promise.js实现nodejs promises库简单介绍

<script>
(function(){
var Promise = exports.Promise = function(){
  this._callbacks = [];
};
  
Promise.prototype.then = function(func, context){
  //处理回调结果的方法
  function doCallbackResults(r) {
    if (r instanceof Promise) {
      r.then(function(err, values){
        p.done(err, values);
      });
    } else {
      p.done(null, r);
    }
  }
  
  var p = new Promise();
  if (this._isdone) {
    var results = func.apply(context, this.results);
    doCallbackResults(results);
  } else {
    this._callbacks.push(function(){
      var results = func.apply(context, arguments);
      doCallbackResults(results);
    });
  }
  return p;
};
  
Promise.prototype.done = function(){
  this.results = arguments;
  this._isdone = true;
  for (var i=0; i<this._callbacks.length; i++) {
    this._callbacks[i].apply(null, arguments);
  }
  this._callbacks = [];
};
  
Promise.join = function(promises){
  var p = new Promise();
  var results = [];
  
  if (!promises || !promises.length) {
    p.done(results);
    return p;
  }
  
  var numdone = 0;
  var total = promises.length;
  
  function notifier(i) {
    return function() {
      numdone += 1;
      results[i] = Array.prototype.slice.call(arguments);
      if (numdone === total) {
        p.done(results);
      }
    };
  }
  
  for (var i = 0; i < total; i++) {
    promises[i].then(notifier(i));
  }
  
  return p;
};
  
Promise.chain = function(funcs, args) {
  var p = new Promise();
  if (!funcs || !funcs.length) {
    p.done.apply(p, args);
  } else {
    funcs[0].apply(null, args).then(function(){
      funcs.splice(0, 1);
      Promise.chain(funcs, arguments).then(function(){
        p.done.apply(p, arguments);
      });
    });
  }
  return p;
};
})();
</script>