The +=
operator performs addition (which is either numeric addition or string concatenation) on the two operands and assigns the result to the left operand.
The +=
operator performs addition (which is either numeric addition or string concatenation) on the two operands and assigns the result to the left operand.
x += y
x += y
is equivalent to x = x + y
, except that the expression x
is only evaluated once.
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"
let x = 1n; // BigInt + BigInt -> addition x += 2n; // 3n // BigInt + Number -> throws TypeError x += 1; // TypeError: Cannot mix BigInt and other types, use explicit conversions
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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Addition_assignment