手写 promise

手写 promise

class Promise { static resolve() {} static reject() {} static PENDING = "pending"; static RESOLVED = "resolved"; static REJECTED = "rejected"; constructor(exector) { this.state = Promise.PENDING; this.value = undefined; this.reason = undefined; this.onResolvedCallbacks = []; this.onRejectedCallbacks = []; const resolve = (value) => { if (this.state === Promise.PENDING) { this.value = value; this.state = Promise.RESOLVED; this.onResolvedCallbacks.forEach((fn) => fn(this.value)); } }; const reject = (reason) => { if (this.state === Promise.PENDING) { this.reason = reason; this.state = Promise.REJECTED; this.onRejectedCallbacks.forEach((fn) => fn(this.reason)); } }; try { exector(resolve, reject); } catch (e) { reject(e); } } then(onFulFilled, onRejected) { // 针对 then 传入的回调函数为 null 的情况，直接将值返回 // p.then(null).then((res) => { // console.log(res); // }); onFulFilled = typeof onFulFilled === "function" ? onFulFilled : (value) => value; onRejected = typeof onRejected === "function" ? onRejected : (reason) => { throw reason; }; // 实现链式调用 let promise = new Promise((resolve, reject) => { if (this.state === Promise.PENDING) { this.onResolvedCallbacks.push(() => { setTimeout(() => { try { let x = onFulFilled(this.value); resolvePromise(promise, x, resolve, reject); } catch (error) { reject(error); } }); }); this.onRejectedCallbacks.push(() => { setTimeout(() => { try { let x = onRejected(this.reason); resolvePromise(promise, x, resolve, reject); } catch (error) { reject(error); } }, 0); }); } if (this.state === Promise.RESOLVED) { setTimeout(() => { try { // 上一个 then 函数里面可能有很多逻辑 // 可能报错 // 可能返回一个固定值 // 可能返回一个同步的 promise // 可能返回一个异步的 promise let x = onFulFilled(this.value); resolvePromise(promise, x, resolve, reject); } catch (error) { reject(error); } }, 0); } if (this.state === Promise.REJECTED) { setTimeout(() => { try { let x = onRejected(this.reason); resolvePromise(promise, x, resolve, reject); } catch (error) { reject(error); } }, 0); } }); return promise; } catch() {}}function resolvePromise(promise, x, resolve, reject) { // let promise = new Promise((resolve) => { // resolve(1); // }).then(() => { // return promise; // }); // 出现循环引用 if (x === promise) { reject( new TypeError("TypeError: Chaining cycle detected for promise #") ); } // 加锁，防止别人的 promise 不遵守 "状态一旦被修改不可再变化" let called = false; if ((typeof x === "object" && x !== null) || typeof x === "function") { try { let then = x.then; if (typeof then === "function") { then.call( x, (y) => { if (called) return; called = true; // 如果返回的 promise 还是一个 promise // 递归解析 y 的值，直至为常量为止 resolvePromise(promise, y, resolve, reject); }, (r) => { if (called) return; called = true; reject(r); } ); } else { // 带有then的对象 // x: { // then: {} // } if (called) return; called = true; resolve(x); } } catch (error) { if (called) return; called = true; // 有可能 x.then 的时候报错 reject(error); } } else { // 上一个 then return 了普通值 resolve(x); }}Promise.deferred = function () { let dfd = {}; dfd.promise = new Promise((resolve, reject) => { dfd.resolve = resolve; dfd.reject = reject; }); return dfd;};module.exports = Promise;

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