Addition (+)

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:

• If one side is a string, the other operand is also converted to a string and they are concatenated.
• If they are both BigInts, BigInt addition is performed. If one side is a BigInt but the other is not, a TypeError is thrown.
• Otherwise, both sides are converted to numbers, and numeric addition is performed.

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'

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. 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 over types, so adding a string to a BigInt produces string concatenation rather than a TypeError.

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

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

Browser compatibility

See also

• Subtraction (-)
• Division (/)
• Multiplication (*)
• Remainder (%)
• Exponentiation (**)
• Increment (++)
• Decrement (--)
• Unary negation (-)
• Unary plus (+)