手写Promise

1. 无链式then

// 先定义三个常量表示状态
var PENDING = 'pending';
var FULFILLED = 'fulfilled';
var REJECTED = 'rejected';

function MyPromise(fn) {
	this.status = PENDING;    // 初始状态为pending
	this.value = null;        // 初始化value
	this.err = null;       // 初始化err

	this.onFulfilledCallbacks = [] // Fulfilled回调数组
	this.onRejectedCallbacks = [] // Rejected回调数组

	// console.log(fn)

	// 存一下this,以便resolve和reject里面访问
	var that = this;
	// resolve方法参数是value
	function resolve(value) {
		if(that.status === PENDING) {
			that.status = FULFILLED;
			that.value = value;

			// 执行从PENDING到Fulfill的方法,如果MyPromise里没有异步任务，同步resolve，这里应该是空的
			that.onFulfilledCallbacks.forEach(cb => {
				cb(value)
			})
		}
	}

	// reject方法参数是err
	function reject(err) {
		if(that.status === PENDING) {
			that.status = REJECTED;
			that.err = err;

			// 执行从PENDING到Reject的方法
			that.onRejectedCallbacks.forEach(cb => {
				cb(err)
			})
		}
	}

	try {
		fn(resolve, reject);
		/*
		(function(resolve,reject)=>{
			setTimeout(()=>{
				resolve('hello')
			},2500)
		})(resolve,reject);
		*/
	} catch (error) {
 		reject(error);
	}
}

// two params of function then
MyPromise.prototype.then = function(onFulfilled,onReject){

	  //没有then链式调用
	  
	  // 状态改变 执行相应回调
	  // 能直接调用回调的两种情况，说明Promise构造时resolve没有异步执行  
	  if(this.status === FULFILLED) {
	    onFulfilled(this.value)
	  }
	  if(this.status === REJECTED) {
	    onRejected(this.err);
	  }

	  // 如果还是PENDING状态，说明Promise构造在异步执行，还没resolve/reject，所以先将回调保存下来
	  if(this.status === PENDING) {
	    this.onFulfilledCallbacks.push(onFulfilled);
	    this.onRejectedCallbacks.push(onReject);
	  } 
}


let p = new MyPromise((resolve,reject)=>{
	// resolve('hello')
	// 异步resolve
	setTimeout(()=>{
		resolve('hello')
	},1000)	
})


p.then(res=>{
	console.log(res);
})

本文地址：https://blog.csdn.net/qq_38765789/article/details/107388727