概念-提炼诺言如何运作？

有人可以在几行中实现最基本的承诺吗？

这里是：

function Promise(fn) {    // takes a function as an argument that gets the fullfiller    var callbacks = [], result;    fn(function fulfill() {        if (result) return;        result = arguments;        for (var c;c=callbacks.shift();) c.apply(null, arguments);    });    this.addCallback = function(c) {        if (result) c.apply(null, result)        else callbacks.push(c);    }}Promise.prototype.then = function(fn) {    var that = this;    return new Promise(function(c){        that.addCallback(function() { var result = fn.apply(null, arguments); if (result instanceof Promise)     result.addCallback(c); else     c(result);        });    });};

这两个摘要有什么关系？

ajax

getPromiseForAjaxResult

function getPromiseForAjaxResult(ressource) {    return new Promise(function(callback) {        ajax({url:ressource}, callback);    });}