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® 2026 Language Specification # sec-assignment-operators |