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`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: ```javascript 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 `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: ```javascript 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. ```js 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: ```js findAuthor() { if (Math.random() > 0.5) { throw new Error(); } return new Author(); } try { return findAuthor(); // succeed or fail } catch(error) { return findOtherAuther(); } finally { // always runs // doesn't affect the return value } ``` Asynchronous example: ```js findAuthor().catch(function(reason){ return findOtherAuther(); }).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: ```javascript 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: ```javascript 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: ```javascript Promise.race([ajax('foo.json'), timeout(5000)]) ```

`Promise.reject` returns a promise rejected with the passed `reason`. It is shorthand for the following: ```javascript 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: ```javascript 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: ```javascript 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: ```javascript let promise = 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. ```js 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. ```js 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. ```js 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*. ```js 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. ```js 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 ```javascript let result; try { result = findResult(); // success } catch(reason) { // failure } ``` Errback Example ```js findResult(function(result, err){ if (err) { // failure } else { // success } }); ``` Promise Example; ```javascript findResult().then(function(result){ // success }, function(reason){ // failure }); ``` Advanced Example -------------- Synchronous Example ```javascript let author, books; try { author = findAuthor(); books = findBooksByAuthor(author); // success } catch(reason) { // failure } ``` Errback Example ```js 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; ```javascript findAuthor(). then(findBooksByAuthor). then(function(books){ // found books }).catch(function(reason){ // something went wrong }); ```