faye/src/util/promise.js

Simple pub/sub messaging for the web

'use strict';

var asap = require('asap');

var PENDING   = -1,
    FULFILLED =  0,
    REJECTED  =  1;

var Promise = function(task) {
  this._state = PENDING;
  this._value = null;
  this._defer = [];

  execute(this, task);
};

Promise.prototype.then = function(onFulfilled, onRejected) {
  var promise = new Promise();

  var deferred = {
    promise:     promise,
    onFulfilled: onFulfilled,
    onRejected:  onRejected
  };

  if (this._state === PENDING)
    this._defer.push(deferred);
  else
    propagate(this, deferred);

  return promise;
};

Promise.prototype['catch'] = function(onRejected) {
  return this.then(null, onRejected);
};

var execute = function(promise, task) {
  if (typeof task !== 'function') return;

  var calls = 0;

  var resolvePromise = function(value) {
    if (calls++ === 0) resolve(promise, value);
  };

  var rejectPromise = function(reason) {
    if (calls++ === 0) reject(promise, reason);
  };

  try {
    task(resolvePromise, rejectPromise);
  } catch (error) {
    rejectPromise(error);
  }
};

var propagate = function(promise, deferred) {
  var state   = promise._state,
      value   = promise._value,
      next    = deferred.promise,
      handler = [deferred.onFulfilled, deferred.onRejected][state],
      pass    = [resolve, reject][state];

  if (typeof handler !== 'function')
    return pass(next, value);

  asap(function() {
    try {
      resolve(next, handler(value));
    } catch (error) {
      reject(next, error);
    }
  });
};

var resolve = function(promise, value) {
  if (promise === value)
    return reject(promise, new TypeError('Recursive promise chain detected'));

  var then;

  try {
    then = getThen(value);
  } catch (error) {
    return reject(promise, error);
  }

  if (!then) return fulfill(promise, value);

  execute(promise, function(resolvePromise, rejectPromise) {
    then.call(value, resolvePromise, rejectPromise);
  });
};

var getThen = function(value) {
  var type = typeof value,
      then = (type === 'object' || type === 'function') && value && value.then;

  return (typeof then === 'function')
         ? then
         : null;
};

var fulfill = function(promise, value) {
  settle(promise, FULFILLED, value);
};

var reject = function(promise, reason) {
  settle(promise, REJECTED, reason);
};

var settle = function(promise, state, value) {
  var defer = promise._defer, i = 0;

  promise._state = state;
  promise._value = value;
  promise._defer = null;

  if (defer.length === 0) return;
  while (i < defer.length) propagate(promise, defer[i++]);
};

Promise.resolve = function(value) {
  try {
    if (getThen(value)) return value;
  } catch (error) {
    return Promise.reject(error);
  }

  return new Promise(function(resolve, reject) { resolve(value) });
};

Promise.reject = function(reason) {
  return new Promise(function(resolve, reject) { reject(reason) });
};

Promise.all = function(promises) {
  return new Promise(function(resolve, reject) {
    var list = [], n = promises.length, i;

    if (n === 0) return resolve(list);

    var push = function(promise, i) {
      Promise.resolve(promise).then(function(value) {
        list[i] = value;
        if (--n === 0) resolve(list);
      }, reject);
    };

    for (i = 0; i < n; i++) push(promises[i], i);
  });
};

Promise.race = function(promises) {
  return new Promise(function(resolve, reject) {
    for (var i = 0, n = promises.length; i < n; i++)
      Promise.resolve(promises[i]).then(resolve, reject);
  });
};

Promise.deferred = function() {
  var tuple = {};

  tuple.promise = new Promise(function(resolve, reject) {
    tuple.resolve = resolve;
    tuple.reject  = reject;
  });
  return tuple;
};

module.exports = Promise;