Order of execution Javascript

Question

I have this code:

setTimeout(function timeout() {
    console.log('timeout');
}, 0);

let p = new Promise(function(resolve, reject) {
    console.log('create promise');
    resolve();
});

p.then(function(){
    console.log('execute promise');
});

console.log('end');

I get this order when execute the code:

create promise
end
execute promise
timeout

Question: Why create promise is executed first? I expect to execute first end, because it is the only one synchrone code so it should be first executed.

the promise function is called immediately in most implementations. there's no need to defer its execution — Daniel A. White, Jan 29 '22 at 15:19
@DanielA.White, then do i understand correct that all that is inside `new Promise(...)` is executed synchronously? — Asking, Jan 29 '22 at 15:24
yes - the standard says so: https://tc39.es/ecma262/multipage/control-abstraction-objects.html#sec-promise-constructor — Daniel A. White, Jan 29 '22 at 15:26

wuarmin · Answer 1 · 2022-01-29T15:31:00.147

1

The callback passed to the Promise constructor is executed immediately and synchronously. But it is possible to start an asynchronous task in a Promise callback. Then the Promise resolves after the asynchronous task has completed. This is the primary use-case of Promises.

In your case there is no asynchronous task started, so all your cody is synchronous.

edited Jan 29 '22 at 15:31

answered Jan 29 '22 at 15:25

wuarmin

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Order of execution Javascript

1 Answers1