Use of promise object in ES6 asynchronous programming

Promise Characteristics of the object :

1. Promise The state of the object is not affected by the outside world ,

         One Promise The object represents an asynchronous operation .

        One Promise Objects have 3 States :

                                 Pending   ---> Have in hand

                                 Fulfilled   ---> Have succeeded

                                 Rejected  ---> Failed

2. The status is changed only once , Once changed, it will not change again , It's going to stay that way (Resolved)

              Pending ------>  Fulfilled       

              Pending ------> Rejected

3. shortcoming : Cannot cancel an executing Promise,  If no callback function is set , Internal errors cannot be thrown to the outside

Promise usage :

  let onePromise =  new Promise((resolve,reject)=>{

        // Execute the code that requires asynchronous operation here

 });

onePromise.then(

        // Successful callback

     (data)=>{

},

// Failed callback

(err)=>{

});

// @ts-ignore
let onePromise = new Promise(function(resolve, reject){
    // Do some asynchronous operations 
    setTimeout(function(){
        console.log(' Execution completed Promise');
        resolve(' The data to be returned can be any data, such as the data returned by the interface ');
    }, 2000);
});

// Handle Promise Execution results 
onePromise.then(value => {
    console.log(value);
},error=>{

});

let promise=new Promise((resolve,reject)=>{
    for (let i = 0; i < 1000; i++) {
        console.log(i,' Asynchronous task execution ...');
        if(i===999){
            resolve({'index':i,data:' Cycle execution is complete '});
        }
    }
    reject();
});

promise.then((data)=>{
    console.log(data);
},(error)=>{

});

// The function returns Promise object 
function  runPromise(){
    // @ts-ignore
    return new Promise((resolve,reject)=>{
        // Asynchronous execution 
        for (let i = 0; i < 10000; i++) {
            console.log(i);
        }
        resolve('Loop Finish');
    });
}
//Promise Callback 
runPromise().then((value => {
    console.log(' Asynchronous execution complete :',value);
}));

// Asynchronous acquisition JSON
let getJsonAsync = (url)=>{
  // @ts-ignore
    let p=new Promise((resolve,reject)=>{
          let c= new XMLHttpRequest();
          c.open('GET',url);
          c.onreadystatechange = handler;
          c.responseType='json';
          c.setRequestHeader('Accept','application/json');
          c.send();
          // Handle request status events 
          var handler = ()=>{
                if(this.readyState !== 4){
                    return;
                }
                // @ts-ignore
                if(this.status === 200){
                    resolve(this.response);
                }else{
                    reject(new Error(this.statusText));
                }
          };
    });

    return p;
};

getJsonAsync('https://img-home.csdnimg.cn/data_json/toolbar/toolbar1105.json').then((json)=>{
    console.log(json);
},(err)=>{
    console.log('HTTP Request error :',err);
});