I would like to calculate how long an async function (async/await) is taking in JavaScript.
One could do:
const asyncFunc = async function () {};
const before = Date.now();
asyncFunc().then(() => {
const after = Date.now();
console.log(after - before);
});
However, this does not work, because promises callbacks are run in a new microtask. I.e. between the end of asyncFunc() and the beginning of then(() => {}), any already queued microtask will be fired first, and their execution time will be taken into account.
E.g.:
const asyncFunc = async function () {};
const slowSyncFunc = function () {
for (let i = 1; i < 10 ** 9; i++) {}
};
process.nextTick(slowSyncFunc);
const before = Date.now();
asyncFunc().then(() => {
const after = Date.now();
console.log(after - before);
});
This prints 1739 on my machine, i.e. almost 2 seconds, because it waits for slowSyncFunc() to complete, which is wrong.
Note that I do not want to modify the body of asyncFunc, as I need to instrument many async functions without the burden of modifying each of them. Otherwise I could just add a Date.now() statement at the beginning and the end of asyncFunc.
Note also that the problem is not about how the performance counter is being retrieved. Using Date.now(), console.time(), process.hrtime() (Node.js only) or performance (browser only) will not change the base of this problem. The problem is about the fact that promise callbacks are run in a new microtask. If you add statements like setTimeout or process.nextTick to the original example, you are modifying the problem.
Any already queued microtask will be fired first, and their execution time will be taken into account.
Yes, and there's no way around that. If you don't want to have other tasks contribute to your measurement, don't queue any. That's the only solution.
This is not a problem of promises (or async functions) or of the microtask queue specifically, it's a problem shared by all asynchronous things which run callbacks on a task queue.