Addition assignment (+=)

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

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

Try it

let a = 2;
let b = "hello";

console.log((a += 3)); // Addition
// Expected output: 5

console.log((b += " world")); // Concatenation
// Expected output: "hello world"

Syntax

js
x += y

Description

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

Examples

Addition assignment using numbers

js
let bar = 5;
bar += 2; // 7

Other non-string, non-BigInt values are coerced to numbers:

js
let baz = true;
baz += 1; // 2
baz += false; // 2

Addition assignment using BigInts

js
let x = 1n;
x += 2n; // 3n

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

Addition assignment using strings

js
let foo = "foo";
foo += false; // "foofalse"
foo += "bar"; // "foofalsebar"

let bar = 5;
bar += "foo"; // "5foo"

Specifications

Specification
ECMAScript® 2025 Language Specification
# sec-assignment-operators

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
Addition assignment (x += y)

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Full support
Full support

See also