rsvp

What is rsvp?

The 'rsvp' npm package is a lightweight library for handling asynchronous operations in JavaScript. It provides a set of tools for working with Promises, which are a way to handle asynchronous operations more gracefully than traditional callback-based approaches.

What are rsvp's main functionalities?

Creating Promises

This feature allows you to create a new Promise. The Promise constructor takes a function with two arguments: resolve and reject. You can perform asynchronous operations inside this function and call resolve when the operation is successful or reject if it fails.

const RSVP = require('rsvp');

let promise = new RSVP.Promise(function(resolve, reject) {
  // Asynchronous operation
  setTimeout(function() {
    resolve('Success!');
  }, 1000);
});

promise.then(function(value) {
  console.log(value); // 'Success!'
});

Chaining Promises

This feature allows you to chain multiple Promises together. Each .then() method returns a new Promise, which can be used to perform further asynchronous operations.

const RSVP = require('rsvp');

let promise = new RSVP.Promise(function(resolve, reject) {
  setTimeout(function() {
    resolve(1);
  }, 1000);
});

promise.then(function(value) {
  return value * 2;
}).then(function(value) {
  console.log(value); // 2
});

Handling Multiple Promises

This feature allows you to handle multiple Promises concurrently. The RSVP.all() method takes an array of Promises and returns a new Promise that resolves when all of the input Promises have resolved.

const RSVP = require('rsvp');

let promise1 = new RSVP.Promise(function(resolve, reject) {
  setTimeout(function() {
    resolve('First');
  }, 1000);
});

let promise2 = new RSVP.Promise(function(resolve, reject) {
  setTimeout(function() {
    resolve('Second');
  }, 2000);
});

RSVP.all([promise1, promise2]).then(function(values) {
  console.log(values); // ['First', 'Second']
});

Handling Promise Rejection

This feature allows you to handle errors in Promises. The .catch() method is used to specify a callback function that will be called if the Promise is rejected.

const RSVP = require('rsvp');

let promise = new RSVP.Promise(function(resolve, reject) {
  setTimeout(function() {
    reject('Error!');
  }, 1000);
});

promise.then(function(value) {
  console.log(value);
}).catch(function(error) {
  console.error(error); // 'Error!'
});

Other packages similar to rsvp

bluebird

Bluebird is a fully-featured Promise library for JavaScript. It offers a wide range of features including Promise cancellation, iteration methods, and more. Compared to RSVP, Bluebird is more feature-rich and has better performance in some cases.

q

Q is another popular Promise library that provides a way to manage asynchronous operations. It offers features like deferred objects and a variety of utility methods for working with Promises. Q is similar to RSVP but has a different API and additional features.

when

When is a lightweight Promise library that focuses on performance and small size. It provides a set of tools for working with Promises and asynchronous operations. When is similar to RSVP but is designed to be more lightweight and performant.

README

RSVP.js

RSVP.js provides simple tools for organizing asynchronous code.

Specifically, it is a tiny implementation of Promises/A and a mixin for turning objects into event targets.

It works in node and the browser. You can get the browser build in browser/rsvp.js and browser/rsvp.min.js.

Promises

RSVP.Promise is an implementation of Promises/A that passes the promises test suite written by Domenic Denicola.

It passes both the primary suite, which tests explicit compliance with the Promises/A spec, and the extension tests, which test compliance with commonly accepted practices around promises in JavaScript.

It delivers all promises asynchronously, even if the value is already available, to help you write consistent code that doesn't change if the underlying data provider changes from synchronous to asynchronous.

It is compatible with TaskJS, a library by Dave Herman of Mozilla that uses ES6 generators to allow you to write synchronous code with promises. It currently works in Firefox, and will work in any browser that adds support for ES6 generators. See the section below on TaskJS for more information.

Basic Usage

var promise = new Promise();

promise.then(function(value) {
  // success
}, function(value) {
  // failure
});

// later...

promise.resolve(value) // triggers first callback
promise.reject(error) // triggers second callback

Once a promise has been resolved or rejected, it cannot be resolved or rejected again.

