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

Description

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:

String concatenation is often thought to be equivalent with template literals or String.prototype.concat(), but they are not. Addition coerces the expression to a primitive, which calls valueOf() in priority; on the other hand, template literals and concat() coerce the expression to a string, which calls toString() in priority. If the expression has a [Symbol.toPrimitive]() method, string concatenation calls it with "default" as hint, while template literals use "string". This is important for objects that have different string and primitive representations — such as 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'

Examples

Addition using numbers

1 + 2; // 3

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

Addition using BigInts

1n + 2n; // 3n

You cannot mix BigInt and number operands in addition. null, undefined, and boolean values are coerced to numbers and are forbidden as well.

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

Strings have priority over other types, so adding a string to a BigInt produces string concatenation rather than a TypeError.

"1" + 2n; // "12"

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"

Addition (+)