Race condition in JavaScript with compound assignment

Question

I\'m not talking about complex race conditions involving the network or events. Rather, I seem to have found out that the += operator is not atomic in V8 (Chrome 58

Answer 1

This is not a race condition, because you are explicitly yielding the execution using await.

The standard defines that a compound assignment such as += is not atomic: The left-hand-side of a compound assignment is evaluated before the right-hand-side.^[1]

So if your RHS changes acc somehow, the changes will be overwritten. Most simple example:

var n = 1;
n += (function () {
    n = 2;
    return 0;
})();

console.log(n);

Answer 2

Indeed, after replacing the acc += await n(c) line with:

const ret = await n(c); acc += ret;

the race condition was avoided.

My guess is V8 didn't optimize acc += await n(c) to an ADD of the n() result over the memory location containing acc, but rather expanded it to acc = acc + await n(c);, and the initial value of acc when nForever(5.3) was first called, was 0.

This is counter-intuitive to me, though not sure the V8 developers would consider it a bug.