SOURCE

function MyPromise(resolver) {
	this._status = 'PENDDING'
	this._value = undefined
	this._resolveCallback = undefined
	this._rejectCallback = undefined
	
	resolver(resolve, reject)
	
	function resolve(val) {
		setTimeout(function () {
			console.log(88)
			this._resolveCallback && this._resolveCallback(val)
		})
	}
	
	function reject(val) {
		setTimeout(function () {
			this._rejectCallback && this._rejectCallback(val)
		})
	}
}

MyPromise.prototype.then = function (onResolve, onReject) {
	this._resolveCallback = onResolve
	this._rejectCallback = onReject
}

var fn=function(resolve, reject){
  console.log('begin to execute!');
	setTimeout(function () {
		var number=Math.random();
		if(number<=0.5){
			resolve('less than 0.5');
		}else{
			reject('greater than 0.5');
		}
	}, 200)
}

var p = new MyPromise(fn);
p.then(function(data){
  console.log('resolve: ', data);
}, function(data){
  console.log('reject: ', data);
})