BFE.devAI解答 - JavaScript 代码题
67. 请自行实现Promise

以下是一个可能的解答：

class MyPromise {  constructor(executor) {    this.state = 'pending';    this.value = undefined;    this.reason = undefined;    this.onResolvedCallbacks = [];    this.onRejectedCallbacks = [];    const resolve = value => {      if (this.state === 'pending') {        this.state = 'fulfilled';        this.value = value;        this.onResolvedCallbacks.forEach(cb => cb(this.value));      }    };    const reject = reason => {      if (this.state === 'pending') {        this.state = 'rejected';        this.reason = reason;        this.onRejectedCallbacks.forEach(cb => cb(this.reason));      }    };    try {      executor(resolve, reject);    } catch (error) {      reject(error);    }  }  then(onFulfilled, onRejected) {    const promise2 = new MyPromise((resolve, reject) => {      if (this.state === 'fulfilled') {        setTimeout(() => {          try {            const x = onFulfilled(this.value);            this.resolvePromise(promise2, x, resolve, reject);          } catch (error) {            reject(error);          }        })      } else if (this.state === 'rejected') {        setTimeout(() => {          try {            const x = onRejected(this.reason);            this.resolvePromise(promise2, x, resolve, reject);          } catch (error) {            reject(error);          }        })      } else if (this.state === 'pending') {        this.onResolvedCallbacks.push(() => {          setTimeout(() => {            try {              const x = onFulfilled(this.value);              this.resolvePromise(promise2, x, resolve, reject);            } catch (error) {              reject(error);            }          })        })        this.onRejectedCallbacks.push(() => {          setTimeout(() => {            try {              const x = onRejected(this.reason);              this.resolvePromise(promise2, x, resolve, reject);            } catch (error) {              reject(error);            }          })        })      }    })    return promise2;  }  catch(onRejected) {    return this.then(null, onRejected);  }  resolvePromise(promise2, x, resolve, reject) {    if (x === promise2) {      return reject(new TypeError('Chaining cycle detected'));    }    let called = false;    if (x instanceof MyPromise) {      x.then(        value => {          if (called) return;          called = true;          this.resolvePromise(promise2, value, resolve, reject);        },        reason => {          if (called) return;          called = true;          reject(reason)        }      )    } else if (x !== null && (typeof x === 'object' || typeof x === 'function')) {      try {        const then = x.then;        if (typeof then === 'function') {          then.call(x,            value => {              if (called) return;              called = true;              this.resolvePromise(promise2, value, resolve, reject);            },            reason => {              if (called) return;              called = true;              reject(reason);            }          )        } else {          resolve(x);        }      } catch (error) {        if (called) return;        called = true;        reject(error);      }    } else {      resolve(x);    }  }  static resolve(value) {    if (value instanceof MyPromise) {      return value;    }    return new MyPromise(resolve => resolve(value));  }  static reject(reason) {    return new MyPromise((_, reject) => reject(reason));  }}

这个实现使用类构造函数创建promise，并存储状态、值和回调函数列表。它实现了两个核心器方法 then() 和 catch() 以支持链式调用。

函数 resolvePromise() 处理可能的快速解决（fulfillment）结果，并将其解析为与当前promises级联的结果。

最后，静态方法 resolve() 、reject() 从构造函数中创建新的promises对外提供了帮助函数。