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promise.js
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promise.js
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//三个状态
const PENDING = "pending";
const FULFILLED = "fulfilled";
const REJECTED = "rejected";
class MyPromise {
FULFILLED_CALLBACK_LIST = [];
REJECTED_CALLBACK_LIST = [];
_status = PENDING;
constructor(fn) {
this.status = PENDING;
this.value = null;
this.reason = null;
try {
fn(this.resolve.bind(this), this.reject.bind(this))
} catch (e) {
this.reject(e);
}
}
get status() {
return this._status;
}
set status(newStatus) {
this._status = newStatus;
switch (newStatus) {
case FULFILLED:
this.FULFILLED_CALLBACK_LIST.forEach(callback => {
callback(this.value);
})
break;
case REJECTED:
this.REJECTED_CALLBACK_LIST.forEach(callback => {
callback(this.reason);
})
}
}
isFunction(param) {
return typeof param === "function"
}
isObject(param) {
return typeof param === "object";
}
resolve(value) {
if (this.status === PENDING) {
this.status = FULFILLED;
this.value = value;
}
}
reject(reason) {
if (this.status === PENDING) {
this.status = REJECTED;
this.reason = reason;
}
}
then(onFulfilled, onRejected) {
const realOnFulfilled = this.isFunction(onFulfilled) ? onFulfilled : (value) => {
return value;
}
const realOnRejected = this.isFunction(onRejected) ? onRejected : (reason) => {
throw reason;
}
const promise2 = new MyPromise((resolve, reject) => {
const fulfilledMicrotask = () => {
queueMicrotask(() => {
try {
const x = realOnFulfilled(this.value);
this.resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
})
}
const rejectedMicrotask = () => {
queueMicrotask(() => {
try {
const x = realOnRejected(this.reason);
this.resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
})
}
switch (this.status) {
case FULFILLED:
fulfilledMicrotask();
break;
case REJECTED:
rejectedMicrotask();
break;
case PENDING:
this.FULFILLED_CALLBACK_LIST.push(fulfilledMicrotask);
this.REJECTED_CALLBACK_LIST.push(rejectedMicrotask);
break;
}
});
return promise2;
}
resolvePromise(promise2, x, resolve, reject) {
if (promise2 === x) {
return reject(new TypeError("promise2和x不能相等"));
}
if (x instanceof MyPromise) {
queueMicrotask(() => {
x.then((y) => {
this.resolvePromise(promise2, y, resolve, reject);
}, reject)
})
} else if (this.isFunction(x) || this.isObject(x)) {
if (x === null) {
return resolve(x);
}
let then = null;
try {
then = x.then
} catch (e) {
return reject(e)
}
if (this.isFunction(then)) {
let called = false;
try {
then.call(x, (value) => {
if (called) return;
called = true;
this.resolvePromise(promise2, value, resolve, reject);
}, (reason) => {
if (called) return;
called = true;
reject(reason);
})
} catch (e) {
if (called) return;
reject(e);
}
} else {
resolve(x);
}
} else {
resolve(x);
}
}
}
module.exports = MyPromise;