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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

js
x += y

Description

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

Examples

Using addition assignment

js
let baz = true;

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

// Number + Boolean -> addition
baz += false; // 2
js
let foo = "foo";

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

// String + String -> concatenation
foo += "bar"; // "foofalsebar"
js
let bar = 5;

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

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

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

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Addition_assignment