异步等待异常行为

Question

I am trying to understand JavaScript Async/Await feature.

So I wrote a short code to understand it but it is giving me unusual behaviour/result.

 var a = 10; function load_data(data) { setTimeout(() => { a = data }, 2000); } function print() { console.log(a); } async function init() { await load_data(40); print(); } init(); 

I expect the value logged to be 40, but its logging 10 with async and await.

Answer 1

Async await depends on Promises , but you aren't making a promise anywhere in your code. As a result, awaiting load_data doesn't wait.

Try using a promise and resolving once the timeout fires:

function load_data(data){
    return new Promise(resolve => setTimeout(() => {
        a=data
        resolve()
    }, 2000))
}

Also, we'll assume this is just for learning async/await…otherwise you should consider all the usual advice agains using global variables this way. It's a recipe for a mess once your code gets any larger

 var a = 10 function load_data(data){ return new Promise(resolve => setTimeout(() => { a=data resolve() }, 2000)) } function print(){ console.log(a) } async function init(){ await load_data(40); print(); } init(); 

Answer 2

Async/Await is syntax used for handling promises, not just any asynchronous behavior. For it to work with load_data , the function would need to return a promise object that resolves 40.

This should work how you expect:

var a = 10

function load_data(data){
  return new Promise((resolve) => {
    setTimeout(() => {
        a=data;
        resolve(a);
    }, 2000)
  });
}

function print(){
    console.log(a)
}

async function init(){

    await load_data(40);

    print();
}

init();

Before learning about async await, learning about promises is essential since it's just a syntax over promises.

Answer 3

You should return a Promise in load_data() . Actually you're returning nothing, so the code is not waiting for any value to be resolved. You should do something like this:

function load_data(data){
    return new Promise( resolve => {
       setTimeout(() => {
           a=data;
           resolve(true);
       }, 2000)
    }
}

Answer 4

Your should create the promise. Furthermore, it is better not to use a global variable, but to let the promise resolve with the desired value:

 function load_data(data){ return new Promise(resolve => { setTimeout(() => { resolve(data); //<-- pass data to resolve }, 2000) }); } async function init(){ let data = await load_data(40); // <--- get the promised value console.log(data); } init(); 

Answer 5

please set promise for the set your value before execute print function

var a = 10;
function load_data(data) {
  return new Promise(resolve => {
      a=data;
      resolve('resolved');
  });
}

async function init() {

  var result = await load_data(40);
  print();
}

function print(){
    console.log(a)
}

init();

Answer 6

See: manual1 manual2

It's work for me:

var a = 10

function load_data(data){
    return new Promise(resolve=>{
        setTimeout(() => {
            a=data;
            resolve('ok');
           }, 2000)    
});

}

function print(){
    console.log(a)
}

async function init(){

await load_data(40);

print();
}

init();