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Addition assignment (+=)

The += operator performs addition (which is either numeric addition or string concatenation) on the two operands and assigns the result to the left operand.

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Syntax

x += y

Description

x += y is equivalent to x = x + y, except that the expression x is only evaluated once.

Examples

Using addition assignment

let baz = true;

// Boolean + Number -> addition
baz += 1; // 2

// Number + Boolean -> addition
baz += false; // 2

let foo = "foo";

// String + Boolean -> concatenation
foo += false; // "foofalse"

// String + String -> concatenation
foo += "bar"; // "foofalsebar"

let bar = 5;

// Number + Number -> addition
bar += 2; // 7

// Number + String -> concatenation
bar += "foo"; // "7foo"

let x = 1n;

// BigInt + BigInt -> addition
x += 2n; // 3n

// BigInt + Number -> throws TypeError
x += 1; // TypeError: Cannot mix BigInt and other types, use explicit conversions

Specifications

Specification
ECMAScript Language Specification # sec-assignment-operators

Browser compatibility

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	Deno	Node.js
`Addition_assignment`	1	12	1	3	1	18	4	10.1	1	1.0	4.4	1.0	0.10.0