files/en-us/web/javascript/reference/global_objects/atomics/add/index.md

metadata

title: Atomics.add()
short-title: add()
slug: Web/JavaScript/Reference/Global_Objects/Atomics/add
page-type: javascript-static-method
browser-compat: javascript.builtins.Atomics.add
sidebar: jsref

The Atomics.add() static method adds a given value at a given position in the array and returns the old value at that position. This atomic operation guarantees that no other write happens until the modified value is written back.

{{InteractiveExample("JavaScript Demo: Atomics.add()")}}

// Create a SharedArrayBuffer with a size in bytes
const buffer = new SharedArrayBuffer(16);
const uint8 = new Uint8Array(buffer);
uint8[0] = 7;

// 7 + 2 = 9
console.log(Atomics.add(uint8, 0, 2));
// Expected output: 7

console.log(Atomics.load(uint8, 0));
// Expected output: 9

Syntax

Atomics.add(typedArray, index, value)

Parameters

• typedArray
  • : An integer typed array. One of {{jsxref("Int8Array")}}, {{jsxref("Uint8Array")}}, {{jsxref("Int16Array")}}, {{jsxref("Uint16Array")}}, {{jsxref("Int32Array")}}, {{jsxref("Uint32Array")}}, {{jsxref("BigInt64Array")}}, or {{jsxref("BigUint64Array")}}.
• index
  • : The position in the typedArray to add a value to.
• value
  • : The number to add.

Return value

The old value at the given position (typedArray[index]).

Exceptions

• {{jsxref("TypeError")}}
  • : Thrown if typedArray is not one of the allowed integer types.
• {{jsxref("RangeError")}}
  • : Thrown if index is out of bounds in the typedArray.

Examples

Using add()

const sab = new SharedArrayBuffer(1024);
const ta = new Uint8Array(sab);

Atomics.add(ta, 0, 12); // returns 0, the old value
Atomics.load(ta, 0); // 12

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• {{jsxref("Atomics")}}
• {{jsxref("Atomics.sub()")}}