions

JavaScript is very flexible about the types of values it requires. We’ve seen this for booleans: when JavaScript expects a boolean value, you may supply a value of any type, and JavaScript will convert it as needed. Some values (“truthy” values) convert to true and others (“falsy” values) convert to false. The same is true for other types: if JavaScript wants a string, it will convert whatever value you give it to a string. If JavaScript wants a number, it will try to convert the value you give it to a number (or to NaN if it cannot perform a meaningful conversion). Some examples:

10 + " objects"     // => "10 objects":  Number 10 converts to a string
"7" * "4"           // => 28: both strings convert to numbers
let n = 1 - "x";    // n == NaN; string "x" can't convert to a number
n + " objects"      // => "NaN objects": NaN converts to string "NaN"

Table 2-2 summarizes how values convert from one type to another in JavaScript. Bold entries in the table highlight conversions that you may find surprising. Empty cells indicate that no conversion is necessary and none is performed.

Table 2-2. JavaScript type conversions
Value to String to Number to Boolean















"" (empty string)



"1.2" (nonempty, numeric)



"one" (nonempty, non-numeric)


















1 (finite, non-zero)



{} (any object)

see §2.9.3

see §2.9.3


[] (empty array)




[9] (one numeric element)




['a'] (any other array)

use join() method



function(){} (any function)

see §2.9.3



The primitive-to-primitive conversions shown in the table are relatively straightforward. Conversion to boolean was already discussed in §2.3. Conversion to strings is well-defined for all primitive values. Conversion to numbers is just a little trickier. Strings that can be parsed as numbers convert to those numbers. Leading and trailing spaces are allowed, but any leading or trailing non-space characters that are not part of a numeric literal cause the string-to-number conversion to produce NaN. Some numeric conversions may seem surprising: true converts to 1, and false and the empty string "" convert to 0.

Object-to-primitive conversion is somewhat more complicated, and it is the subject of §2.9.3.

2.9.1 Conversions and Equality

Because JavaScript can convert values flexibly, its == equality operator is also flexible with its notion of equality. All of the following comparisons are true, for example:

null == undefined // => true: These two values are treated as equal.
"0" == 0          // => true: String converts to a number before comparing.
0 == false        // => true: Boolean converts to number before comparing.
"0" == false      // => true: Both operands convert to 0 before comparing!

§3.9.1 explains exactly what conversions are performed by the == operator in order to determine whether two values should be considered equal, and it also describes the strict equality operator === that does not perform conversions when testing for equality.

Keep in mind that convertibility of one value to another does not imply equality of those two values. If undefined is used where a boolean value is expected, for example, it will convert to false. But this does not mean that undefined == false. JavaScript operators and statements expect values of various types, and perform conversions to those types. The if statement converts undefined to false, but the == operator never attempts to convert its operands to booleans.

2.9.2 Explicit Conversions

Although JavaScript performs many type conversions automatically, you may sometimes need to perform an explicit conversion, or you may prefer to make the conversions explicit to keep your code clearer.

The simplest way to perform an explicit type conversion is to use the Boolean(), Number(), and String() functions:

Number("3")    // => 3
String(false)  // => "false":  Or use false.toString()
Boolean([])    // => true

Any value other than null or undefined has a toString() method and the result of this method is usually the same as that returned by the String() function.

As an aside, note that the Boolean(), Number() and String() functions can also be invoked, with new, as constructor. If you use them this way, you’ll get a “wrapper” object that behaves just like a primitive boolean, number or string value. These wrapper objects are a historical leftover from the earliest days of JavaScript, and there is never really any good reason to use them.

Certain JavaScript operators perform implicit type conversions, and are sometimes used for the purposes of type conversion. If one operand of the + operator is a string, it converts the other one to a string. The unary + operator converts its operand to a number. And the unary ! operator converts its operand to a boolean and negates it. These facts lead to the following type conversion idioms that you may see in some code:

x + ""   // => String(x)
+x       // => Number(x)
x-0      // => Number(x)
!!x      // => Boolean(x): Note double !

Formatting and parsing numbers are common tasks in computer programs and JavaScript has specialized functions and methods that provide more precise control over number-to-string and string-to-number conversions.

