JavaScript Reference

The JavaScript Reference serves as a repository of facts about the JavaScript language. The entire language is described here in detail. As you write JavaScript code, you'll refer to these pages often (thus the title "JavaScript Reference"). If you're learning JavaScript, or need help understanding some of its capabilities or features, check out the JavaScript Guide. 

What you should already know

The JavaScript language is intended to be used within some larger environment, be it a browser, server-side scripts, or similar. For the most part, this reference attempts to be environment-agnostic and does not target a web browser environment. For demonstration purposes, this reference uses a function, println, which is not part of JavaScript and can be mapped to environment-specific functionality to display given values. For example, in a web browser println might have been defined as follows:

function println(string) {
  console.log(string);
}

Mapping to equivalent functionality in other environments is left as an exercise for the reader.

Formatting conventions

This reference includes descriptive syntax sections to demonstrate appropriate or common usage of the subject of documentation. Within these sections, all text literals to be reproduced verbatim are non-italicized, with the exception of ellipses. Words in italics represent user-defined names or statements. Any portions enclosed in square brackets ([ and ]) are optional. A comma-delimited sequence that includes an ellipsis (...) indicates that the sequence is a list and all items in the sequence except the first are optional (e.g. only param1 is required in "param1, param2, ..., paramN").

JavaScript history

Recent versions of Mozilla-based browsers support newer versions of JavaScript. The following table lists the JavaScript version supported by different Mozilla-based browser versions.

Browsers that do not support at least JavaScript 1.5 are very rare today, since JavaScript 1.5 was introduced back in 1999. If you're interested in historic information, please refer to the Wikipedia article on ECMAScript.

JavaScript/Browser support history

Where to find JavaScript information

JavaScript documentation of core language features (pure ECMAScript, for the most part) includes the following:

• The JavaScript Guide
• The JavaScript Reference

If you are new to JavaScript, start with the Guide. Once you have a firm grasp of the fundamentals, you can use the Reference to get more details on individual objects and language constructs.

JavaScript statements consist of keywords used with the appropriate syntax. A single statement may span multiple lines. Multiple statements may occur on a single line if each statement is separated by a semicolon. This isn't a keyword, but a group of keywords.

Generators

Generators functions enable writing iterators more easily.

block

A block statement is used to group zero or more statements. The block is delimited by a pair of curly brackets.

break

Terminates the current loop, switch, or label statement and transfers program control to the statement following the terminated statement.

const

Declares a read-only named constant.

continue

Terminates execution of the statements in the current iteration of the current or labelled loop, and continues execution of the loop with the next iteration.

debugger

Invokes any available debugging functionality. If no debugging functionality is available, this statement has no effect.

do...while

Creates a loop that executes a specified statement until the test condition evaluates to false. The condition is evaluated after executing the statement, resulting in the specified statement executing at least once.

export

Allows a signed script to provide properties, functions, and objects to other signed or unsigned scripts. This feature is not in ECMA-262, Edition 3.

for

Creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by a statement executed in the loop.

for each...in

Iterates a specified variable over all values of object's properties. For each distinct property, a specified statement is executed.

for...in

Iterates over the enumerable properties of an object, in arbitrary order. For each distinct property, statements can be executed.

for...of

Iterates over iterable objects (including arrays, array-like objects, iterators and generators), invoking a custom iteration hook with statements to be executed for the value of each distinct property.

function

Declares a function with the specified parameters.

if...else

Executes a statement if a specified condition is true. If the condition is false, another statement can be executed.

import

Allows a script to import properties, functions, and objects from a signed script that has exported the information. This feature is not in ECMA 262, Edition 3.

label

Provides a statement with an identifier that you can refer to using a break or continue statement.

let

Declares a block scope local variable, optionally initializing it to a value.

return

Specifies the value to be returned by a function.

switch

Evaluates an expression, matching the expression's value to a case clause, and executes statements associated with that case.

throw

Throws a user-defined exception.

try...catch

Marks a block of statements to try, and specifies a response, should an exception be thrown.

var

Declares a variable, optionally initializing it to a value.

while

Creates a loop that executes a specified statement as long as the test condition evaluates to true. The condition is evaluated before executing the statement.

with

Extends the scope chain for a statement.

yield

See New_in_JavaScript 1.7 & Iterators and generators

There are several special operators that do not fit into any other category:

Arithmetic Operators

(+, -, *, /, %, ++, --, unary -, unary +)

Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value.

Assignment Operators

(=, *=, /=, %=, +=, -=, <<=, >>=, >>>=, &=, ^=, |=)

An assignment operator assigns a value to its left operand based on the value of its right operand.

Bitwise Operators

(&, |, ^, ~, <<, >>, >>>)

Bitwise operators treat their operands as a set of 32 bits (zeros and ones) and return standard JavaScript numerical values.

Comparison Operators

(==, !=, ===, !==, >, >=, <, <=)

A comparison operator compares its operands and returns a logical value based on whether the comparison is true.

Logical Operators

(&&, ||, !)

Logical operators are typically used with boolean (logical) values, and when they are, they return a boolean value.

String Operators

(+ and +=)

The string operators concatenate two string values together, returning another string that is the union of the two strings.

Member Operators

(object.property and object["property"])

Member operators provide access to a property or method of an object.

Special Operators

Conditional Operator

(condition ? ifTrue : ifFalse)

The conditional operator returns one of two values based on the logical value of the condition.

Comma Operator

(,)

The comma operator allows multiple expressions to be evaluated in a single statement and returns the result of the last expression.

delete Operator

(delete)

The delete operator deletes a property from an object.

in Operator

(in)

The in operator determines whether an object has a given property.

instanceof Operator

(instanceof)

The instanceof operator determines whether an object is an instance of another object.

new Operator

(new)

The new operator creates an instance of a constructor.

typeof Operator

(typeof)

The typeof operator determines the type of a given object.

void Operator

(void)

The void operator discards an expression's return value.

yield Operator

(yield)

The yield operator determines what is returned in a generator by that generator's iterator.

Operator Precedence

Operator precedence defines the order in which operators are evaluated.

Keywords

get Modifier

(get)

The get modifier defines a property in an object literal to be a getter.

set Modifier

(set)

The set modifier defines a property in an object literal to be a setter.

function Keyword

(function)

The function keyword defines a function expression.

this Keyword

(this)

The this keyword refers to the function's execution context.