Flattening promises in javascript - Stack Overflow

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The bluebird library seems to automagically use Promise::then both as an equivalent of "map" and "flatMap" on the promise, eg see this example.

var Promise;

Promise = require('bluebird').Promise;

Promise.resolve(1).then(function(x) {
  return Promise.resolve(x + 1);
}).then(function(x) {
  return console.log(x); // => `2` (not a promise)
});

Promise.resolve(1).then(function(x) {
  return x + 1;
}).then(function(x) {
  return console.log(x); // => `2`
});

Promise.reject('hi').catch(function(x) {
  return Promise.reject('hi2');
}).catch(function(x) {
  return console.error(x); //  => `hi2` (not a promise)
});

Is this a contract of the es6 Promise API? I see no mention of this flattening behavior here or here, for example.

The bluebird library seems to automagically use Promise::then both as an equivalent of "map" and "flatMap" on the promise, eg see this example.

var Promise;

Promise = require('bluebird').Promise;

Promise.resolve(1).then(function(x) {
  return Promise.resolve(x + 1);
}).then(function(x) {
  return console.log(x); // => `2` (not a promise)
});

Promise.resolve(1).then(function(x) {
  return x + 1;
}).then(function(x) {
  return console.log(x); // => `2`
});

Promise.reject('hi').catch(function(x) {
  return Promise.reject('hi2');
}).catch(function(x) {
  return console.error(x); //  => `hi2` (not a promise)
});

Is this a contract of the es6 Promise API? I see no mention of this flattening behavior here or here, for example.

• javascript
• promise
• ecmascript-6
• es6-promise

Share Improve this question Follow edited Jun 28, 2015 at 19:55 George Simms asked Jun 28, 2015 at 19:48 George SimmsGeorge Simms 4,06044 gold badges2323 silver badges3636 bronze badges 1

• Uh, those docs are very sparse. MSDN doesn't even mention that then returns a promise :-/ – Bergi Commented Jun 28, 2015 at 20:54

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Is this a contract of the es6 Promise API?

Yes, it is a contract established by Promises/A+, and has made its way from there into the ES6 specification. You will find some discussions here, here and here.

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