[手写系列] 带你实现一个简单的Promise

简介学习之前需要先对Promise有个基本了解哦,这里都默认大家都是比较熟悉Promise的
本次将带小伙伴们实现Promise的基本功能

Promise的基本骨架
Promise的then
Promise.then的多次调用
then链式调用
catch的实现
finally的实现

01-搭建基本骨架

const PROMISE_STATUS_PENDING = "PROMISE_STATUS_PENDING";const PROMISE_STATUS_FULFILLED = "PROMISE_STATUS_FULFILLED";const PROMISE_STATUS_REJECTED = "PROMISE_STATUS_REJECTED";class ZXPromise {constructor(executor) {this.status = PROMISE_STATUS_PENDING;const resolve = (value) => {if (this.status === PROMISE_STATUS_PENDING) {this.status = PROMISE_STATUS_FULFILLED;console.log(value);}}const rejected = (reason) => {if (this.status === PROMISE_STATUS_PENDING) {this.status = PROMISE_STATUS_REJECTED;console.log(reason);}}executor(resolve, rejected)}}// 初步搭建好Promise的construtor结构const promise = new ZXPromise((resolve, rejected) => {resolve("123");rejected("wushichu")})

因为Promise有三种状态pending,fulfilled,rejected,我们这里就声明三个常量来代表这三种状态
Promise中需要传递一个回调函数,他的参数中包含了resolve和rejected,调用resolve之后,状态会变为fulfilled,调用rejected,状态会变成rejected
我定义了一个类,我们在constructor中定义所需要的resolve和rejected函数,然后将这两个函数传入那个executor中去,这样Promise的基本骨架就已经搭建完成了,非常简单.

02-实现Promise的then功能

const PROMISE_STATUS_PENDING = "PROMISE_STATUS_PENDING";const PROMISE_STATUS_FULFILLED = "PROMISE_STATUS_FULFILLED";const PROMISE_STATUS_REJECTED = "PROMISE_STATUS_REJECTED";class ZXPromise {	constructor(executor) {this.status = PROMISE_STATUS_PENDING;const resolve = (value) => {if (this.status === PROMISE_STATUS_PENDING) {queueMicrotask(() => {//因为只有pending状态才能进行变化if(this.status!==PROMISE_STATUS_PENDING) returnthis.status = PROMISE_STATUS_FULFILLED;if (this.onfufilled)this.onfufilled(value);})}}const rejected = (reason) => {if (this.status === PROMISE_STATUS_PENDING) {queueMicrotask(() => {if(this.status!==PROMISE_STATUS_PENDING) returnthis.status = PROMISE_STATUS_REJECTED;if (this.onrejected)this.onrejected(reason);})}}executor(resolve, rejected)	}	then(onfufilled, onrejected) {this.onfufilled = onfufilled;this.onrejected = onrejected;	}}// 接下来开始写then方法const promise = new ZXPromise((resolve, rejected) => {	resolve("123");	rejected("wushichu");})promise.then((res) => {	console.log("res", res);}, (err) => {	console.log("err", err);})

then方法中接受两个参数,分别是onfulfilled和onrejected两个函数,分别对应着状态fulfilled和rejected
这里要注意一个点我在resolve和rejected中都使用了queueMicrotask,这里使用的目的是为了保证顺序执行的一致性,确保在then方法执行过后,再去执行相关代码,这里需要大家熟悉微任务队列和宏任务队列,推荐大家看下这篇文章
在JS中使用queueMicroTask

03-Promise.then多次调用大家可以用上一部分的代码实验一下,如果多次调用,会发现只有最后一个输出,并且在定时器中使用,会出现结果为undefined
【[手写系列] 带你实现一个简单的Promise】

p1.then((res) => {	console.log("res1", res);});p1.then((res) => {	console.log('res2: ', res);});setTimeout(() => {	p1.then((res) => {console.log("res4", res);	})}, 1000);

现在我们来解决下上述问题,看代码

const PROMISE_STATUS_PENDING = "PROMISE_STATUS_PENDING";const PROMISE_STATUS_FULFILLED = "PROMISE_STATUS_FULFILLED";const PROMISE_STATUS_REJECTED = "PROMISE_STATUS_REJECTED";class ZXPromise {	constructor(executor) {this.status = PROMISE_STATUS_PENDING;this.value = https://tazarkount.com/read/undefined;this.reason = undefined;this.onfufilled = [];this.onrejected = [];const resolve = (value) => {if (this.status === PROMISE_STATUS_PENDING) {queueMicrotask(() => {if (this.status !== PROMISE_STATUS_PENDING) returnthis.status = PROMISE_STATUS_FULFILLED;this.value = value;this.onfufilled.forEach(fn => {fn(value);});})}}const rejected = (reason) => {if (this.status === PROMISE_STATUS_PENDING) {queueMicrotask(() => {if (this.status !== PROMISE_STATUS_PENDING) returnthis.status = PROMISE_STATUS_REJECTED;this.reason = reason;this.onrejected.forEach(fn => {fn(reason);})})}}executor(resolve, rejected)	}	// 接下来为了Promise能够多次调用 进行优化	then(onfufilled, onrejected) {if (this.status === PROMISE_STATUS_FULFILLED) {onfufilled(this.value);}if (this.status === PROMISE_STATUS_REJECTED) {onrejected(this.value);}if (this.status === PROMISE_STATUS_PENDING) {this.onfufilled.push(onfufilled);this.onrejected.push(onrejected);}	}}

