本文实例讲述了JS手写一个自定义Promise操作。分享给大家供大家参考，具体如下：经常在面试题中会看到，让你实现一个Promsie,或者问你实现Promise的原理，所以今天就尝试利用class类的形式来实现一个Promise为了不与原生的Promise命名冲突，这里就简单命名为MyPromise.
class MyPromise {
constructor(executor) {

let _this = this

this.state = 'pending' // 当前状态

this.value = undefined // 存储成功的值

this.reason = undefined // 存储失败的值

// 利用发布订阅模式，让Promise支持异步

this.onFulfilledFunc = [] // 存储成功的回调

this.onRejectedFunc = [] // 存储失败的回调


function resolve (value) {

// Promise对象已经由pending状态改变为了成功态（resolved）或是失败态（rejected）就不能再次更改状态了。因此我们在更新状态时要判断，如果当前状态是pending（等待态）才可更新

if (_this.state === 'pending') {

_this.value = value

//依次执行成功回调

_this.onFulfilledFunc.forEach(fn => fn(value))

_this.state = 'resolved'

}

}


function reject (reason) {

// Promise对象已经由pending状态改变为了成功态（resolved）或是失败态（rejected）就不能再次更改状态了。因此我们在更新状态时要判断，如果当前状态是pending（等待态）才可更新

if (_this.state === 'pending') {

_this.reason = reason

//依次执行失败回调

_this.onRejectedFunc.forEach(fn => fn(reason))

_this.state = 'rejected'

}

}


try {

// 当实例化Promise时，构造函数中就要马上调用传入的executor函数执行

executor(resolve, reject)

} catch (error) {

reject(error)

}
}
_resolvePromise (promise2, x, resolve, reject) {

// 如果返回了自己的Promise对象，状态永远为等待态（pending），再也无法成为resolved或是rejected，程序会死掉，因此首先要处理它

if (promise2 === x) {

reject(new TypeError('Promise存在循环引用'))

}

if (x !== null && (typeof x === 'object' || typeof x === 'function')) {

// x可能是一个promise

try {

let then = x.then

if (typeof then === 'function') {

then.call(x, (y) => {

_resolvePromise(promise2, y, resolve, reject)

})

} else {

resolve(x)

}

} catch (err) {

reject(err)

}

} else {

//否则是个普通值

resolve(x)

}
}
then (onFulfilled, onRejected) {

let promise2

onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : function (val) { return val }

onRejected = typeof onRejected === 'function' ? onRejected : function (reason) { throw reason }


if (this.state === 'resolved') {

promise2 = new MyPromise((resolve, reject) => {

setTimeout(() => {

try {

let x = onFulfilled(this.value)

this._resolvePromise(promise2, x, resolve, reject)

} catch (error) {

reject(error)

}

}, 0);

})

}


if (this.state === 'rejected') {

promise2 = new MyPromise((resolve, reject) => {

setTimeout(() => {

try {

let x = onRejected(this.reason)

this._resolvePromise(promise2, x, resolve, reject)

} catch (error) {

reject(error)

}

}, 0);

})

}


if (this.state === 'pending') {

promise2 = new MyPromise((resolve, reject) => {

this.onFulfilledFunc.push(() => {

setTimeout(() => {

try {

let x = onFulfilled(this.value)

this._resolvePromise(promise2, x, resolve, reject)

} catch (error) {

reject(error)

}

}, 0);

})


this.onRejectedFunc.push(() => {

setTimeout(() => {

try {

let x = onRejected(this.reason)

this._resolvePromise(promise2, x, resolve, reject)

} catch (error) {

reject(error)

}

}, 0);

})

})

}


return promise2
}
}


class MyPromise {
constructor(executor) {

let _this = this

this.state = 'pending' // 当前状态

this.value = undefined // 存储成功的值

this.reason = undefined // 存储失败的值

// 利用发布订阅模式，让Promise支持异步

this.onFulfilledFunc = [] // 存储成功的回调

this.onRejectedFunc = [] // 存储失败的回调


function resolve (value) {

// Promise对象已经由pending状态改变为了成功态（resolved）或是失败态（rejected）就不能再次更改状态了。因此我们在更新状态时要判断，如果当前状态是pending（等待态）才可更新

if (_this.state === 'pending') {

_this.value = value

//依次执行成功回调

_this.onFulfilledFunc.forEach(fn => fn(value))

_this.state = 'resolved'

}

}


function reject (reason) {

// Promise对象已经由pending状态改变为了成功态（resolved）或是失败态（rejected）就不能再次更改状态了。因此我们在更新状态时要判断，如果当前状态是pending（等待态）才可更新

if (_this.state === 'pending') {

_this.reason = reason

//依次执行失败回调

_this.onRejectedFunc.forEach(fn => fn(reason))

_this.state = 'rejected'

}

}


try {

// 当实例化Promise时，构造函数中就要马上调用传入的executor函数执行

executor(resolve, reject)

} catch (error) {

reject(error)

}
}
_resolvePromise (promise2, x, resolve, reject) {

// 如果返回了自己的Promise对象，状态永远为等待态（pending），再也无法成为resolved或是rejected，程序会死掉，因此首先要处理它

if (promise2 === x) {

reject(new TypeError('Promise存在循环引用'))

}

if (x !== null && (typeof x === 'object' || typeof x === 'function')) {

// x可能是一个promise

try {

let then = x.then

if (typeof then === 'function') {

then.call(x, (y) => {

_resolvePromise(promise2, y, resolve, reject)

})

} else {

resolve(x)

}

} catch (err) {

reject(err)

}

} else {

//否则是个普通值

resolve(x)

}
}
then (onFulfilled, onRejected) {

let promise2

onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : function (val) { return val }

onRejected = typeof onRejected === 'function' ? onRejected : function (reason) { throw reason }


if (this.state === 'resolved') {

promise2 = new MyPromise((resolve, reject) => {

setTimeout(() => {

try {

let x = onFulfilled(this.value)

this._resolvePromise(promise2, x, resolve, reject)

} catch (error) {

reject(error)

}

}, 0);

})

}


if (this.state === 'rejected') {

promise2 = new MyPromise((resolve, reject) => {

setTimeout(() => {

try {

let x = onRejected(this.reason)

this._resolvePromise(promise2, x, resolve, reject)

} catch (error) {

reject(error)

}

}, 0);

})

}


if (this.state === 'pending') {

promise2 = new MyPromise((resolve, reject) => {

this.onFulfilledFunc.push(() => {

setTimeout(() => {

try {

let x = onFulfilled(this.value)

this._resolvePromise(promise2, x, resolve, reject)

} catch (error) {

reject(error)

}

}, 0);

})


this.onRejectedFunc.push(() => {

setTimeout(() => {

try {

let x = onRejected(this.reason)

this._resolvePromise(promise2, x, resolve, reject)

} catch (error) {

reject(error)

}

}, 0);

})

})

}


return promise2
}
}

运行测试：
var promise = new MyPromise((resolve, reject) => {
console.log(1)
setTimeout(() => {

resolve(2)
}, 1000);
console.log(3)
}).then(value => console.log(value))


var promise = new MyPromise((resolve, reject) => {
console.log(1)
setTimeout(() => {

resolve(2)
}, 1000);
console.log(3)
}).then(value => console.log(value))

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