/** * MyPromise — 基于 queueMicrotask 的 Promises/A+ 规范实现 * * 核心设计： *   - 三种状态：pending, fulfilled, rejected，单向不可逆 *   - 异步回调通过 queueMicrotask 调度，满足规范 2.2.4 *   - Promise Resolution Procedure 处理 thenable 链 *   - then 始终返回新的 Promise */const PENDING = 'pending'const FULFILLED = 'fulfilled'const REJECTED = 'rejected'// ── Promise Resolution Procedure ──────────────────────────────────────────// 规范: [[Resolve]](promise, x)// 负责处理 thenable 递归解析，防止循环引用和多次调用function resolvePromise(promise2, x, resolve, reject) {  // 2.3.1: 如果 promise2 和 x 指向同一对象，抛出 TypeError  // 注意：promise2 在 executor 场景中为 null，排除 null === null 的误判  if (promise2 !== null && promise2 === x) {    return reject(new TypeError('Chaining cycle detected for promise'))  }  // 2.3.2: 如果 x 是 MyPromise 实例，采用其状态  if (x instanceof MyPromise) {    x.then(resolve, reject)    return  }  // 2.3.3: 如果 x 是对象或函数（可能是 thenable）  let called = false  if (x !== null && (typeof x === 'object' || typeof x === 'function')) {    try {      // 2.3.3.1: 获取 then（只访问一次，防止 getter 副作用）      const then = x.then      if (typeof then === 'function') {        // 2.3.3.3: then 是函数，按 thenable 处理        then.call(          x,          // onFulfilled          (y) => {            if (called) return            called = true            // 2.3.3.3.1: 递归解析            resolvePromise(promise2, y, resolve, reject)          },          // onRejected          (r) => {            if (called) return            called = true            reject(r)          }        )      } else {        // 2.3.3.4: then 不是函数，直接 fulfill        resolve(x)      }    } catch (e) {      // 2.3.3.2 / 2.3.3.3.4: 访问 then 或调用时抛出异常      if (called) return      called = true      reject(e)    }  } else {    // 2.3.4: x 不是对象也不是函数，直接 fulfill    resolve(x)  }}// ── MyPromise 类 ──────────────────────────────────────────────────────────class MyPromise {  constructor(executor) {    // 初始状态    this._state = PENDING    this._value = undefined    this._reason = undefined    // 回调队列：每个元素是 { onFulfilled, onRejected, promise2, resolve, reject }    this._callbacks = []    // 绑定内部方法    // settled 标志：resolve / reject 只能有一个生效    // 使用独立标志而非 _state 检查，因为 resolve(thenable) 时    // _fulfill 可能是异步的，此时 _state 仍为 PENDING    let settled = false    const resolve = (value) => {      if (settled) return      settled = true      // 如果 value 是自身，直接 reject（虽然 executor 中不太可能出现）      if (value === this) {        return this._reject(new TypeError('Cannot resolve promise with itself'))      }      // 使用 Promise Resolution Procedure 处理 value      try {        resolvePromise(          null, // executor 场景没有 promise2 引用          value,          (val) => this._fulfill(val),          (reason) => this._reject(reason)        )      } catch (e) {        this._reject(e)      }    }    const reject = (reason) => {      if (settled) return      settled = true      this._reject(reason)    }    // 执行 executor    try {      executor(resolve, reject)    } catch (e) {      // executor 同步抛出异常      if (!settled) {        settled = true        this._reject(e)      }    }  }  // ── 内部方法 ────────────────────────────────────────────────────────────  /**   * 内部 fulfill：直接设置值，不经过 thenable 解析   */  _fulfill(value) {    if (this._state !== PENDING) return    this._state = FULFILLED    this._value = value    // 2.2.4: 异步执行所有挂起的回调    this._flush()  }  /**   * 内部 reject：直接设置 reason   */  _reject(reason) {    if (this._state !== PENDING) return    this._state = REJECTED    this._reason = reason    // 2.2.4: 异步执行所有挂起的回调    this._flush()  }  /**   * 通过 queueMicrotask 调度所有挂起回调的执行   */  _flush() {    if (this._callbacks.length === 0) return    const callbacks = this._callbacks    this._callbacks = []    queueMicrotask(() => {      for (const cb of callbacks) {        this._invokeCallback(cb)      }    })  }  /**   * 执行单个回调，处理 onFulfilled/onRejected 的返回值或异常   */  _invokeCallback(cb) {    const { onFulfilled, onRejected, promise2, resolve, reject } = cb    try {      if (this._state === FULFILLED) {        if (typeof onFulfilled === 'function') {          const result = onFulfilled(this._value)          resolvePromise(promise2, result, resolve, reject)        } else {          // 2.2.7.3: onFulfilled 不是函数，值穿透          resolve(this._value)        }      } else {        // REJECTED        if (typeof onRejected === 'function') {          const result = onRejected(this._reason)          resolvePromise(promise2, result, resolve, reject)        } else {          // 2.2.7.4: onRejected 不是函数，异常穿透          reject(this._reason)        }      }    } catch (e) {      // 2.2.7.2: 回调抛出异常，reject promise2      reject(e)    }  }  // ── 实例方法 ────────────────────────────────────────────────────────────  /**   * then(onFulfilled, onRejected)   *   * 2.2: 返回一个新的 Promise   * 2.2.4: onFulfilled / onRejected 异步执行（通过 queueMicrotask）   */  then(onFulfilled, onRejected) {    let resolve, reject    const promise2 = new MyPromise((res, rej) => {      resolve = res      reject = rej    })    if (this._state === PENDING) {      // 2.2.6: 状态未确定，加入回调队列      this._callbacks.push({        onFulfilled,        onRejected,        promise2,        resolve,        reject,      })    } else {      // 2.2.4: 状态已确定，通过 queueMicrotask 异步执行      queueMicrotask(() => {        this._invokeCallback({          onFulfilled,          onRejected,          promise2,          resolve,          reject,        })      })    }    return promise2  }  /**   * catch(onRejected) — then(undefined, onRejected) 的语法糖   */  catch(onRejected) {    return this.then(undefined, onRejected)  }  /**   * finally(onFinally) — 无论 fulfilled 还是 rejected 都会执行   * 返回一个新的 Promise，其状态与当前 promise 一致（除非 onFinally 抛出异常）   */  finally(onFinally) {    return this.then(      (value) =>        MyPromise.resolve(typeof onFinally === 'function' ? onFinally() : undefined).then(          () => value        ),      (reason) =>        MyPromise.resolve(typeof onFinally === 'function' ? onFinally() : undefined).then(          () => {            throw reason          }        )    )  }  // ── 静态方法 ────────────────────────────────────────────────────────────  /**   * MyPromise.resolve(value)   * 返回一个 fulfilled 的 Promise（如果 value 是 thenable 则跟随其状态）   */  static resolve(value) {    if (value instanceof MyPromise) return value    return new MyPromise((resolve) => resolve(value))  }  /**   * MyPromise.reject(reason)   * 返回一个 rejected 的 Promise（不解析 thenable）   */  static reject(reason) {    return new MyPromise((_, reject) => reject(reason))  }}