promise_easy.js

/*
 from http://www.cnblogs.com/iamzhanglei/archive/2013/02/24/2924680.html
 */

var Promise = function() {
  this.thens = [];
};

Promise.prototype = {
  resolve: function() {
    var t = this.thens.shift(), n;
    n = t.apply(null, arguments); // n instanceof Promise && (n.thens = this.thens));
    if(n instanceof Promise) {
      n.thens = this.thens;
    }
  },
  then: function(n) {
    this.thens.push(n);
    return this;
  }
};

function f1() {
  var promise = new Promise();
  setTimeout(function() {
    console.log(1);
    promise.resolve();
  }, 1000);
  return promise;
}

function f2() {
  var promise = new Promise();
  setTimeout(function() {
    console.log(2);
    promise.resolve();
  }, 1000);
  return promise;
}

function f3() {
  var promise = new Promise();
  setTimeout(function() {
    console.log(3);
    promise.resolve();
  }, 1000);
  return promise;
}

function f4() {
  console.log(4);
}

f1().then(f2).then(f3).then(f4);