This part of the JavaScript section on MDN serves as a repository of facts about the JavaScript language. Read more about this reference.
Global Objects
This chapter documents all the JavaScript standard built-in objects, along with their methods and properties.
Value properties
Global properties returning a simple value.
Function properties
Global functions returning the result of a specific routine.
eval()
uneval()
isFinite()
isNaN()
parseFloat()
parseInt()
decodeURI()
decodeURIComponent()
encodeURI()
encodeURIComponent()
escape()
unescape()
Fundamental objects
General language objects, functions and errors.
Object
Function
Boolean
Symbol
Error
EvalError
InternalError
RangeError
ReferenceError
StopIteration
SyntaxError
TypeError
URIError
Numbers and dates
Objects dealing with numbers, dates and mathematical calculations.
Text processing
Objects for manipulating texts.
Indexed collections
Collections ordered by an index. Array-type objects.
Array
Float32Array
Float64Array
Int16Array
Int32Array
Int8Array
Uint16Array
Uint32Array
Uint8Array
Uint8ClampedArray
ParallelArray
Keyed collections
Collections of objects as keys. Elements iterable in insertion order.
Structured data
Data buffers and JavaScript Object Notation.
Control abstraction objects
Reflection
Internationalization
Additions to the ECMAScript core for language-sensitive functionalities.
Other
Statements
This chapter documents all the JavaScript statements and declarations.
For an alphabetical listing see the sidebar on the left.
Control flow
-
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.
-
continue
- Terminates execution of the statements in the current iteration of the current or labeled loop, and continues execution of the loop with the next iteration.
-
Empty
- An empty statement is used to provide no statement, although the JavaScript syntax would expect one.
-
if...else
- Executes a statement if a specified condition is true. If the condition is false, another statement can be executed.
-
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.
Declarations
-
var
- Declares a variable, optionally initializing it to a value.
-
let
- Declares a block scope local variable, optionally initializing it to a value.
-
const
- Declares a read-only named constant.
Functions
-
function
- Declares a function with the specified parameters.
-
function*
- Generators functions enable writing iterators more easily.
-
return
- Specifies the value to be returned by a function.
-
yield
- See New_in_JavaScript 1.7 & Iterators and generators
Iterations
-
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.
-
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.
-
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.
Others
-
debugger
- Invokes any available debugging functionality. If no debugging functionality is available, this statement has no effect.
-
export
- Used to allow a signed script to provide properties, functions, and objects to other signed or unsigned scripts. This ancient Netscape functionality has been removed and will be redefined by ECMAScript 6 modules.
-
import
- Used to allow a script to import properties, functions, and objects from a signed script that has exported the information. This ancient Netscape functionality has been removed and will be redefined by ECMAScript 6 modules.
-
label
-
Provides a statement with an identifier that you can refer to using a
break
orcontinue
statement.
-
with
- Extends the scope chain for a statement.
Expressions and operators
This chapter documents all the JavaScript expressions and operators.
For an alphabetical listing see the sidebar on the left.
Primary expressions
Basic keywords and general expressions in JavaScript.
-
this
-
The
this
keyword refers to the function's execution context. -
function
-
The
function
keyword defines a function expression. -
[]
- Array literal syntax.
-
{}
- Object literal syntax.
-
/ab+c/i
- Regular expression literal syntax.
-
[for (x of y) x]
- Array comprehensions.
-
(for (x of y) y)
- Generator comprehensions.
-
( )
- Grouping operator.
Left-hand-side expressions
Left values are the destination of an assignment.
- Property accessors
-
Member operators provide access to a property or method of an object
(object.property
andobject["property"]
). -
new
-
The
new
operator creates an instance of a constructor. -
super
-
The
super
keyword calls the parent constructor. -
...obj
- The spread operator allows an expression to be expanded in places where multiple arguments (for function calls) or multiple elements (for array literals) are expected.
Increment and decrement
Postfix/prefix increment and postfix/prefix decrement operators.
-
A++
- Postfix increment operator.
-
A--
- Postfix decrement operator.
-
++A
- Prefix increment operator.
-
--A
- Prefix decrement operator.
Unary operators
A unary operation is operation with only one operand.
-
delete
-
The
delete
operator deletes a property from an object. -
void
-
The
void
operator discards an expression's return value. -
typeof
-
The
typeof
operator determines the type of a given object. -
+
- The unary plus operator converts its operand to Number type.
-
-
- The unary negation operator converts its operand to Number type and then negates it.
-
~
- Bitwise NOT operator.
-
!
- Logical NOT operator.
Arithmetic operators
Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value.
-
+
- Addition operator.
-
-
- Subtraction operator.
-
/
- Division operator.
-
*
- Multiplication operator.
-
%
- Remainder operator.
Relational operators
A comparison operator compares its operands and returns a Boolean
value based on whether the comparison is true.
-
in
-
The
in
operator determines whether an object has a given property. -
instanceof
-
The
instanceof
operator determines whether an object is an instance of another object. -
<
- Less than operator.
-
>
- Greater than operator.
-
<=
- Less than or equal operator.
-
>=
- Greater than or equal operator.
Equality operators
The result of evaluating an equality operator is always of type Boolean
based on whether the comparison is true.
Bitwise shift operators
Operations to shift all bits of the operand.
-
<<
- Bitwise left shift operator.
-
>>
- Bitwise right shift operator.
-
>>>
- Bitwise unsigned right shift operator.
Binary bitwise operators
Bitwise operators treat their operands as a set of 32 bits (zeros and ones) and return standard JavaScript numerical values.
Binary logical operators
Logical operators are typically used with boolean (logical) values, and when they are, they return a boolean value.
Conditional (ternary) operator
-
(condition ? ifTrue : ifFalse)
-
The conditional operator returns one of two values based on the logical value of the condition.
Assignment operators
An assignment operator assigns a value to its left operand based on the value of its right operand.
-
=
- Assignment operator.
-
*=
- Multiplication assignment.
-
/=
- Division assignment.
-
%=
- Remainder assignment.
-
+=
- Addition assignment.
-
-=
- Subtraction assignment
-
<<=
- Left shift assignment.
-
>>=
- Right shift assignment.
-
>>>=
- Unsigned right shift assignment.
-
&=
- Bitwise AND assignment.
-
^=
- Bitwise XOR assignment.
-
|=
- Bitwise OR assignment.
-
[a, b] = [1, 2]
{a, b} = {a:1, b:2}
-
Destructuring assignment allows you to assign the properties of an array or object to variables using syntax that looks similar to array or object literals.
Comma operator
-
,
-
The comma operator allows multiple expressions to be evaluated in a single statement and returns the result of the last expression.
Functions
This chapter documents how to work with JavaScript functions to develop your applications.
Additional reference pages