写一个Promise方法

好的，我可以帮您写一个Promise方法。以下是示例代码：

function myPromise() {
  return new Promise(function(resolve, reject) {
    // 这里可以放入异步操作
    setTimeout(function() {
      // 如果操作成功
      resolve('操作成功');
      // 如果操作失败
      // reject('操作失败');
    }, 2000);
  });
}

您可以使用myPromise()这个方法来执行异步操作，并且使用then()方法获取操作成功后返回的结果，或者使用catch()方法捕获操作失败时的错误。例如：

myPromise()
  .then(function(result) {
    console.log(result);
  })
  .catch(function(error) {
    console.log(error);
  });

注：以上代码仅为示例，实际使用时需要根据具体场景进行改写。

相关问题

手写一个promise

Promise 是一种异步编程的解决方案，它可以避免回调地狱，让异步操作更加简洁和可读。下面是手写一个 Promise 的基本实现：

class MyPromise {
  constructor(executor) {
    this.status = 'pending';
    this.value = undefined;
    this.reason = undefined;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];

    const resolve = (value) => {
      if (this.status === 'pending') {
        this.status = 'fulfilled';
        this.value = value;
        this.onResolvedCallbacks.forEach(fn => fn());
      }
    };

    const reject = (reason) => {
      if (this.status === 'pending') {
        this.status = 'rejected';
        this.reason = reason;
        this.onRejectedCallbacks.forEach(fn => fn());
      }
    };

    try {
      executor(resolve, reject);
    } catch (error) {
      reject(error);
    }
  }

  then(onFulfilled, onRejected) {
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
    onRejected = typeof onRejected === 'function' ? onRejected : reason => { throw reason };

    const promise2 = new MyPromise((resolve, reject) => {
      if (this.status === 'fulfilled') {
        setTimeout(() => {
          try {
            const x = onFulfilled(this.value);
            this.resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      } else if (this.status === 'rejected') {
        setTimeout(() => {
          try {
            const x = onRejected(this.reason);
            this.resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      } else {
        this.onResolvedCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onFulfilled(this.value);
              this.resolvePromise(promise2, x, resolve, reject);
            } catch (error) {
              reject(error);
            }
          }, 0);
        });

        this.onRejectedCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onRejected(this.reason);
              this.resolvePromise(promise2, x, resolve, reject);
            } catch (error) {
              reject(error);
            }
          }, 0);
        });
      }
    });

    return promise2;
  }

  resolvePromise(promise2, x, resolve, reject) {
    if (promise2 === x) {
      return reject(new TypeError('Chaining cycle detected for promise'));
    }

    let called = false;

    if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
      try {
        const then = x.then;

        if (typeof then === 'function') {
          then.call(x, y => {
            if (called) return;
            called = true;
            this.resolvePromise(promise2, y, resolve, reject);
          }, r => {
            if (called) return;
            called = true;
            reject(r);
          });
        } else {
          resolve(x);
        }
      } catch (error) {
        if (called) return;
        called = true;
        reject(error);
      }
    } else {
      resolve(x);
    }
  }

  catch(onRejected) {
    return this.then(null, onRejected);
  }

  finally(onFinally) {
    return this.then(
      value => MyPromise.resolve(onFinally()).then(() => value),
      reason => MyPromise.resolve(onFinally()).then(() => { throw reason })
    );
  }

  static resolve(value) {
    return new MyPromise(resolve => resolve(value));
  }

  static reject(reason) {
    return new MyPromise((resolve, reject) => reject(reason));
  }

  static all(promises) {
    return new MyPromise((resolve, reject) => {
      const results = [];
      let count = 0;

      const processResult = (index, result) => {
        results[index] = result;
        count++;

        if (count === promises.length) {
          resolve(results);
        }
      };

      for (let i = 0; i < promises.length; i++) {
        promises[i].then(result => processResult(i, result), reject);
      }
    });
  }

  static race(promises) {
    return new MyPromise((resolve, reject) => {
      for (let i = 0; i < promises.length; i++) {
        promises[i].then(resolve, reject);
      }
    });
  }
}

