js promise 实现

Promise 的基本实现原理

Promise 是 JavaScript 中处理异步操作的一种机制，其核心在于状态管理、回调队列和链式调用。以下是 Promise 的简化实现：

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

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

    return promise2;
  }
}

function 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;
            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);
  }
}

Promise 的核心特性

状态管理：Promise 有三种状态 - pending、fulfilled 和 rejected。状态一旦改变就不可逆转。

回调队列：在 pending 状态时，将成功和失败的回调分别存入对应的队列中，待状态改变时依次执行。

异步执行：通过 setTimeout 确保 then 方法的回调总是异步执行，符合 Promise/A+ 规范。

链式调用：通过返回新的 Promise 实例实现链式调用，每个 then 方法都可以处理前一个 Promise 的结果。

Promise 的静态方法实现

MyPromise.resolve = function(value) {
  if (value instanceof MyPromise) {
    return value;
  }
  return new MyPromise(resolve => resolve(value));
};

MyPromise.reject = function(reason) {
  return new MyPromise((_, reject) => reject(reason));
};

MyPromise.all = function(promises) {
  return new MyPromise((resolve, reject) => {
    const results = [];
    let count = 0;
    promises.forEach((promise, index) => {
      MyPromise.resolve(promise).then(
        value => {
          results[index] = value;
          count++;
          if (count === promises.length) resolve(results);
        },
        reject
      );
    });
  });
};

MyPromise.race = function(promises) {
  return new MyPromise((resolve, reject) => {
    promises.forEach(promise => {
      MyPromise.resolve(promise).then(resolve, reject);
    });
  });
};

错误处理机制

Promise 通过 catch 方法捕获错误，实际上是 then(null, onRejected) 的语法糖：

MyPromise.prototype.catch = function(onRejected) {
  return this.then(null, onRejected);
};

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

Promise 的应用示例

const p = new MyPromise((resolve, reject) => {
  setTimeout(() => resolve('success'), 1000);
});

p.then(value => {
  console.log(value); // 'success'
  return 'next value';
}).then(value => {
  console.log(value); // 'next value'
  throw new Error('error occurred');
}).catch(error => {
  console.error(error.message); // 'error occurred'
});