The toString() method defined by the Number class accepts an optional argument that specifies a radix, or base, for the conversion. If you do not specify the argument, the conversion is done in base 10. However, you can also convert numbers in other bases (between 2 and 36). For example:

let n = 17;
let binary = "0b" + n.toString(2);  // binary == "0b10001"
let octal = "0o" + n.toString(8);   // octal == "0o21"
let hex = "0x" + n.toString(16);    // hex == "0x11"

When working with financial or scientific data, you may want to convert numbers to strings in ways that give you control over the number of decimal places or the number of significant digits in the output, or you may want to control whether exponential notation is used. The Number class defines three methods for these kinds of number-to-string conversions. toFixed() converts a number to a string with a specified number of digits after the decimal point. It never uses exponential notation. toExponential() converts a number to a string using exponential notation, with one digit before the decimal point and a specified number of digits after the decimal point (which means that the number of significant digits is one larger than the value you specify). toPrecision() converts a number to a string with the number of significant digits you specify. It uses exponential notation if the number of significant digits is not large enough to display the entire integer portion of the number. Note that all three methods round the trailing digits or pad with zeros as appropriate. Consider the following examples:

let n = 123456.789;
n.toFixed(0)         // => "123457"
n.toFixed(2)         // => "123456.79"
n.toFixed(5)         // => "123456.78900"
n.toExponential(1)   // => "1.2e+5"
n.toExponential(3)   // => "1.235e+5"
n.toPrecision(4)     // => "1.235e+5"
n.toPrecision(7)     // => "123456.8"
n.toPrecision(10)    // => "123456.7890"

In addition to the number formatting methods shown here, the Intl.NumberFormat class defines a more general internationalized number formatting method. See §9.7.1 for details.

If you pass a string to the Number() conversion function, it attempts to parse that string as an integer or floating-point literal. That function only works for base-10 integers, and does not allow trailing characters that are not part of the literal. The parseInt() and parseFloat() functions (these are global functions, not methods of any class) are more flexible. parseInt() parses only integers, while parseFloat() parses both integers and floating-point numbers. If a string begins with “0x” or “0X”, parseInt() interprets it as a hexadecimal number.2 Both parseInt() and parseFloat() skip leading whitespace, parse as many numeric characters as they can, and ignore anything that follows. If the first non-space character is not part of a valid numeric literal, they return NaN:

parseInt("3 blind mice")     // => 3
parseFloat(" 3.14 meters")   // => 3.14
parseInt("-12.34")           // => -12
parseInt("0xFF")             // => 255
parseInt("0xff")             // => 255
parseInt("-0XFF")            // => -255
parseFloat(".1")             // => 0.1
parseInt("0.1")              // => 0
parseInt(".1")               // => NaN: integers can't start with "."
parseFloat("$72.47")         // => NaN: numbers can't start with "$"

parseInt() accepts an optional second argument specifying the radix (base) of the number to be parsed. Legal values are between 2 and 36. For example:

parseInt("11", 2)     // => 3: (1*2 + 1)
parseInt("ff", 16)    // => 255: (15*16 + 15)
parseInt("zz", 36)    // => 1295: (35*36 + 35)
parseInt("077", 8)    // => 63: (7*8 + 7)
parseInt("077", 10)   // => 77: (7*10 + 7)

2.9.3 Object to Primitive Conversions

The rules for converting objects to primitive values are complicated. One reason for the complexity is that some types of objects have more than one primitive representation. Date objects, for example can be represented as strings or as numeric timestamps. The JavaScript specification defines three fundamental algorithms for converting objects to primitive values:

  • prefer-string: this algorithm returns a primitive value, preferring a string value, if a conversion to string is possible

  • prefer-number: this algorithm returns a primitive value, preferring a number, if such a conversion is possible.

  • no-preference: this algorithm expresses no preference about what type of primitive value is desired, and classes can define their own conversions. Of the built-in JavaScript types, all except Date implement this algorithm as prefer-number. The Date class implements this algorithm as prefer-string.

The implementation of these object-to-primitive conversion algorithms, is explained at the end of this section. First, however, we explain how the algorithms are used in JavaScript.

