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.
Theaddition assignment (+=) operator performsaddition (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: 5console.log((b += " world")); // Concatenation// Expected output: "hello world"Syntax
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x += yDescription
x += y is equivalent tox = x + y, except that the expressionx is only evaluated once.
Examples
Addition assignment using numbers
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let bar = 5;bar += 2; // 7Other non-string, non-BigInt values are coerced to numbers:
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let baz = true;baz += 1; // 2baz += false; // 2Addition assignment using BigInts
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let x = 1n;x += 2n; // 3nx += 1; // TypeError: Cannot mix BigInt and other types, use explicit conversionsAddition assignment using strings
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let foo = "foo";foo += false; // "foofalse"foo += "bar"; // "foofalsebar"let bar = 5;bar += "foo"; // "5foo"Specifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-assignment-operators |