Async/await for ES6

Async/Await The role of

Async It means ： asynchronous
Await It means ： wait for

stay ES6 in , Use async/await It means ： Wait for the asynchrony to complete before continuing code execution .JS in , The execution code runs on the main thread first , When encountering asynchrony , Asynchronously goes to the message queue , Wait until the main thread finishes running before executing the asynchronous task of the message queue .

Async/Await Use

Async/Await The use of is not complicated .

async function a(){
    
	await ···// Asynchronous code 
}

This is it. async/await Use . Although it's very convenient to use , But there are many things to pay attention to .

Async/Await It's used together , Use await You have to add async Embellish , And used async Can not be applied in the function of await

async The return value of the function will be encapsulated as Promise object

async function a(){
    
	return "a"
}
console.log(a());

Output at console ：

It is Promise object , We can use then To get the results

async function a(){
    
	return "a"
}
a().then((item)=>{
    
	console.log(item);
});
// Console output  a
// Note that the function uses return Return value ,return The value of is in then Function 
// If you want to make the value between catch Function , should return One promise.reject();
// for example ：
//async function a(){
    
// return Promise.resolve("a1"); Here with return equally , The value will be in then In the callback 
// return Promise.reject("a2"); The value is in catch Calling back 
//}

await Wait for the asynchronous execution to complete

await It should be noted that it is necessary to combine Promise Use it together

//async function a(){
    
// await  Execute asynchronous code 
//}
// First contact Async/Await It's easy to get into the misunderstanding when you're in the middle of the world ：await With no Promise Use it together 
// The wrong way to write 
//(async function a(){
    
// await setTimeout(()=>{
    
// console.log(1);
// },3000);
// console.log(2);
//})();
// First contact await The output will be considered as 1 Until then 2, It's actually wrong . Console or output 2 Then the output 1, The reason is that await With no Promise Use it together 

(async function a(){
    
  await new Promise((resolve,reject)=>{
    
  	setTimeout(()=>{
    
    	resolve(1);
  	},1000);
  }).then((item)=>{
    
     console.log(item);
   });
   console.log(2);
})()
// Console output 1 Then the output 2