SOURCE

class MyPromise{
    constructor(func){
        this.status='pending';
        this.value='';
        this.resolveCallbacks=[];
        this.rejectCallbacks=[];
        const resolve=(value)=>{
            if(this.status==='pending'){
                this.status='fulfilled'
                this.value = value;
                this.resolveCallbacks.forEach(callback=>callback(this.value))
            }
        }
        const reject=(error)=>{
            if(this.status==='pending'){
                this.status='rejected'
                this.value = error;
                this.rejectCallbacks.forEach(callback=>callback(this.value))
            }
        }
        try{
            func(resolve,reject)
        }catch(error){
            reject(error)
        }
    }
    then(onResolve,onReject){
        if(this.status==='fulfilled'){
            onResolve(this.value)
        }else if(this.status==='rejected'){
            onReject(this.value)
        }else{
            this.resolveCallbacks.push(onResolve)
            this.rejectCallbacks.push(onReject)
        }
    }
}
const promise = new MyPromise((resolve, reject) => {
  // 异步操作，比如请求数据
  setTimeout(() => {
    resolve('成功');
    // 或者 reject('失败');
  }, 1000);
});
promise.then(
  (value) => {
    console.log('成功:', value);
  },
  (reason) => {
    console.log('失败:', reason);
  }
);