摘要：pending（进行中）：初始状态，既没有被 fulfilled，也没有被 rejected。fulfilled（已成功）：操作成功完成，并返回一个值（用 resolve 触发）。rejected（已失败）：操作失败，并返回失败原因（用 reject 触发）。
很好 你问的是 Promise 的状态、方法，以及实现原理。我帮你拆成三部分来讲：
Promise 内部有 三种状态（[[PromiseState]]）：
pending（进行中）：初始状态，既没有被 fulfilled，也没有被 rejected。fulfilled（已成功）：操作成功完成，并返回一个值（用 resolve 触发）。rejected（已失败）：操作失败，并返回失败原因（用 reject 触发）。⚠️ 特点：
状态 只能从 pending → fulfilled 或 pending → rejected，一旦变化就 不可逆。成功和失败只能发生一次，后续调用 resolve / reject 不会再改变状态。then(onFulfilled, onRejected)注册成功和失败的回调。返回一个新的 Promise（可链式调用）。如果 onFulfilled / onRejected 返回值是 Promise，会进行 状态递归解析。catch(onRejected)相当于 .then(undefined, onRejected)。用于错误捕获。finally(onFinally)无论成功/失败都会执行。不会改变链式调用的结果，只做副作用处理。Promise.resolve(value)返回一个 fulfilled 状态的 Promise。如果参数本身是 Promise，直接返回。Promise.reject(reason)返回一个 rejected 状态的 Promise。Promise.all(iterable)并行执行多个 Promise，所有都成功才成功，否则第一个失败就失败。Promise.allSettled(iterable)等待所有 Promise 执行完，不管成功/失败，返回每个结果的数组。Promise.race(iterable)谁先有结果（成功或失败）就返回谁的结果。Promise.any(iterable)只要有一个成功就返回成功，如果全部失败则返回 AggregateError。一个 Promise 对象要有：
state：存储当前状态（pending / fulfilled / rejected）。value / reason：存储成功的值或失败的原因。handlers：存放 then 注册的回调函数。class MyPromise {constructor(executor) {this.state = 'pending';this.value = undefined;this.reason = undefined;this.onFulfilledCallbacks = ;this.onRejectedCallbacks = ;const resolve = (value) => {if (this.state === 'pending') {this.state = 'fulfilled';this.value = value;this.onFulfilledCallbacks.forEach(fn => fn);}};const 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) {onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v;onRejected = typeof onRejected === 'function' ? onRejected : e => { throw e };const promise2 = new MyPromise((resolve, reject) => {if (this.state === 'fulfilled') {queueMicrotask( => { // 保证异步try {const x = onFulfilled(this.value);resolvePromise(promise2, x, resolve, reject);} catch (e) {reject(e);}});}if (this.state === 'rejected') {queueMicrotask( => {try {const x = onRejected(this.reason);resolvePromise(promise2, x, resolve, reject);} catch (e) {reject(e);}});}if (this.state === 'pending') {this.onFulfilledCallbacks.push( => {queueMicrotask( => {try {const x = onFulfilled(this.value);resolvePromise(promise2, x, resolve, reject);} catch (e) {reject(e);}});});this.onRejectedCallbacks.push( => {queueMicrotask( => {try {const x = onRejected(this.reason);resolvePromise(promise2, x, resolve, reject);} catch (e) {reject(e);}});});}});return promise2;}}处理 then 回调返回值的逻辑：
如果返回的是 普通值，直接 resolve。如果返回的是 Promise，递归解析其状态。避免 循环引用。function resolvePromise(promise2, x, resolve, reject) {if (promise2 === x) {return reject(new TypeError('Chaining cycle detected'));}if (x && (typeof x === 'object' || typeof x === 'function')) {let called = false;try {let then = x.then;if (typeof then === 'function') {then.call(x, y => {if (called) return;called = true;resolvePromise(promise2, y, resolve, reject);}, e => {if (called) return;called = true;reject(e);});} else {resolve(x);}} catch (e) {if (called) return;called = true;reject(e);}} else {resolve(x);}}来源：科学新学生
