用es5复写 Promise

好的，以下是使用 ES5 复写 Promise 的代码示例：

function Promise(fn) {
  var state = 'pending';
  var value;
  var deferred = null;

  function resolve(newValue) {
    if (newValue && typeof newValue.then === 'function') {
      newValue.then(resolve);
      return;
    }
    value = newValue;
    state = 'resolved';

    if (deferred) {
      handle(deferred);
    }
  }

  function reject(reason) {
    state = 'rejected';
    value = reason;

    if (deferred) {
      handle(deferred);
    }
  }

  function handle(handler) {
    if (state === 'pending') {
      deferred = handler;
      return;
    }

    var handlerCallback;
    if (state === 'resolved') {
      handlerCallback = handler.onResolved;
    } else {
      handlerCallback = handler.onRejected;
    }

    if (!handlerCallback) {
      if (state === 'resolved') {
        handler.resolve(value);
      } else {
        handler.reject(value);
      }
      return;
    }

    var ret;
    try {
      ret = handlerCallback(value);
    } catch (e) {
      handler.reject(e);
      return;
    }

    handler.resolve(ret);
  }

  this.then = function(onResolved, onRejected) {
    return new Promise(function(resolve, reject) {
      handle({
        onResolved: onResolved,
        onRejected: onRejected,
        resolve: resolve,
        reject: reject
      });
    });
  };

  fn(resolve, reject);
}

上述代码是一个最基本的 Promise 实现，支持 `then` 方法，但没有支持 `catch` 和 `finally` 方法，也没有提供 `Promise.resolve` 和 `Promise.reject` 静态方法。同时，这个实现中还没有处理异步行为，所以是不完整的。