Question

20

Why doesn't the code after await run right away? Isn't it supposed to be non-blocking?

rated 0 times [ 23] [ 3] / answers: 1 / hits: 41482 / 7 Years ago, sun, april 9, 2017, 12:00:00

I have a hard time understanding how async and await works behind the scenes. I know we have promises which make our non blocking code by using the "then" function we can place all the work we need to do after the promise is resolved. and the work we want to do parallel to promise we just write it outside our then function. Hence the code becomes non blocking. However i don't understand how the async await makes non-blocking code.

async function myAsyncFunction() {

  try {

    let data = await myAPICall('https://jsonplaceholder.typicode.com/posts/1');

    // It will not run this line until it resolves await.

    let result = 2 + 2;

    return data;

  }catch (ex){

    return ex;

  }

}

See the above code. I cannot move forward until the API call is resolved. If it makes my code blocking code, how is it any better then promises? Or is there something I missed about async and await? Where do i put my code that is not dependent to the await call? so it can keep on working without await holding the execution?

I am adding a Promise code that i would like to replicate in an async await example.

function myPromiseAPI() {

  myAPICall('https://jsonplaceholder.typicode.com/posts/1')

    .then(function (data) {

        // data

    });

   // runs parallel

  let result = 2 + 2;

}

Answers

Only authorized users can answer the question. Please sign in first, or register a free account.

joseluispauld

Add To Favorites

Follow

Total Points: 13

Total Questions: 132

Total Answers: 98

Location: Venezuela

Member since Sat, Apr 24, 2021

3 Years ago

answered 7 Years ago devinjadong · Accepted Answer

Just as its name implies, the await keyword will cause the function to "wait" until the corresponding promise resolves before executing the next line. The whole point of await is to provide a way to wait for an asynchronous operation to complete before continuing.

The difference between this and blocking code is that the world outside the function can continue executing while the function is waiting for the asynchronous operations to finish.

async and await are just syntactic sugar on top of promises. They allow you to write code that looks a lot like ordinary synchronous code even though it uses promises under the covers. If we translated your example there to something that explicitly worked with the promises, it would look something like:

function myAsyncFunction() {

  return myAPICall('https://jsonplaceholder.typicode.com/posts/1')

    .then(function (data) {

       let result = 2 + 2;

       return data;

    })

    .catch(function (ex) {

        return ex;

    });

}

As we can see here, the let result = 2 + 2; line is inside a .then() handler, which means it's not going to execute until myAPICall() has resolved. It's the same when you use await. await just abstracts away the .then() for you.

One thing to bear in mind (and I think the point you're looking for) is that you don't have to use await right away. If you wrote your function like this, then you could execute your let result = 2 + 2; line right away:

const timeout = 

    seconds => new Promise(res => setTimeout(res, seconds * 1000));



function myAPICall() {

  // simulate 1 second wait time

  return timeout(1).then(() => 'success');

}



async function myAsyncFunction() {

  try {

    console.log('starting');



    // just starting the API call and storing the promise for now. not waiting yet

    let dataP = myAPICall('https://jsonplaceholder.typicode.com/posts/1');



    let result = 2 + 2;



    // Executes right away

    console.log('result', result);



    // wait now

    let data = await dataP;



    // Executes after one second

    console.log('data', data);



    return data;

  } catch (ex) {

    return ex;

  }

}



myAsyncFunction();

After some clarification, I can see that what you really wanted to know about is how to avoid having to wait for two async operations one by one and instead have them execute in parallel. Indeed, if you use one await after the other, the second won't start executing until the first has finished:

const timeout = 

    seconds => new Promise(res => setTimeout(res, seconds * 1000));



function myAPICall() {

  // simulate 1 second wait time

  return timeout(1).then(() => 'success');

}



async function myAsyncFunction() {

  try {

    console.log('starting');



    let data1 = await myAPICall('https://jsonplaceholder.typicode.com/posts/1');

    // logs after one second

    console.log('data1', data1);



    let data2 = await myAPICall('https://jsonplaceholder.typicode.com/posts/2');

    // logs after one more second

    console.log('data2', data2);

  } catch (ex) {

    return ex;

  }

}



myAsyncFunction();

To avoid this, what you can do is start both async operations by executing them without awaiting them, assigning their promises to some variables. Then you can await both promises:

const timeout = 

    seconds => new Promise(res => setTimeout(res, seconds * 1000));



function myAPICall() {

  // simulate 1 second wait time

  return timeout(1).then(() => 'success');

}



async function myAsyncFunction() {

  try {

    console.log('starting');

    // both lines execute right away

    let dataP1 = myAPICall('https://jsonplaceholder.typicode.com/posts/1');

    let dataP2 = myAPICall('https://jsonplaceholder.typicode.com/posts/2');



    let data1 = await dataP1;

    let data2 = await dataP2;



    // logs after one second

    console.log('data1', data1);

    console.log('data2', data2);

  } catch (ex) {

    return ex;

  }

}



myAsyncFunction();

One alternative way to do this is to use Promise.all() with some array decomposition:

const timeout = 

    seconds => new Promise(res => setTimeout(res, seconds * 1000));



function myAPICall() {

  // simulate 1 second wait time

  return timeout(1).then(() => 'success');

}



async function myAsyncFunction() {

  try {

    console.log('starting');



    // both myAPICall invocations execute right away

    const [data1, data2] = await Promise.all([

        myAPICall('https://jsonplaceholder.typicode.com/posts/1'), 

        myAPICall('https://jsonplaceholder.typicode.com/posts/2'),

    ]);



    // logs after one second

    console.log('data1', data1);

    console.log('data2', data2);

  } catch (ex) {

    return ex;

  }

}



myAsyncFunction();