Catalog

One 、async function

Concept ：  

give an example ：

Two 、await expression

1.await Must be on async Function

2.await The expression on the right is usually promise object

3.await You can return to the right side promise The value of success

4.await On the right side of the promise If it fails , Will throw an exception , Need to pass through try…catch Capture processing

give an example ：

give an example ： Failed code await Wrong code Need to use try catch Capture

3、 ... and 、async await ajax Based on using

One 、async function

Concept ：  

async It's a modifier ,async The defined function will return a by default Promise object resolve（ Success value ） Value , So right. async Functions can be used directly then operation , The value returned is then Method .

give an example ：

async function demo(){
// 1: When the return value is not promise object   When the function is called, it is the successful value 
// return "succ";
// 2: When it comes back to promise object   So the function (promise object ) Results of and returned promise In the same state 
return  new Promise((resolve,reject)=>{   //Promise  Please refer to the previous work , Focus on Columns .
			let flag = true;
				if(flag){
				resolve("succ");
			}else{
				reject("error");
				}
				})
			}
const MyPromise = demo();
MyPromise.then((resolve)=>{
	console.log(resolve);
},(reject)=>{
	console.log(reject);
})

Two 、await expression

It's also a modifier ,await keyword Only on the async Internal function , await The role of keywords Is to get Promise Content returned in , What you get is Promise Function resolve.

1.await Must be on async Function

2.await The expression on the right is usually promise object

3.await You can return to the right side promise The value of success

4.await On the right side of the promise If it fails , Will throw an exception , Need to pass through try…catch Capture processing

give an example ：

// 1:await It needs to be written in async Inside the function 	
// 2:await  Embellished Promise  The value returned is resolve Value 
// 3: The following code needs to wait  await After the results of the 
async function demo(){
const a = await "a";
const b = await new Promise((resolve,reject)=>{
	setTimeout(()=>{
		console.log(" The timer executed ....");
	resolve("b");
		},3000);
	});
const c =  await "c";
console.log(a,b,c);
			}
			demo();

give an example ： Failed code await Wrong code Need to use try catch Capture

async  function demo(){
	try{
	const a = await new Promise((relsolve,reject)=>{
		reject(" The data doesn't exist ");	
})
	}catch(error){
		console.log(error);
				}
			}
demo();

3、 ... and 、async await ajax Based on using

function mark (url){
return new Promise((resolve,reject)=>{
      const ajax = new XMLHttpRequest();
      ajax.open("GET",url)
      ajax.send();
      ajax.onreadystatechange=function(){
if(ajax.readyState==4){
      if(ajax.status==200){
       resolve(JSON.parse(ajax.response));
                        }
                    }
                }
            })
        }
async function demo(){
const res = await mark("http://127.0.0.1:5500/test.json")
return new Promise((resolve,reject)=>{
    if(res.code==200){
        resolve(" There's data ")
     }else{
        reject(" No data ")
            }
})
        }
demo().then(resolve=>console.log(resolve),reject=>console.log(reject))