手写promise函数

好的，关于手写实现Promise函数，其实Promise是一种异步编程的解决方案，它的本质是对回调函数的封装，可以更加优雅和简洁地处理异步操作。以下是一个简单的手写Promise函数示例：

function MyPromise(fn) {
  var self = this;
  self.status = 'pending';
  self.value = null;
  self.reason = null;
  self.onFulfilledCallbacks = [];
  self.onRejectedCallbacks = [];

  function resolve(value) {
    if (self.status === 'pending') {
      self.status = 'fulfilled';
      self.value = value;
      self.onFulfilledCallbacks.forEach(function(callback) {
        callback(self.value);
      });
    }
  }

  function reject(reason) {
    if (self.status === 'pending') {
      self.status = 'rejected';
      self.reason = reason;
      self.onRejectedCallbacks.forEach(function(callback) {
        callback(self.reason);
      });
    }
  }

  try {
    fn(resolve, reject);
  } catch (e) {
    reject(e);
  }
}

MyPromise.prototype.then = function(onFulfilled, onRejected) {
  var self = this;
  return new MyPromise(function(resolve, reject) {
    if (self.status === 'fulfilled') {
      try {
        var x = onFulfilled(self.value);
        resolve(x);
      } catch (e) {
        reject(e);
      }
    } else if (self.status === 'rejected') {
      try {
        var x = onRejected(self.reason);
        resolve(x);
      } catch (e) {
        reject(e);
      }
    } else if (self.status === 'pending') {
      self.onFulfilledCallbacks.push(function(value) {
        try {
          var x = onFulfilled(value);
          resolve(x);
        } catch (e) {
          reject(e);
        }
      });
      self.onRejectedCallbacks.push(function(reason) {
        try {
          var x = onRejected(reason);
          resolve(x);
        } catch (e) {
          reject(e);
        }
      });
    }
  });
}

以上就是一个简单的手写Promise函数的实现，它是基于回调函数的封装实现的异步编程方案。