Class RSVP.Promise

catch (onRejection, label) Promise

Defined in node_modules/rsvp/lib/rsvp/promise.js:154

onRejection

Function

label

String

optional string for labeling the promise. Useful for tooling.

returns

Promise

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 (callback, label) Promise

Defined in node_modules/rsvp/lib/rsvp/promise.js:186

callback

Function

label

String

optional string for labeling the promise. Useful for tooling.

returns

Promise

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
});

then (onFulfillment, onRejection, label) Promise

Defined in node_modules/rsvp/lib/rsvp/promise.js:243

onFulfillment

Function

onRejection

Function

label

String

optional string for labeling the promise. Useful for tooling.

returns

Promise

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
});