Here is an example of a simple XHR2 wrapper written using RSVP.js:

var getJSON = function(url) {
  var promise = new RSVP.Promise();

  var client = new XMLHttpRequest();
  client.open("GET", url);
  client.onreadystatechange = handler;
  client.responseType = "json";
  client.setRequestHeader("Accept", "application/json");
  client.send();

  function handler() {
    if (this.readyState === this.DONE) {
      if (this.status === 200) { promise.resolve(this.response); }
      else { promise.reject(this); }
    }
  };

  return promise;
};

getJSON("/posts.json").then(function(json) {
  // continue
}, function(error) {
  // handle errors
});

Chaining

One of the really awesome features of Promises/A promises are that they can be chained together. In other words, the return value of the first resolve handler will be passed to the second resolve handler.

If you return a regular value, it will be passed, as is, to the next handler.

getJSON("/posts.json").then(function(json) {
  return json.post;
}).then(function(post) {
  // proceed
});;

The really awesome part comes when you return a promise from the first handler:

getJSON("/post/1.json").then(function(post) {
  // save off post
  return getJSON(post.commentURL);
}).then(function(comments) {
  // proceed with access to posts and comments
});;

This allows you to flatten out nested callbacks, and is the main feature of promises that prevents "rightward drift" in programs with a lot of asynchronous code.

Errors also propagate:

getJSON("/posts.json").then(function(posts) {

}).then(null, function(error) {
  // even though no error callback was passed to the
  // first `.then`, the error propagates
});

You can use this to emulate try/catch logic in synchronous code. Simply chain as many resolve callbacks as a you want, and add a failure handler at the end to catch errors.

getJSON("/post/1.json").then(function(post) {
  return getJSON(post.commentURL);
}).then(function(comments) {
  // proceed with access to posts and comments
}).then(null, function(error) {
  // handle errors in either of the two requests
});

Arrays of promises

Sometimes you might want to work with many promises at once. If you pass an array of promises to the all() method it will return a new promise that will be fulfilled when all of the promises in the array have been fulfilled; or rejected immediately if any promise in the array is rejected.

var postIds = [2, 3, 5, 7, 11, 13];
var promises = [];

for(var i = 0; i < postIds.length; i++) {
	promises.push(getJSON("/post/" + postIds[i] + ".json"));
}

RSVP.all(promises).then(function(posts) {
	// posts contains an array of results for the given promises
});

TaskJS

The TaskJS library makes it possible to take promises-oriented code and make it synchronous using ES6 generators.

Let's review an earlier example:

getJSON("/post/1.json").then(function(post) {
  return getJSON(post.commentURL);
}).then(function(comments) {
  // proceed with access to posts and comments
}).then(null, function(error) {
  // handle errors in either of the two requests
});

Without any changes to the implementation of getJSON, you could write the following code with TaskJS:

spawn(function *() {
  try {
    var post = yield getJSON("/post/1.json");
    var comments = yield getJSON(post.commentURL);
  } catch(error) {
    // handle errors
  }
});

In the above example, function * is new syntax in ES6 for generators. Inside a generator, yield pauses the generator, returning control to the function that invoked the generator. In this case, the invoker is a special function that understands the semantics of Promises/A, and will automatically resume the generator as soon as the promise is resolved.

The cool thing here is the same promises that work with current JavaScript using .then will work seamlessly with TaskJS once a browser has implemented it!

Event Target

RSVP also provides a mixin that you can use to convert any object into an event target. The promises implementation uses RSVP.EventTarget, so RSVP exposes it for your own use.

Basic Usage

The basic usage of RSVP.EventTarget is to mix it into an object, then use on and trigger to register listeners and trigger them.

var object = {};

RSVP.EventTarget.mixin(object);

object.on("finished", function(event) {
  // handle event
});

object.trigger("finished", { detail: value });

Prototypes

You can mix RSVP.EventTarget into a prototype and it will work as expected.

var Person = function() {};
RSVP.EventTarget.mixin(Person.prototype);

var yehuda = new Person();
var tom = new Person();

yehuda.on("poke", function(event) {
  console.log("Yehuda says OW");
});

tom.on("poke", function(event) {
  console.log("Tom says OW");
});

yehuda.trigger("poke");
tom.trigger("poke");

The example will work as expected. If you mix RSVP.EventTarget into a constructor's prototype, each instance of that constructor will get its own callbacks.