手写 Promise() 相关函数：

Promise()、then()、catch()、finally()

// 定义三种状态常量
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'class MyPromise {/*定义状态和结果两个私有属性:1.使用 # 语法（ES2022+ 官方私有字段）：在类中通过 # 前缀声明属性，该属性仅在类的内部可访问2.Symbol 作为属性键：用 Symbol 作为属性名，外部无法直接获取 Symbol 引用。*/#state = PENDING // 状态#result = undefined // 结果#thenables = [] // 存储 then 方法回调的队列constructor(executor) {// resolve 和 reject 主要功能即为改变状态，设置结果const resolve = (value) => { // 解决时调用this.#changeState(FULFILLED, value)}const reject = (reason) => { // 拒绝时调用this.#changeState(REJECTED, reason)}try { // 只能捕获同步错误，无法捕获异步错误，如 setTimeout 里的错误executor(resolve, reject)} catch (err) {reject(err)}}// 改变状态和设置结果的私有方法#changeState(state, result) {if (this.#state !== PENDING) returnthis.#state = statethis.#result = resultthis.#run()}// 处理 then 回调的私有方法#handleCallback(callback, resolve, reject) {if (typeof callback !== 'function') { // 状态穿透，即 then 方法返回的 Promise 状态与当前 Promise 状态保持一致// then 回调是微任务，需要放到微任务队列中执行queueMicrotask(() => { // 参考：https://developer.mozilla.org/zh-CN/docs/Web/API/Window/queueMicrotaskconst settled = this.#state === FULFILLED ? resolve : rejectsettled(this.#result)})return}queueMicrotask(() => {try {const result = callback(this.#result)// 如果返回值是一个 Promise，则等待它完成后再 resolveif (result instanceof MyPromise) {result.then(resolve, reject)} else {resolve(result)}} catch (err) {reject(err)}})}/*执行队列的私有方法，两个执行时机：1.Promise 状态改变时2.then 方法被调用时*/#run() {if (this.#state === PENDING) return// 取出队列中的回调函数，依次执行(先进先出原则)while (this.#thenables.length) {const { onFulfilled, onRejected, resolve, reject } = this.#thenables.shift()try {if (this.#state === FULFILLED) { // 执行 onFulfilled 回调函数this.#handleCallback(onFulfilled, resolve, reject)} else { // 执行 onRejected 回调函数this.#handleCallback(onRejected, resolve, reject)}} catch (err) {reject(err)}}}/*onFulfilled 可选：一个在此 Promise 对象被兑现时异步执行的函数。它的返回值将成为 then() 返回的 Promise 对象的兑现值。onRejected 可选：一个在此 Promise 对象被拒绝时异步执行的函数。它的返回值将成为 catch() 返回的 Promise 对象的兑现值。*/then(onFulfilled, onRejected) {return new MyPromise((resolve, reject) => {// 将四个回调函数放入队列，以便立即或将来处理this.#thenables.push({onFulfilled,onRejected,resolve,reject})// 启动队列处理this.#run()})}catch(onRejected) {return this.then(undefined, onRejected)}finally(onFinally) {this.then((value) => {MyPromise.resolve(onFinally()).then(() => value)},(reason) => {MyPromise.resolve(onFinally()).then(() => { throw reason })})}
}

验证测试 MyPromise() 函数

const p1 = new Promise((resolve, reject) => {resolve(1)reject(2)
})
console.log('p1', p1)
const p2 = new Promise(() => { throw 123 })
console.log('p2', p2)
const p3 = new Promise((resolve, reject) => {setTimeout(() => {resolve('setTimeout') // 无法捕获}, 0)
})
console.log('p3', p3)
const myP1 = new MyPromise((resolve, reject) => {resolve(1)reject(2)
})
console.log('myP1', myP1)
myP1.then((res) => {console.log('res', res) // 1return res + 1},(err) => {console.log('err', err) // 1}
).then((res) => {console.log('res', res) // 2// throw 'error'return res + 1
}).then((res) => {console.log('res', res) // 3
}).catch((err) => {console.log('err', err)
}).finally(() => {console.log('finally')
})
const myP2 = new MyPromise(() => { throw 123 })
console.log('myP2', myP2)
const myP3 = new MyPromise((resolve, reject) => {setTimeout(() => {resolve('setTimeout') // 无法捕获}, 0)
})
console.log('myP3', myP3)

执行结果