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javascript operator

Addition assignment (+=)

View on MDN ↗

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.

Interactive exampleOpen the canonical MDN source to run this embedded demo.
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

SpecificationsStandards references are available on the canonical MDN page.

Browser compatibility

Browser compatibilityCompatibility data is available on the canonical MDN page.

See also