Addition (+)

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 (+) operator produces the sum of numeric operands or string concatenation.

Try it

console.log(2 + 2);
/ Expected output: 4

console.log(2 + true);
/ Expected output: 3

console.log("hello " + "everyone");
/ Expected output: "hello everyone"

console.log(2001 + ": A Space Odyssey");
/ Expected output: "2001: A Space Odyssey"

Syntax

x + y

The + operator is overloaded for two distinct operations: numeric addition and string concatenation. When evaluating, it first coerces both operands to primitives. Then, the two operands' types are tested:

If one side is a string, the other operand is also converted to a string and they are concatenated.
If they are both TypeError is thrown.
Otherwise, both sides are converted to numbers, and numeric addition is performed.

String concatenation is often thought to be equivalent with toString() in priority. If the expression has a Temporal, whose objects' valueOf() methods all throw.

const t = Temporal.Now.instant();
"" + t; / Throws TypeError
`${t}`; / '2022-07-31T04:48:56.113918308Z'
"".concat(t); / '2022-07-31T04:48:56.113918308Z'

You are advised to not use "" + x to perform string coercion.

Examples

Addition using numbers

1 + 2; / 3

Other non-string, non-BigInt values are coerced to numbers:

true + 1; / 2
false + false; / 0

Addition using BigInts

1n + 2n; / 3n

You cannot mix BigInt and number operands in addition.

1n + 2; / TypeError: Cannot mix BigInt and other types, use explicit conversions
2 + 1n; / TypeError: Cannot mix BigInt and other types, use explicit conversions
"1" + 2n; / TypeError: Cannot mix BigInt and other types, use explicit conversions

To do addition with a BigInt and a non-BigInt, convert either operand:

1n + BigInt(2); / 3n
Number(1n) + 2; / 3

Addition using strings

If one of the operands is a string, the other is converted to a string and they are concatenated:

"foo" + "bar"; / "foobar"
5 + "foo"; / "5foo"
"foo" + false; / "foofalse"
"2" + 2; / "22"

Specifications

Specification
ECMAScript® 2026 Language Specification # sec-addition-operator-plus