Why does javascript ES6 Promises continue execution after a resolve?

Question

As I understand a promise is something that can resolve() or reject() but I was suprised to find out that code in the promise continues to execute after a resolve or reject is called.

I considered resolve or reject being an async-friendly version of exit or return , that would halt all immediate function execution.

Can someone explain the thought behind why the following example sometimes shows the console.log after a resolve call:

var call = function() {
    return new Promise(function(resolve, reject) {
        resolve();
        console.log("Doing more stuff, should not be visible after a resolve!");
    });
};

call().then(function() {
    console.log("resolved");
});

jsbin

Reasonable question, but then again, JS just executes one statement after another like you tell to it to. resolve() is not a JS control statement that magically would have the effect of return, it's just a function call, and yes, execution continues after it. — user663031
This is a good question, and even after reading all the responses, I'm not sure about the best practices... — Gabriel Glenn
I think the misunderstanding comes from what exactly you are terminating with resolve(): the promise IS resolved just after you call resolve(), but as already said by others, this does not mean that the function that have terminated the promise had terminated its duty too, so it continues until it reaches a "normal" termination. — Giuseppe Bertone

Felix Kling Felix Kling · Accepted Answer · 2015-03-06T09:57:53

JavaScript has the concept of "run to completion". Unless an error is thrown, a function is executed until a return statement or its end is reached. Other code outside of the function can't interfere with that (unless, again, an error is thrown).

If you want resolve() to exit your initializer function, you have to prepend it by return:

return new Promise(function(resolve, reject) {
    return resolve();
    console.log("Not doing more stuff after a return statement");
});

Why does javascript ES6 Promises continue execution after a resolve?

3 Answers