promise实现

思路就是把then的所有方法都加到一个队列里面，然后resolve的时候依次执行，

基础实现

function getUserID(){
	return new Promise(function(resolve){
		setTimeout(function(){
			resolve(123);
		},100);
	});
}
getUserID().then(function(id){
	console.log(id);
});
function Promise(fn) {
	var deferreds = [];
	this.then = function(onFulfilled){
		deferreds.push(onFulfilled);
		return this;
	};
	var resolve = function(value){
		deferreds.forEach(function(deferred){
			deferred(value);
		});
	};
	fn(resolve);
}

Promise内如果是同步代码会有问题，会直接走resove

function getUserID(){
	return new Promise(function(resolve){
		resolve(123);
	});
}
getUserID().then(function(id){
	console.log(id);
});
function Promise(fn) {
	var deferreds = [];
	this.then = function(onFulfilled){
		deferreds.push(onFulfilled);
		return this;
	};
	var resolve = function(value){
		setTimeout(function(){//加入任务队列，让主线程先执行
			deferreds.forEach(function(deferred){
			deferred(value);
			});
		},0);
	};
	fn(resolve);
}

如果promise已经完成，之后then的回调不再执行，引入状态pending（等待）、fulfilled（完成）、rejected（拒绝）

function Promise(fn){
	var deferreds = [];
	var status = "pending";
	var value = null;
	this.then = function(onFulfilled){
		if(status === "pending"){
			deferreds.push(onFulfilled);\\加入队列
			return this;
		}
		onFulfilled(value);\\直接执行
		return this;
	};

	function resolove(newValue){
		value = newValue;
		status = "fulfilled";
		setTimeout(function(){
			deferreds.forEach(function(deferred){
				deferred(value);
			});
		},0);
	}

	fn(resolve);
}