Promise的源码实现中有哪些关键步骤

根据Promise的规范，Promise的源码实现中需要包含以下关键步骤：

1. Promise构造函数：Promise构造函数接受一个executor函数作为参数，executor函数有两个参数resolve和reject，分别表示Promise的两种状态，resolve用于将Promise状态从pending变为fulfilled，reject用于将Promise状态从pending变为rejected。

2. Promise状态：Promise有三种状态，分别是pending、fulfilled和rejected。Promise的状态只能从pending变为fulfilled或rejected，且状态一旦改变就不能再次改变。

3. then方法：Promise实例的then方法用于注册回调函数，当Promise状态发生改变时，then方法会被调用。then方法接受两个参数，onFulfilled和onRejected，分别表示Promise状态变为fulfilled和rejected时的回调函数。

4. then方法返回值：then方法返回一个新的Promise实例，该实例的状态由onFulfilled和onRejected的返回值决定。

5. Promise链式调用：Promise的then方法可以链式调用，即一个then方法的返回值可以作为下一个then方法的参数。

6. 异常处理：Promise的异常处理可以通过catch方法实现，catch方法用于捕获Promise链中的异常。

以下是一个简单的Promise实现示例：

class Promise {
  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 Promise((resolve, reject) => {
      if (this.status === 'fulfilled') {
        setTimeout(() => {
          try {
            const x = onFulfilled(this.value);
            this.resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      }

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

      if (this.status === 'pending') {
        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;
  }

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

  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);
    }
  }
}