因为改进之后,需要存储resolve和rejected的value和reason值,所以我们定义了这两个值
为了满足多次调用,我们需要将promise中的onfulfilled和onrejected改为数组存储以用来满足我们的多次调用
定时器的问题我这边说下,因为setTimeout属于宏任务,在同步代码执行完毕之后,会接着执行微任务,所以宏任务是最后来执行的,所以也就造成了promise中的代码执行完了,但是包裹在定时器中的then方法没有获取到结果
所以呢,在这里我决定让处于定时器中的代码直接执行而不压入数组中去,因为定时器之前的代码已经执行完毕了,promise的状态也已经发生了改变,所以我就在then方法中判断promise的状态,如果是fulfilled和rejected状态的话,传过来的函数就直接执行

04-then方法的链式调用要想实现链式调用,那么then方法肯定是将Promise对象又给返回出来了,说到这了大家有没有思路呢?

const PROMISE_STATUS_PENDING = "PROMISE_STATUS_PENDING";const PROMISE_STATUS_FULFILLED = "PROMISE_STATUS_FULFILLED";const PROMISE_STATUS_REJECTED = "PROMISE_STATUS_REJECTED";class ZXPromise {	constructor(executor) {this.status = PROMISE_STATUS_PENDING;this.value = https://tazarkount.com/read/undefined;this.reason = undefined;this.onfufilled = [];this.onrejected = [];const resolve = (value) => {if (this.status === PROMISE_STATUS_PENDING) {queueMicrotask(() => {if (this.status !== PROMISE_STATUS_PENDING) returnthis.status = PROMISE_STATUS_FULFILLED;this.value = value;this.onfufilled.forEach(fn => {fn(value);});})}}const rejected = (reason) => {if (this.status === PROMISE_STATUS_PENDING) {queueMicrotask(() => {if (this.status !== PROMISE_STATUS_PENDING) returnthis.status = PROMISE_STATUS_REJECTED;this.reason = reason;this.onrejected.forEach(fn => {fn(reason);})})}}try{executor(resolve, rejected)}catch(err){console.log(err);}}then(onfufilled, onrejected) {return new ZXPromise((resolve, rejected) => {if (this.status === PROMISE_STATUS_FULFILLED) {try {//如果then中有返回值,就会作为下一个then所接收的值const value = onfufilled(this.value);resolve(value);} catch (err) {rejected(err);}}if (this.status === PROMISE_STATUS_REJECTED) {try {const value = onrejected(this.value);resolve(value);} catch (err) {rejected(err);}}if (this.status === PROMISE_STATUS_PENDING) {try {this.onfufilled.push(() => {const value = onfufilled(this.value);resolve(value);});} catch (err) {rejected(err);}try {this.onrejected.push(() => {const value = onrejected(this.value);resolve(value);});} catch (err) {rejected(err);}}})	}}const promise = new ZXPromise((resolve, rejected) => {	resolve("123");	rejected("wushichu");})promise.then((res) => {	console.log("res1:", res);	return "abc";}, (err) => {	console.log("err1", err);}).then((res) => {	console.log("res2", res);}, (err) => {	console.log("err2", err);})

变化最大的就是then方法了,大家可以看到我又把ZXPromise返回出去了,代码中我写的很清楚了

05-catch方法实现catch方法实际上是then第二个参数的语法糖,说到这里大家有没有明白什么呢?