Object to Boolean Conversions

Object-to-boolean conversions are trivial: all objects convert to true. Notice that this conversion does not require the use of the object-to-primitive algorithms described above, and that it literally applies to all objects, including empty arrays and even the wrapper object new Boolean(false).

Object to String Conversions

When an object needs to be converted to a string, JavaScript first converts it to a primitive using the prefer-string algorithm, and then converts the resulting primitive value to a string, if necessary, following the rules in Table 2-2.

This kind of conversion happens, for example, if you pass an object to a built-in function that expects a string argument, if you call String() as a conversion function, and when you interpolate objects into template literals (§2.3.4).

Object to Number Conversions

When an object needs to be converted to a number, JavaScript first converts it to a primitive value using the prefer-number algorithm, and then converts the resulting primitive value to a number, if necessary, following the rules in Table 2-2.

Built-in JavaScript functions and methods that expect numeric arguments convert object arguments to numbers in this way, and most (but see the exceptions below) JavaScript operators that expect numeric operands convert objects to numbers in this way as well.

Special Case Operator Conversions

Operators are covered in detail in Chapter 3. Here we explain the special case operators that do not use the basic object-to-string and object-to-number conversions described above.

The + operator in JavaScript performs numeric addition and string concatenation. If either of its operands is an object, JavaScript converts them to primitive values using the no-preference algorithm. Once it has two primitive values, it checks their types. If either argument is a string, it converts the other to a string and concatenates the strings. Otherwise it converts both arguments to numbers and adds them.

The == and != operators perform equality and inequality testing in a loose way that allows type conversions. If one operand is an object and the other is a primitive value these operators convert the object to primitive using the no-preference algorithm and then compare the two primitive values.

Finally, the relational operators <, <=, > and >= compare the order of their operands, and can be used to compare both numbers and strings. If either operand is an object, it is converted to a primitive value using the prefer-number algorithm. Note, however, that unlike the object-to-number conversion, the primitive values returned by the prefer-number conversion are not then converted to numbers.

Note that the numeric representation of Date objects is meaningfully comparable with < and >, but the string representation is not. For Date objects, the no-preference algorithm converts to a string, so the fact that JavaScript uses the prefer-number algorithm for these operators means that we can use them compare the order of two Date objects.

The toString() and valueOf() methods

All objects inherit two conversion methods that are used by object-to-primitive conversions, and before we can explain the prefer-string, prefer-number and no-preference conversion algorithms, we have to explain these two methods.

The first method is toString(), and its job is to return a string representation of the object. The default toString() method does not return a very interesting value (though we’ll find it useful in [Link to Come]):

({x: 1, y: 2}).toString()    // => "[object Object]"

Many classes define more specific versions of the toString() method. The toString() method of the Array class, for example, converts each array element to a string and joins the resulting strings together with commas in between. The toString() method of the Function class returns converts user-defined functions to strings of JavaScript source code. The Date class defines a toString() method that returns a human-readable (and JavaScript-parsable) date and time string. The RegExp class defines a toString() method that converts RegExp objects to a string that looks like a RegExp literal:

[1,2,3].toString()                  // => "1,2,3"
(function(x) { f(x); }).toString()  // => "function(x) { f(x); }"
/\d+/g.toString()                   // => "/\\d+/g"
let d = new Date(2020,0,1);
d.toString()  // => "Wed Jan 01 2020 00:00:00 GMT-0800 (Pacific Standard Time)"

The other object conversion function is called valueOf(). The job of this method is less well-defined: it is supposed to convert an object to a primitive value that represents the object, if any such primitive value exists. Objects are compound values, and most objects cannot really be represented by a single primitive value, so the default valueOf() method simply returns the object itself rather than returning a primitive. Wrapper classes such as String, Number and Boolean define valueOf() methods that simply return the wrapped primitive value. Arrays, functions, and regular expressions simply inherit the default method. Calling valueOf() for instances of these types simply returns the object itself. The Date class defines a valueOf() method that returns the date in its internal representation: the number of milliseconds since January 1, 1970:

let d = new Date(2010, 0, 1);   // January 1st, 2010, (Pacific time)
d.valueOf()                     // => 1262332800000

Object to Primitive Conversion Algorithms

With the toString() and valueOf() methods explained, we can now explain approximately how the three object-to-primitive algorithms works. (The complete details are deferred until [Link to Come])

  • The prefer-string algorithm first tries the toString() method. If the method is defined and returns a primitive value, then JavaScript uses that primitive value (even if it is not a string!). If toString() does not exist or if it returns an object, then JavaScript tries the valueOf() method. If that method exists and returns a primitive value, then JavaScript uses that value. Otherwise the conversion fails with a TypeError.

