手写一个Promise，彻底掌握异步原理

手写 Promise 实现

Promise 是 JavaScript 中处理异步操作的核心机制，手写一个简单的 Promise 有助于深入理解其原理。以下是一个基础的 Promise 实现：

javascript 复制代码

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 : value => value;
    onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err; };

    const promise2 = new MyPromise((resolve, reject) => {
      if (this.state === 'fulfilled') {
        setTimeout(() => {
          try {
            const x = onFulfilled(this.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        }, 0);
      }

      if (this.state === 'rejected') {
        setTimeout(() => {
          try {
            const x = onRejected(this.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        }, 0);
      }

      if (this.state === 'pending') {
        this.onFulfilledCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onFulfilled(this.value);
              resolvePromise(promise2, x, resolve, reject);
            } catch (e) {
              reject(e);
            }
          }, 0);
        });

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

    return promise2;
  }
}

function resolvePromise(promise2, x, resolve, reject) {
  if (x === promise2) {
    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 (e) {
      if (called) return;
      called = true;
      reject(e);
    }
  } else {
    resolve(x);
  }
}

Promise 核心原理

状态管理：Promise 有三种状态（pending/fulfilled/rejected），状态一旦改变就不可逆。

异步处理：通过回调函数队列（onFulfilledCallbacks/onRejectedCallbacks）实现异步调用。

链式调用：then 方法返回新 Promise 实现链式调用，通过 resolvePromise 处理返回值。

错误处理：通过 try-catch 捕获执行器（executor）和回调函数中的错误。

使用方法示例

javascript 复制代码

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

p.then(res => {
  console.log(res);
  return 'chain';
}).then(res => {
  console.log(res);
});

关键实现点

微任务模拟：使用 setTimeout 模拟微任务队列（实际 Promise 使用微任务）。

值穿透：then 方法的参数如果不是函数，需要实现值穿透。

循环引用检测：防止 thenable 对象返回自身导致无限循环。

这个实现涵盖了 Promise 的核心功能，包括状态管理、异步处理、链式调用和错误处理机制。实际使用中建议直接使用原生 Promise，这个实现主要用于学习原理。