node.js - Returning values from Javascript Promise chain

Question

Modern JS/Promise newbie here. I'm having trouble finding the answer to a simple question, despite the overwhelming amount of content on this subject.

I believe I have a decent understanding of JS Promises (thanks to various sources, including mdn and https://spin.atomicobject.com/2016/02/16/how-javascript-promises-work/)

I have consumed and produced Promises regularly in their simplest form, however I keep stumbling on methods in my inherited projects with the following pattern:

const doAsyncOp = () => 
  $http.get(uri)
    .then(
      value => value,
      reject => //... 
    );

My big question is: What happens when you simply return a value from your success handler? I need to consume this method, and need access to 'value' in the client code. Is this technically unhandled? Should I rewrite the implementation?

Answer 1

My big question is: What happens when you simply return a value from your success handler?

When you return a value from a .then() handler, that value becomes the resolved value of the parent promise chain: So, in this code:

// utility function that returns a promise that resolves after a delay
function delay(t, v) {
    return new Promise(resolve => {
        setTimeout(resolve.bind(null, v), t);
    });
}

function test() {
    return delay(100, 5).then(val => {
        // this return value becomes the resolved value of the promise chain
        // that is returned from this function
        return 10;
    });
}

test().then(result => {
    // logs 10
    console.log(result);
});