  • The prefer-number algorithm works like the prefer-string algorithm, except that it tries valueOf() first and toString() second.

  • The no-preference algorithm depends on the class of the object being converted. If the object is a Date object, then JavaScript uses the prefer-string algorithm. For any other object JavaScript uses the prefer-number algorithm.

The rules described above are true for all built-in JavaScript types, and are the default rules for any classes you define yourself. [Link to Come] explains how you can define your own object-to-primitive conversion algorithms for the classes you define.

Before we leave this topic it is worth noting that the details of the prefer-number conversion explain why empty arrays convert to the number 0 and single-element arrays can also convert to numbers:

Number([])    // => 0: this is unexpected!
Number([99])  // => 99: really?

The object-to-number conversion first converts the object to a primitive using the prefer-number algorithm, and then converts the resulting primitive value to a number. The prefer-number algorithm tries valueOf() first and then falls back on toString(). But the Array class inherits the default valueOf() method which does not return a primitive value. So when we try to convert an Array to a number, we end up invoking the toString() method of the array. Empty arrays convert to the empty string. And the empty string converts to the number 0. An array with a single element converts to the same string that that one element does. If an array contains a single number, that number is converted to a string, and then back to a number.

2.9 Variable Declaration and Assignment

One of the most fundamental techniques of computer programming is the use of names–or identifiers–to represent values. Binding a name to a value gives us a way to refer to that value and use it in the programs we write. When we do this, we typically say that we are assigning a value to a variable. The term “variable” implies that new values can be assigned: that the value associated with the variable may vary as our program runs. If we permanently assign a value to a name, then we call that name a constant instead of a variable.

Before you can use a variable or constant in a JavaScript program, you must declare it. In ECMAScript 6 and later, this is done with the let and const keywords, which we explain immediately below. Prior to ES6, variables are declared with var, which is more idiosyncratic and is explained later on in this section.

2.10.1 Declarations with let and const

In modern JavaScript (ES6 and later), variables are declared with the let keyword, like this:

let i;
let sum;

You can also declare multiple variables in a single let statement:

let i, sum;

It is a good programming practice to assign an initial value to your variables when you first declare them:

let message = "hello";
let i = 0, j = 0, k = 0;
let x = 2, y = x*x; // Initializers can use previously declared variables

If you don’t specify an initial value for a variable with the let statement, the variable is declared, but its value is undefined until your code stores a value into it.

To declare a constant instead of a variable, use const instead of let. const works just like let except that you must initialize the constant when you declare it:

const H0 = 74;         // Hubble constant (km/s/Mpc)
const C = 299792.458;  // Speed of light in a vacuum (km/s)
const AU = 1.496E8;    // Astronomical Unit: distance to the sun (km)

It is a common convention to use capital letters for constants as a way to distinguish them from variables. This is only a convention, however, and is not required.

When to use const

There are two schools of thought about the use of the const keyword. One approach is to use const only for values that are fundamentally unchanging, like the physical constants shown above, or program version numbers, or byte sequences used to identify file types, for example. Another approach recognizes that many of the so-called variables in our program don’t actually ever change as our program runs. In this approach, we declare everything with const, and then if we find that we do actually want to allow the value to vary, we switch the declaration to let. This may help to prevent bugs by ruling out accidental changes to variables that we did not intend.

In one approach we use const only for values that must not change. In the other we use const for any value that does not happen to change.

