const PENDING = 'pending',
FULFILLED = 'fulfilled',
REJECTED = 'rejected' class MyPromise {
constructor(executor) {
this.state = PENDING
this.value = undefined
this.reason = undefined
this.onResolvedCallbacks = []
this.onRejectedCallbacks = [] let resolve = (value) => {
if (this.state === PENDING) {
this.state = FULFILLED
this.value = value
this.onResolvedCallbacks.forEach((fn) => fn())
}
} let reject = (reason) => {
if (this.state === PENDING) {
this.state = REJECTED
this.reason = reason
this.onRejectedCallbacks.forEach((fn) => fn())
}
}
try {
executor(resolve, reject)
} catch (err) {
reject(err)
}
} then(onFulFilled, onRejected) {
let p2 = new MyPromise((resolve, reject) => {
let x
if (this.state === FULFILLED) {
setTimeout(() => {
x = onFulFilled(this.value)
//resolve(x); resolvePromise(p2, x, resolve, reject) // x 決定 了 p2 的狀態, resolve(x)或者 reject(x)；
}, 0)
} if (this.state === REJECTED) {
x = onRejected(this.reason)
resolvePromise(p2, x, resolve, reject)
} if (this.state === PENDING) {
this.onResolvedCallbacks.push(() => {
x = onFulFilled(this.value)
resolvePromise(p2, x, resolve, reject)
})
this.onRejectedCallbacks.push(() => {
x = onRejected(this.reason)
resolvePromise(p2, x, resolve, reject)
})
}
}) return p2
}
} function resolvePromise(p2, x, resolve, reject) {
if (p2 === x) {
return new Error('引用錯誤')
}
// thenable 物件; blueBird q;
if ((typeof x === 'object' && x !== null) || typeof x === 'function') {
try {
let then = x.then
if (typeof then === 'function') {
then.call(
x,
(y) => {
resolvePromise(p2, y, resolve, reject)
},
(err) => {
reject(err)
}
)
}
} catch (err) {
reject(err)
}
} else {
resolve(x)
}
}
function test() {
return new MyPromise((resolve, reject) => {
setTimeout(() => {
resolve(100)
}, 1000)
})
} var p1 = test()
// p1 => resolve(100) => p1.then(res) res => 100;
// p2 => resolve(res) => p2.then(res) => res ?? var p2 = p1.then(
(res) => {
// return {
// then(resolve, reject) {
// reject(100000);
// }
// };
return new MyPromise((resolve, reject) => {
resolve(
new MyPromise((resolve, reject) => {
resolve(
new MyPromise((resolve, reject) => {
resolve(100000000000)
})
)
})
)
})
},
(err) => console.log(err)
) p2.then(
(res) => {
console.log(res)
},
(err) => {
console.log(err)
}
)
總結：實現原始碼需要解決的問題
promise 原始碼~
1. 非同步的問題;
2. 鏈式呼叫 => return this ？
3.在初始化之前呼叫值的?
4. 遞迴的問題;