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

Atomics.add(typedArray, index, value)

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

Atomics.add(ta, 0, 12); //  0
Atomics.load(ta, 0); // 12