In Chapter 4 we’ll learn about the for, for/in, and for/of loop statements in JavaScript. Each of these loops includes a loop variable that gets a new value assigned to it on each iteration of the loop. JavaScript allows us to declare the loop variable as part of the loop syntax itself, and this is another common way to use let:

for(let i = 0, len = data.length; i < len; i++) console.log(data[i]);
for(let datum of data) console.log(datum);
for(let property in object) console.log(property);

It may seem surprising, but you an also use const to declare the loop “variables” for for/in and for/of loops, as long as the body of the loop does not re-assign a new value. In this case, the const declaration is just saying that the value is constant for the duration of one loop iteration:

for(const datum of data) console.log(datum);
for(const property in object) console.log(property);

Variable and Constant Scope

The scope of a variable is the region of your program source code in which it is defined. Variables and constants declared with let and const are block scoped. This means that they are only defined within the block of code in which the let or const statement appears. JavaScript class and function definitions are blocks, and so are the bodies of if/else statements, while loops, for loops, and so on. Roughly speaking, if a variable or constant is declared within a set of curly braces, then those curly braces are the region of code in which the variable or constant is defined (though of course it is not legal to reference a variable or constant from lines of code that execute before the let or const statement that declares the variable). Variables and constants declared as part of a for, for/in or for/of loop have the loop body as their scope even though they technically appear outside of the curly braces.

When a declaration appears at the top level, outside of any code blocks, we say it is a global variable or constant and has global scope. In Node, and in client-side JavaScript modules (see [Link to Come]), the scope of a global variable is the file that it is defined in. In traditional client-side JavaScript, however, the scope of a global variable is the HTML document in which it is defined. That is: if one <script> declares a global variable or constant, that variable or constant is defined in all of the <script> elements in that document (or at least all of the scripts that execute after the let or const statement executes).

Repeated Declarations

It is a syntax error to use the same name with more than one let or const declaration in the same scope. It is legal (though a practice best avoided) to re-declare a variable in a nested scope:

const x = 1;        // Declare x as a global constant
if (x === 1) {
    let x = 2;      // Inside a block x can refer to a different value
    console.log(x); // Prints 2
console.log(x);     // Prints 1: we're back in the global scope now
let x = 3;          // ERROR! Syntax error trying to re-declare x

Declarations and Types

If you’re used to statically typed languages such as C or Java, you may think that the primary purpose of variable declarations is to specify the type of values that may be assigned to a variable. But JavaScript is an untyped language, and there is no type associated with JavaScript’s variable declarations.3 A JavaScript variable can hold a value of any type. For example, it is perfectly legal in JavaScript to assign a number to a variable and then later assign a string to that variable:

let i = 10;
i = "ten";

2.10.2 Variable Declarations with var

In versions of JavaScript before ES6, the only way to declare a variable is with the var keyword, and there is no way to declare constants. The syntax of var is just like the syntax of let:

var x;
var data = [], count = data.length;
for(var i = 0; i < count; i++) console.log(data[i]);

Although var and let have the same syntax, there are important differences in the way they work:

  • Variables declared with var do not have block scope. Instead, they are scoped to the body of the containing function no matter how deeply nested they are inside that function.

  • If you use var outside of a function body, it declares a global variable. But global variables declared with var differ from globals declared with let in an important way. Globals declared with var are implemented as properties of the global object. The global object can be referenced as window in client-side JavaScript and as global in Node. So if you write var x = 2; outside of a function, it is like you wrote global.x = 2; or window.x = 2;. (The analogy is not perfect, however: the properties created with global var declarations cannot be deleted with the delete operator.)

  • Unlike variables declared with let, it is legal to declare the same variable multiple times with var. And because var variables have function scope instead of block scope, it is actually common to do this kind of redeclaration. The variable i is frequently used for integer values, and especially as the index variable of for loops. In a function with multiple for loops, it is typical for each one to begin for(var i = 0; .... Because var does not scope these variables to the loop body, each of these loops is (harmlessly) re-declaring and re-initializing the same variable.

  • One of the most unusual features of var declarations is known as “hoisting”. When a variable is declared with var, the declaration is lifted up (or “hoisted”) to the top of the enclosing function. The initialization of the variable remains where you wrote it, but the definition of the variable moves to the top of the function. So variables declared with var can be used, without error, anywhere in the enclosing function. If the initialization code has not run yet, then the value of the variable may be undefined but you won’t get an error if you use the variable before it is initialized. (This can be a source of bugs and is one of the important misfeatures that let corrects: if you declare a variable with let but attempt to use it before the let statement runs, you will get an actual error instead of just seeing an undefined value.)

