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Does awaiting a non-Promise have any detectable effect?

rated 0 times [ 149] [ 6] / answers: 1 / hits: 24389 / 5 Years ago, wed, march 20, 2019, 12:00:00

One can await a non-Promise and that's good so.

All these expressions are valid and cause no error:

await 5

await 'A'

await {}

await null

await undefined

Is there any detectable effect of awaiting a non-Promise? Is there any difference in behavior one should be aware of to avoid a potential error? Any performance differences?

Are the following two lines completely same or do they theoretically differ?:

var x = 5

var x = await 5

How? Any example to demonstrate the difference?

PS: According TypeScript authors, there is a difference:

var x = await 5; is not the same as var x = 5;; var x = await 5; will assign x 5 in the next tern, where as var x = 5; will evaluate immediately.

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janjadonb

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answered 5 Years ago jaquelineh · Accepted Answer

await is not a no-op. If the awaited thing is not a promise, it is wrapped in a promise and that promise is awaited. Therefore await changes the execution order (but you should not rely on it nevertheless):

The following outputs 1, 2, 3:

console.log(1);



(async function() {

  var x = await 5;

  console.log(3);

})();



console.log(2);

With the await removed it's 1, 3, 2:

    console.log(1);



    (async function() {

      console.log(3);

    })();



    console.log(2);

Additionally await does not only work on instanceof Promises 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).