Does awaiting a non-Promise have any detectable effect?
Completely agreed with Jonas's statements. One thing that was not answered in his question was Are the following two lines completely same or do they theoretically differ?:
following two lines are not completely same, they're theoretically different.
- var x = 5
- var x = await 5
execution time in my console for 1st and 2nd statement is 0.008056640625ms and 0.055908203125ms respectively.
async/await, setTimeOut etc are APIs provided by Run time in which JavaScript Run time is running.
Putting await on a non-promise will be executed in event-loop
. Line 1 will be executed right after reaching the stack
but the line 2 will take few time(milliseconds) as it will 1st go to the stack
and then to the task queue
after skipping webAPI waiting section because there's no promise to be resolved & finally after that control will be given to stack
again for execution.
await
is not a no-op. If the awaited thing is not a promise, it is wrapped in a promise, that promise is awaited. Therefore await
changes the execution order (but you should not rely on it nevertheless):
console.log(1);
(async function() {
var x = await 5; // remove await to see 1,3,2
console.log(3);
})();
console.log(2);
Additionally await
does not only work on instanceof Promise
s but on every object with a .then
method:
await { then(cb) { /* nowhere */ } };
console.log("will never happen");
Is there any detectable effect of awaiting a non-Promise?
Sure, .then
gets called if it exists on the awaited thing.
Is there any difference in behavior one should be aware of to avoid a potential error?
Don't name a method "then" if you don't want it to be a Promise.
Any performance differences?
Sure, if you await things you will always defer the continuation to a microtask. But as always: You won't probably notice it (as a human observing the outcome).