const PROMISE_STATUS_PENDING = "PROMISE_STATUS_PENDING";const PROMISE_STATUS_FULFILLED = "PROMISE_STATUS_FULFILLED";const PROMISE_STATUS_REJECTED = "PROMISE_STATUS_REJECTED";const execFnWithCatchError = (execFn, value, resolve, reject) => {try {const result = execFn(value);resolve(result);} catch (err) {reject(err);}}class ZXPromise {constructor(executor) {this.status = PROMISE_STATUS_PENDING;this.value = https://tazarkount.com/read/undefined;this.reason = undefined;this.onfufilled = [];this.onrejected = [];const resolve = (value) => {if (this.status === PROMISE_STATUS_PENDING) {queueMicrotask(() => {if (this.status !== PROMISE_STATUS_PENDING) returnthis.status = PROMISE_STATUS_FULFILLED;this.value = value;this.onfufilled.forEach(fn => {fn(value);});})}}const rejected = (reason) => {if (this.status === PROMISE_STATUS_PENDING) {queueMicrotask(() => {if (this.status !== PROMISE_STATUS_PENDING) returnthis.status = PROMISE_STATUS_REJECTED;this.reason = reason;this.onrejected.forEach(fn => {fn(reason);})return this.reason;})}}executor(resolve, rejected)}then(onfufilled, onrejected) {//这一段是为了将错误代码传递下去的const defaultOnRejected = err => { throw err }onrejected = onrejected || defaultOnRejectedreturn new ZXPromise((resolve, rejected) => {if (this.status === PROMISE_STATUS_FULFILLED && onfufilled) {execFnWithCatchError(onfufilled, this.value, resolve, rejected);}if (this.status === PROMISE_STATUS_REJECTED && onrejected) {execFnWithCatchError(onrejected, this.reason, resolve, rejected);}if (this.status === PROMISE_STATUS_PENDING) {if (onfufilled)this.onfufilled.push(() => {execFnWithCatchError(onfufilled, this.value, resolve, rejected);});if (onrejected) {this.onrejected.push(() => {execFnWithCatchError(onrejected, this.reason, resolve, rejected);});}}})}catch(onrejected) {return this.then(undefined, onrejected);}}

大家可以看到catch代码实际上就只有一行,就是将then方法进行了调用,是不是相当简单呢
然后我觉得那个try catch代码重复性比较高,所以我将它提取了出来复用
然后大家看下那个then里面的开头,onrejected函数被给予了一个默认值,如果then没有传递第二个参数,那么会被赋予一个错误处理函数的默认值,抛出错误后,会自动被try catch捕获进行reject,这样子错误会被层层传递,一直到最后被catch函数所执行.

06-finally的实现finally就是要在最后执行的函数,无论什么情况,实现起来也是非常简单
finally(fn) {return this.then(() => { fn() }, () => { fn() });}

在类中加上这一段代码就好了,因为finally是无法接收任何resolve和rejected的值的,所以我们在传递的函数中执行fn,就是避免resolve的值和rejected的值被传递到finally上去

07-完整代码总览

const PROMISE_STATUS_PENDING = "PROMISE_STATUS_PENDING";const PROMISE_STATUS_FULFILLED = "PROMISE_STATUS_FULFILLED";const PROMISE_STATUS_REJECTED = "PROMISE_STATUS_REJECTED";const execFnWithCatchError = (execFn, value, resolve, reject) => {try {const result = execFn(value);resolve(result);} catch (err) {reject(err);}}class ZXPromise {constructor(executor) {this.status = PROMISE_STATUS_PENDING;this.value = https://tazarkount.com/read/undefined;this.reason = undefined;this.onfufilled = [];this.onrejected = [];const resolve = (value) => {if (this.status === PROMISE_STATUS_PENDING) {queueMicrotask(() => {if (this.status !== PROMISE_STATUS_PENDING) returnthis.status = PROMISE_STATUS_FULFILLED;this.value = value;this.onfufilled.forEach(fn => {fn(value);});})}}const rejected = (reason) => {if (this.status === PROMISE_STATUS_PENDING) {queueMicrotask(() => {if (this.status !== PROMISE_STATUS_PENDING) returnthis.status = PROMISE_STATUS_REJECTED;this.reason = reason;this.onrejected.forEach(fn => {fn(reason);})return this.reason;})}}executor(resolve, rejected)}then(onfufilled, onrejected) {//这一段是为了将错误代码传递下去的const defaultOnRejected = err => { throw err }onrejected = onrejected || defaultOnRejectedreturn new ZXPromise((resolve, rejected) => {if (this.status === PROMISE_STATUS_FULFILLED && onfufilled) {execFnWithCatchError(onfufilled, this.value, resolve, rejected);}if (this.status === PROMISE_STATUS_REJECTED && onrejected) {execFnWithCatchError(onrejected, this.reason, resolve, rejected);}if (this.status === PROMISE_STATUS_PENDING) {if (onfufilled)this.onfufilled.push(() => {execFnWithCatchError(onfufilled, this.value, resolve, rejected);});if (onrejected) {this.onrejected.push(() => {execFnWithCatchError(onrejected, this.reason, resolve, rejected);});}}})}catch(onrejected) {return this.then(undefined, onrejected);}finally(fn) {return this.then(() => { fn() }, () => { fn() });}}

大家可以自行进行测试

本文来自博客园，作者：CodeSpirit，转载请注明原文链接：https://www.cnblogs.com/codespirit-zx/p/15979455.html