大前端

const promise = new Promise((resolve, reject) => {
    setTimeout(()=>{
      console.log('promise resolve');
      resolve()
    },1000)
  })

  promise.then(()=>{
    console.log("promise resolve callback");
  })

    promise 有三种状态： pending，resolve，reject，进行中状态是可以修改的，其余的都是无法修改的
    then可以执行完成后的回调函数
    已拒绝的，可以catch进行捕获

 function ajaxAsync(methods="GET",url){
  return new Promise((resolve, reject) => {
    let xml = new XMLHttpRequest();
    xml.open(methods,url);
    xml.onreadystatechange = function (){
      if (this.readyState !== 4){
        return;
      }
      if (this.status === 200){
        resolve(this.response)
      }else {
        reject(new Error(this.statusText))
      }
    };
    xml.send();
  })
}
ajaxAsync('GET','./ajax.json').then(res=>{
  console.log('成功',JSON.parse(res).data)
}).catch(err=>{
  console.log('失败', err);
})

{
  "data": {
    "name": "wangyang",
    "age": 18
  }
}

 // 构造函数
  function Person(name,age){
    this.name = name;
    this.age = age;
    this.sayHi = function (){
      console.log(`hello; my name is ${this.name},now ${this.age}`)
    }
  }

  const p1 = new Person('wang',14);
  const p2 = new Person('yang',18);

  console.log(p1 === p2)

// 缺点： 每次都是要开辟一个内存空间