sync_promise.js

var SyncPromise = function(callback) {
  this._resolveCallbacks = [];
  this._rejectCallbacks = [];

  var resolve = (function resolve(value) {
    if (this.state !== 'pending') {
      return;
    }

    this.state = 'fulfilled';

    var callback;
    while (callback = this._resolveCallbacks.shift()) {
      callback();
    }
    this._resolveCallbacks = this._rejectCallbacks = null;
  }).bind(this);

  var reject = (function reject(error) {
    if (this.state !== 'pending') {
      return;
    }

    this.state = 'fulfilled';

    var callback;
    while (callback = this._rejectCallbacks.shift()) {
      callback();
    }
    this._resolveCallbacks = this._rejectCallbacks = null;
  }).bind(this);

  callback(resolve, reject);
};

SyncPromise.prototype.state = 'pending';

SyncPromise.prototype.then = function(onFulfill, onReject) {
  var nextResolve, nextReject;
  var p = new SyncPromise(function(resolve, reject) {
    nextResolve = resolve;
    nextReject = reject;
  });

  switch (this.state) {
    case 'pending':
      (typeof onFulfill === 'function') && 
        this._resolveCallbacks.push(onFulfill);
      this._resolveCallbacks.push(nextResolve);

      (typeof onReject === 'function') && 
        this._resolveCallbacks.push(onReject);
      this._resolveCallbacks.push(nextReject);

      break;

    case 'fulfilled':
      (typeof onFulfill === 'function') && onFulfill();
      nextResolve();

      break;

    case 'rejected':
      (typeof onReject === 'function') && onReject();
      nextReject();

      break;
  }

  return p;
};

SyncPromise.prototype.catch = function(onReject) {
  return this.then(null, onRject);
};