SOURCE

function MyPromise(f){
    this.status = 'pending'
    this.value = ''
    this.reason = ''
    this.onResolveCB = []
    this.onRejectCB = []
    this.then = (onFulfilled, onRejected) => {
        if (this.status === 'fulfilled') {
            onFulfilled(this.value)
        }

        if (this.status === 'rejected') {
            onRejected(this.reason)
        }
        if (this.status === 'pending') {
            // 如果promise的状态是 pending，需要将 onFulfilled 和 onRejected 函数存放起来，等待状态确定后，再依次将对应的函数执行
            this.onResolveCB.push(() => {
                onFulfilled(this.value)
            });

            // 如果promise的状态是 pending，需要将 onFulfilled 和 onRejected 函数存放起来，等待状态确定后，再依次将对应的函数执行
            this.onRejectCB.push(()=> {
                onRejected(this.reason);
            })
        }
    }

    this.resolve = (val) => {
        if(this.status === 'pending'){
            this.status = 'fulfilled'
            this.value = val
            this.onResolveCB.forEach(fn => fn())
        }
    }
    this.reject = (err) => {
        if(this.status === 'pending'){
            this.status = 'rejected'
            this.reason = err
            this.onRejectCB.forEach(fn => fn())
        }
    }

    try{
        f(this.resolve,this.reject)
    }catch(err){
        console.log(err)
    }
}

const promise = new MyPromise((resolve, reject) => {
    setTimeout(() => {
    resolve('成功');
  },1000);
}).then(
  (data) => {
    console.log('success', data)
  },
  (err) => {
    console.log('faild', err)
  }
)

//无链式调用