Return from a promise then()

javascript promise
Priscy picture Priscy · Dec 4, 2015 · Viewed 178.1k times · Source

I have got a javascript code like this:

function justTesting() {
  promise.then(function(output) {
    return output + 1;
  });
}

var test = justTesting();

I have got always an undefined value for the var test. I think that it is because the promises are not resolved yet..there is a way to return a value from a promise?

Answer

c0ming picture c0ming · Apr 1, 2017

When you return something from a then() callback, it's a bit magic. If you return a value, the next then() is called with that value. However, if you return something promise-like, the next then() waits on it, and is only called when that promise settles (succeeds/fails).

Source: https://developers.google.com/web/fundamentals/getting-started/primers/promises#queuing-asynchronous-actions

Related questions

Using async/await with a forEach loop

Are there any issues with using async/await in a forEach loop? I'm trying to loop through an array of files and await on the contents of each file. import fs from 'fs-promise' async function printFiles () { const files = await getFilePaths() // …

Read More
Getting a UnhandledPromiseRejectionWarning when testing using mocha/chai

So, I'm testing a component that relies on an event-emitter. To do so I came up with a solution using Promises with Mocha+Chai: it('should transition with the correct event', (done) => { const cFSM = new CharacterFSM({}, emitter, transitions); let …

Read More
How do I wait for a promise to finish before returning the variable of a function?

I'm still struggling with promises, but making some progress thanks to the community here. I have a simple JS function which queries a Parse database. It's supposed to return the array of results, but obviously due to the asynchronous nature …

Read More