这个 Promise 实现包含了基本的状态管理、链式调用、错误处理、异步执行、静态方法等功能。当然，这只是一个简单的实现，还有很多细节和边界情况需要考虑。

手写一个promise实现

好的，以下是一个简单的手写Promise实现：

class MyPromise {
  constructor(executor) {
    this.state = 'pending';
    this.value = null;
    this.reason = null;
    this.onFulfilledCallbacks = [];
    this.onRejectedCallbacks = [];

    const resolve = (value) => {
      if (this.state === 'pending') {
        this.state = 'fulfilled';
        this.value = value;
        this.onFulfilledCallbacks.forEach((callback) => callback(this.value));
      }
    };

    const reject = (reason) => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
        this.onRejectedCallbacks.forEach((callback) => callback(this.reason));
      }
    };

    try {
      executor(resolve, reject);
    } catch (error) {
      reject(error);
    }
  }

  then(onFulfilled, onRejected) {
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : (value) => value;
    onRejected = typeof onRejected === 'function' ? onRejected : (reason) => { throw reason; };

    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);
          }
        }, 0);
      } else if (this.state === 'rejected') {
        setTimeout(() => {
          try {
            const x = onRejected(this.reason);
            this.resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      } else {
        this.onFulfilledCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onFulfilled(this.value);
              this.resolvePromise(promise2, x, resolve, reject);
            } catch (error) {
              reject(error);
            }
          }, 0);
        });

        this.onRejectedCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onRejected(this.reason);
              this.resolvePromise(promise2, x, resolve, reject);
            } catch (error) {
              reject(error);
            }
          }, 0);
        });
      }
    });

    return promise2;
  }

  catch(onRejected) {
    return this.then(null, onRejected);
  }

  resolvePromise(promise2, x, resolve, reject) {
    if (promise2 === x) {
      reject(new TypeError('Chaining cycle detected'));
    }

    let called = false;

    if (x instanceof MyPromise) {
      x.then((value) => {
        this.resolvePromise(promise2, value, resolve, reject);
      }, (reason) => {
        if (!called) {
          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) {
              called = true;
              this.resolvePromise(promise2, value, resolve, reject);
            }
          }, (reason) => {
            if (!called) {
              called = true;
              reject(reason);
            }
          });
        } else {
          resolve(x);
        }
      } catch (error) {
        if (!called) {
          called = true;
          reject(error);
        }
      }
    } else {
      resolve(x);
    }
  }

  static resolve(value) {
    return new MyPromise((resolve) => {
      resolve(value);
    });
  }

  static reject(reason) {
    return new MyPromise((resolve, reject) => {
      reject(reason);
    });
  }

  static all(promises) {
    const results = [];

    return new MyPromise((resolve, reject) => {
      for (let i = 0; i < promises.length; i++) {
        const promise = promises[i];

        if (!(promise instanceof MyPromise)) {
          promise = MyPromise.resolve(promise);
        }

        promise.then((value) => {
          results[i] = value;

          if (results.length === promises.length) {
            resolve(results);
          }
        }, (reason) => {
          reject(reason);
        });
      }
    });
  }

  static race(promises) {
    return new MyPromise((resolve, reject) => {
      for (let i = 0; i < promises.length; i++) {
        const promise = promises[i];

        if (!(promise instanceof MyPromise)) {
          promise = MyPromise.resolve(promise);
        }

        promise.then((value) => {
          resolve(value);
        }, (reason) => {
          reject(reason);
        });
      }
    });
  }
}

这个实现包含了Promise的基本功能，包括resolve、reject、then、catch、resolvePromise等方法，并且还实现了Promise.all和Promise.race方法。但是这个实现并不完整，还有很多细节需要考虑和完善。