When is the body of a Promise executed?

Question

Suppose I have the following Promise:

function doSomethingAsynchronous() {   return new Promise((resolve) => {     const result = doSomeWork();      setTimeout(() => {       resolve(result);    }), 100);   }); } 

At which point in time is doSomeWork() called? Is it immediately after or as the Promise is constructed? If not, is there something additional I need to do explicitly to make sure the body of the Promise is run?

Answer 1

Immediately, yes, by specification.

From the MDN:

The executor function is executed immediately by the Promise implementation, passing resolve and reject functions (the executor is called before the Promise constructor even returns the created object)

Here it is in the ECMAScript specification (of course harder to read...): http://www.ecma-international.org/ecma-262/6.0/#sec-promise-executor

This guarantee may be important, for example when you're preparing several promises you then pass to all or race, or when your executors have synchronous side effects.

Answer 2

Yes, when you construct a Promise the first parameter gets executed immediately.

In general, you wouldn't really use a promise in the way you did, as with your current implementation, it would still be synchronous.

You would rather implement it with a timeout, or call the resolve function as part of an ajax callback

function doSomethingAsynchronous() {   return new Promise((resolve) => {     setTimeout(function() {       const result = doSomeWork();       resolve(result);     }, 0);   }); } 

The setTimeout method would then call the function at the next possible moment the event queue is free