Using Undeclared Variables

In strict mode (§4.6.3) if you attempt to use an undeclared variable, you’ll get a reference error when you run your code. Outside of strict mode, however, if you assign a value to a name that has not been declared with let, const or var, you’ll end up creating a new global variable. It will be a global no matter now deeply nested within functions and blocks your code is, which is almost certainly not what you want, is bug-prone, and is one of the best reasons for using strict mode!

Global variables created in this accidental way are like global variables declared with var: they define properties of the global object. But unlike the properties defined by proper var declarations, these properties can be deleted with the delete operator.

2.10.3 Destructuring Assignment

ECMAScript 6 implements a kind of compound declaration and assignment syntax known as destructuring assignment. In a destructuring assignment, the value on the right-hand side of the equals sign is an array or object (a “structured” value) and the left-hand side specifies one or more variable names using a syntax that mimics array and object literal syntax. When a destructuring assignment occurs, one or more values are extracted (“destructured”) from the value on the right and stored into the variables named on the left. Destructuring assignment is perhaps most commonly used to initialize variables as part of a const, let, or var declaration statement, but it can also be done in regular assignment expressions (with variables that have already been declared). And, as we’ll see in §7.3.5, destructuring can also be used when defining the parameters to a function.

Here are simple destructuring assignments using arrays of values:

let [x,y] = [1,2];  // Same as let x=1, y=2
[x,y] = [x+1,y+1];  // Same as x = x + 1, y = y+1
[x,y] = [y,x];      // Swap the value of the two variables
[x,y]               // => [3,2]: the incremented and swapped values

Notice how destructuring assignment makes it easy to work with functions that return arrays of values:

// Convert [x,y] coordinates to [r,theta] polar coordinates
function toPolar(x, y) {
    return [Math.sqrt(x*x+y*y), Math.atan2(y,x)];

// Convert polar to Cartesian coordinates
function toCartesian(r, theta) {
    return [r*Math.cos(theta), r*Math.sin(theta)];

let [r,theta] = toPolar(1.0, 1.0);  // r == Math.sqrt(2); theta == Math.PI/4
let [x,y] = toCartesian(r,theta);   // [x, y] == [1.0, 1,0]

We saw above that variables and constants can be declared as part of JavaScript’s various for loops. It is possible to use variable destructuring in this context as well. Here is a code that loops over the name/value pairs of all properties of an object, and uses destructuring assignment to convert those pairs from two-element arrays into individual variables:

let o = { x: 1, y: 2 }; // The object we'll loop over
for(const [name, value] of Object.entries(o)) {
    console.log(name, value); // Prints "x 1" and "y 2"

The number of variables on the left of a destructuring assignment does not have to match the number of array elements on the right. Extra variables on the left are set to undefined, and extra values on the right are ignored. The list of variables on the left can include extra commas to skip certain values on the right:

let [x,y] = [1];     // x == 1; y == undefined
[x,y] = [1,2,3];     // x == 1; y == 2
[,x,,y] = [1,2,3,4]; // x == 2; y == 4

If you want to collect all unused or remaining values into a single variable when destructuring an array, use three dots ... before the last variable name on the left hand side:

let [x, ...y] = [1,2,3,4];  // y == [2,3,4]

We’ll see three dots used this way again in §7.3.2 where they are used to indicate that all remaining function arguments should be collected into a single array.

