Class Promise

Promise.all accepts an array of promises, and returns a new promise which is fulfilled with an array of fulfillment values for the passed promises, or rejected with the reason of the first passed promise to be rejected. It casts all elements of the passed iterable to promises as it runs this algorithm.

Example:

import Promise, { resolve } from 'rsvp';

let promise1 = resolve(1);
let promise2 = resolve(2);
let promise3 = resolve(3);
let promises = [ promise1, promise2, promise3 ];

Promise.all(promises).then(function(array){
  // The array here would be [ 1, 2, 3 ];
});

If any of the promises given to RSVP.all are rejected, the first promise that is rejected will be given as an argument to the returned promises's rejection handler. For example:

Example:

import Promise, { resolve, reject } from 'rsvp';

let promise1 = resolve(1);
let promise2 = reject(new Error("2"));
let promise3 = reject(new Error("3"));
let promises = [ promise1, promise2, promise3 ];

Promise.all(promises).then(function(array){
  // Code here never runs because there are rejected promises!
}, function(error) {
  // error.message === "2"
});

catch is simply sugar for then(undefined, onRejection) which makes it the same as the catch block of a try/catch statement.

function findAuthor(){
  throw new Error('couldn\'t find that author');
}

// synchronous
try {
  findAuthor();
} catch(reason) {
  // something went wrong
}

// async with promises
findAuthor().catch(function(reason){
  // something went wrong
});

finally will be invoked regardless of the promise's fate just as native try/catch/finally behaves

Synchronous example:

findAuthor() {
  if (Math.random() > 0.5) {
    throw new Error();
  }
  return new Author();
}

try {
  return findAuthor(); // succeed or fail
} catch(error) {
  return findOtherAuthor();
} finally {
  // always runs
  // doesn't affect the return value
}

Asynchronous example:

findAuthor().catch(function(reason){
  return findOtherAuthor();
}).finally(function(){
  // author was either found, or not
});

Promise.race returns a new promise which is settled in the same way as the first passed promise to settle.

Example:

import Promise from 'rsvp';

let promise1 = new Promise(function(resolve, reject){
  setTimeout(function(){
    resolve('promise 1');
  }, 200);
});

let promise2 = new Promise(function(resolve, reject){
  setTimeout(function(){
    resolve('promise 2');
  }, 100);
});

Promise.race([promise1, promise2]).then(function(result){
  // result === 'promise 2' because it was resolved before promise1
  // was resolved.
});

Promise.race is deterministic in that only the state of the first settled promise matters. For example, even if other promises given to the promises array argument are resolved, but the first settled promise has become rejected before the other promises became fulfilled, the returned promise will become rejected:

import Promise from 'rsvp';

let promise1 = new Promise(function(resolve, reject){
  setTimeout(function(){
    resolve('promise 1');
  }, 200);
});

let promise2 = new Promise(function(resolve, reject){
  setTimeout(function(){
    reject(new Error('promise 2'));
  }, 100);
});

Promise.race([promise1, promise2]).then(function(result){
  // Code here never runs
}, function(reason){
  // reason.message === 'promise 2' because promise 2 became rejected before
  // promise 1 became fulfilled
});

An example real-world use case is implementing timeouts:

import Promise from 'rsvp';

Promise.race([ajax('foo.json'), timeout(5000)])

Promise.reject returns a promise rejected with the passed reason. It is shorthand for the following:

import Promise from 'rsvp';

let promise = new Promise(function(resolve, reject){
  reject(new Error('WHOOPS'));
});

promise.then(function(value){
  // Code here doesn't run because the promise is rejected!
}, function(reason){
  // reason.message === 'WHOOPS'
});

Instead of writing the above, your code now simply becomes the following:

import Promise from 'rsvp';

let promise = Promise.reject(new Error('WHOOPS'));

promise.then(function(value){
  // Code here doesn't run because the promise is rejected!
}, function(reason){
  // reason.message === 'WHOOPS'
});

Promise.resolve returns a promise that will become resolved with the passed value. It is shorthand for the following:

import Promise from 'rsvp';

let promise = new Promise(function(resolve, reject){
  resolve(1);
});

promise.then(function(value){
  // value === 1
});

Instead of writing the above, your code now simply becomes the following:

import Promise from 'rsvp';

let promise = RSVP.Promise.resolve(1);

promise.then(function(value){
  // value === 1
});

The primary way of interacting with a promise is through its then method, which registers callbacks to receive either a promise's eventual value or the reason why the promise cannot be fulfilled.

findUser().then(function(user){
  // user is available
}, function(reason){
  // user is unavailable, and you are given the reason why
});

Chaining

The return value of then is itself a promise. This second, 'downstream' promise is resolved with the return value of the first promise's fulfillment or rejection handler, or rejected if the handler throws an exception.

findUser().then(function (user) {
  return user.name;
}, function (reason) {
  return 'default name';
}).then(function (userName) {
  // If `findUser` fulfilled, `userName` will be the user's name, otherwise it
  // will be `'default name'`
});

findUser().then(function (user) {
  throw new Error('Found user, but still unhappy');
}, function (reason) {
  throw new Error('`findUser` rejected and we\'re unhappy');
}).then(function (value) {
  // never reached
}, function (reason) {
  // if `findUser` fulfilled, `reason` will be 'Found user, but still unhappy'.
  // If `findUser` rejected, `reason` will be '`findUser` rejected and we\'re unhappy'.
});

If the downstream promise does not specify a rejection handler, rejection reasons will be propagated further downstream.

findUser().then(function (user) {
  throw new PedagogicalException('Upstream error');
}).then(function (value) {
  // never reached
}).then(function (value) {
  // never reached
}, function (reason) {
  // The `PedgagocialException` is propagated all the way down to here
});

Assimilation

Sometimes the value you want to propagate to a downstream promise can only be retrieved asynchronously. This can be achieved by returning a promise in the fulfillment or rejection handler. The downstream promise will then be pending until the returned promise is settled. This is called assimilation.

findUser().then(function (user) {
  return findCommentsByAuthor(user);
}).then(function (comments) {
  // The user's comments are now available
});

If the assimliated promise rejects, then the downstream promise will also reject.

findUser().then(function (user) {
  return findCommentsByAuthor(user);
}).then(function (comments) {
  // If `findCommentsByAuthor` fulfills, we'll have the value here
}, function (reason) {
  // If `findCommentsByAuthor` rejects, we'll have the reason here
});

Simple Example

Synchronous Example

let result;

try {
  result = findResult();
  // success
} catch(reason) {
  // failure
}

Errback Example

findResult(function(result, err){
  if (err) {
    // failure
  } else {
    // success
  }
});

Promise Example;

findResult().then(function(result){
  // success
}, function(reason){
  // failure
});

Advanced Example

Synchronous Example

let author, books;

try {
  author = findAuthor();
  books  = findBooksByAuthor(author);
  // success
} catch(reason) {
  // failure
}

Errback Example

function foundBooks(books) {

}

function failure(reason) {

}

findAuthor(function(author, err){
  if (err) {
    failure(err);
    // failure
  } else {
    try {
      findBoooksByAuthor(author, function(books, err) {
        if (err) {
          failure(err);
        } else {
          try {
            foundBooks(books);
          } catch(reason) {
            failure(reason);
          }
        }
      });
    } catch(error) {
      failure(err);
    }
    // success
  }
});

Promise Example;

findAuthor().
  then(findBooksByAuthor).
  then(function(books){
    // found books
}).catch(function(reason){
  // something went wrong
});