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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 += 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

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

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

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

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

Addition assignment using BigInts

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

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

Addition assignment using strings

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

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

Specifications

Specification
ECMAScript® 2026 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	WebView on iOS	Bun	Deno	Node.js
`Addition_assignment`	1	12	1	3	1	18	4	10.1	1	1.0	4.4	1	1.0.0	1.0	0.10.0