Destructuring assignment can be used with nested arrays. In this case, the left-hand side of the assignment should look like a nested array literal:

let [a, [b, c]] = [1, [2,2.5], 3]; // a == 1; b == 2; c == 2.5

A powerful feature of array destructuring is that it does not actually require an array! You can use any iterable object (Chapter 10) on the right-hand side of the assignment: any object that can be used with a for/of loop (§4.4.4) can also be destructured:

let [first,] = "Hello"; // first == "H"; rest == ["e","l","l","o"]

Destructuring assignment can also be performed when the right-hand side is an object value. In this case, the left-hand side of the assignment looks something like an object literal: a comma-separated list of variable names within curly braces:

let transparent = {r: 0.0, g: 0.0, b: 0.0, a: 1.0}; // A RGBA color
let {r, g, b} = transparent;  // r == 0.0; g == 0.0; b == 0.0

The next example copies global functions of the Math object into variables, which might simplify code that does a lot of trigonometry:

// Same as let sin=Math.sin, cos=Math.cos, tan=Math.tan
let {sin, cos, tan} = Math;

Notice in the code above that the Math object has many properties other than the three that are destructured into individual variables. Those that are not named are simply ignored. If the left-hand side of the assignment above had included a variable whose name was not a property of Math, that variable would simply be assigned undefined.

In each of the object destructuring examples above, we have chosen variable names that match the property names of the object we’re destructuring. This keeps the syntax simple and easy to understand, but it is not required. Each of the identifiers on the left-hand side of an object destructuring assignment can also be a colon-separated pair of identifiers, where the first is the name of the property whose value is to be assigned and the second is the name of the variable to assign it to:

// Same as let cosine = Math.cos, tangent = Math.tan;
let { cos: cosine, tan: tangent } = Math;

I find that object destructuring syntax becomes too complicated to be useful when the variable names and property names are not the same, and I tend to avoid the shorthand in this case. If you choose to use it, remember that property names are always on the left of the colon, in both object literals and on the left of an object destructuring assignment.

Destructuring assignment becomes even more complicated when it is used with nested object, or arrays of objects, or objects of arrays, but it is legal:

let points = [{x: 1, y: 2}, {x: 3, y: 4}];     // An array of two point objects
let [{x: x1, y: y1}, {x: x2, y: y2}] = points; // destructured into 4 variables.
(x1 === 1 && y1 === 2 && x2 === 3 && y2 === 4) // => true

Or, instead of destructuring an array of object, we could destructure an object of arrays:

let points = { p1: [1,2], p2: [3,4] };         // An object with 2 array props
let { p1: [x1, y1], p2: [x2, y2] } = points;   // destructured into 4 vars
(x1 === 1 && y1 === 2 && x2 === 3 && y2 === 4) // => true

Complex destructuring syntax like this can be hard to write and hard to read, and you may be better off just writing out your assignments explicitly with traditional code like let x1 = points.p1[0];.

If you find yourself working with code that uses complex destructuring like this, however, there is a useful regularity that can help you to make sense of the complex cases. Think first about a regular (single-value) assignment. After the assignment is done, you can take the variable name from the left-hand side of the assignment and use it as an expression in your code, where it will evaluate to whatever value you assigned it. In destructuring assignment, we’ve said that the left-hand side uses a syntax like array literal syntax or like object literal syntax. But notice that after the destructuring assignment is done, the code that looks like an array literal or object literal from the left-hand side will actually work as a valid array literal or object literal elsewhere in your code: all the necessary variables have been defined so that you can cut-and-paste the text on the left of the equals sign and use it as an array or object literal (possibly with nested array and object literals) in your code.

2.10 Summary

This chapter explained:

  • How to write and manipulate numbers and strings of text in JavaScript

  • How to work with JavaScript’s other primitive types: booleans, symbols, null and undefined.

  • The differences between immutable primitive types and mutable reference types.

  • How JavaScript converts values implicitly from one type to another and how you can do so explicitly in your programs.

  • How to declare and initialize constants and variables (including with destructuring assignment), and the lexical scope of the variables and constants you declare.

1 This is the format for numbers of type double in Java, C++, and most modern programming languages.

2 In ECMAScript 3, parseInt() may parse a string that begins with “0” (but not “0x” or “0X”) as an octal number or as a decimal number. Because the behavior is unspecified, you should never use parseInt() to parse numbers with leading zeros, unless you explicitly specify the radix to be used! In ECMAScript 5, parseInt() only parses octal numbers if you explicitly pass 8 as the second argument.

3 There are JavaScript extensions, such as TypeScript and Flow that allow types to be specified as part of variable declarations with syntax like let x: number = 0;