Addition assignment (+=)

The addition assignment (+=) operator adds the value of the right operand to a variable and assigns the result to the variable. The types of the two operands determine the behavior of the addition assignment operator. Addition or concatenation is possible.

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Syntax

x += y // x = x + y

Examples

Using addition assignment

let baz = true;

// Boolean + Number -> addition
baz += 1; // 2

// Number + Boolean -> addition
baz += false; // 2

let foo = "foo";

// String + Boolean -> concatenation
foo += false; // "foofalse"

// String + String -> concatenation
foo += "bar"; // "foofalsebar"

let bar = 5;

// Number + Number -> addition
bar += 2; // 7

// Number + String -> concatenation
bar += "foo"; // "7foo"

Specifications

Specification
ECMAScript Language Specification
# sec-assignment-operators

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See also