手撕promise

promise

// 手写promise
  function myPromise(fn) {
   let self = this;
   self.status = 'pending';
   self.value = undefined;
   self.reason = undefined;

   function resolve(value) {
    if (self.status === 'pending') {
     self.value = value;
     self.status = 'resolved';
    }
   }

   function reject(reason) {
    if (self.status === 'pending') {
     self.reason = reason;
     self.status = 'rejected';
    }
   }
   try {
    fn(resolve, reject)
   } catch (e) {
    reject(e);
   }
   
   myPromise.prototype.then = (resolve, reject)=> {
    switch (self.status) {
     case 'resolved':
      return resolve?resolve(self.value):null;
      break;
     case 'rejected':
      return reject?reject(self.reason):null;
      break;
     default:
      break;
    }
   }
   myPromise.prototype.catch = reject=> {
    return myPromise.prototype.then(null, reject);
   };
  }
  let test = new myPromise((resolve,reject)=>{
   // resolve(8888);
   reject(4444)
  })
  test.then(res => {
   console.log('res',res)
  },err=>{
   console.log('err',err)
  })
  test.catch(err=>{
   console.log('err',err)
  })