Catalog

One 、async function

1. Definition

2. demonstration

Two 、await expression

1. Definition

2. Be careful

3. demonstration

3、 ... and 、async await ajax Use

One 、async function

1. Definition

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

2. demonstration

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)=>{   
			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

1. Definition

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 perhaps reject Value .

2. Be careful

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 .

3. demonstration

(1) success

// 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();

(2) Failure ： 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 Use

First create the object , use get The method of requests the following incoming address , Resend request , By judging whether there is data output .

function sendajax(url){
	return new Promise((resolve,reject)=>{				
		  const http =  new XMLHttpRequest();// Create objects 
		  http.open("GET",url);// use get Method request address 
		  http.send();// Send a request 				
		  http.onreadystatechange = function(){					  
			  if(http.readyState==4){							  
				  if(http.status==200){
					  resolve(JSON.parse(http.response));
				  }							  
			  }						  
		  }  										  
	})				
}			
async function demo(){				
	const res = await sendajax("http://127.0.0.1:8848/web2209/ES6/test.json");
	if(res.code==200){
		console.log(" There's data ");
	}else{
		console.log(" No data ");
	}		
}
demo();