promise.js

function Promise(){
  this.callbacks = [];
  this.value = undefined;
}

Promise.prototype.success = function(callback){
    this.callbacks.push(callback);
    if(this.value)
      this.execute_callbacks();
}

Promise.prototype.execute_callbacks = function(){
    var that = this;
    this.callbacks.forEach(function(callback){
      callback(that.value);
    });
}

Promise.prototype.resolve = function(value){
    if(this.value)
      console.log("Error! Promise already resolved to value.");
    else {
      this.value = value;
      if(this.callbacks)
        this.execute_callbacks();
    }
}

var foo = new Promise();
var bar = new Promise();

foo.resolve('hello');

setTimeout(function(){
    bar.resolve('world');
}, 500);

foo.success(function(result){
    console.log(result);
});

foo.success(function(result){
    console.log("second callback");
});

bar.success(function(result){
    console.log(result);
});