CODEWITHSUNDEEP
javascriptasync-await
Théo Lavaux
Théo Lavaux

Reputation: 1454

Async/await syntax: Is there a way to define a block of code to execute after a function without blocking the execution?

I've been questionning myself recently on how to reproduce a behaviour of then/catch with an async/await syntax.

With then/catch, I can define a callback which is only executed when the Promise resolves and then continue the execution like so .

function test() {
    getUsersFromDB().then(users => console.log(users));
    console.log('The rest of the code here continues to execute');
    [...]
    // Promise resolves and logs the users value
}

For me, with async/await you can have 2 possible behaviours.

1. Wait for the function and block the rest of the execution

async function test() {
   const users = await getUsersFromDB();
    // Code in here is not executed until promises returns
    console.log(users);
}

2. Don't wait for the return value but don't expect you promise to be fulfilled when the rest of the code executes

function test() {
    const users = getUsersFromDB();
    // May log undefined
    console.log(users);
}

Can I reproduce the first use case using async/await ?

Upvotes: 1

Views: 1783

Answers (2)

CoderFF
CoderFF

Reputation: 298

So what you need is basically split the code. One piece should be executed in async/await syntax, and another one should be as usual.

First what I want to say, if you do as follows

async function test() {
   console.log('The rest of the code here continues to execute');   
   const users = await getUsersFromDB();
   // Code in here is not executed until promises returns
   console.log(users);
}

that will do the trick. This may appear a little strange because we just moved the line a bit up, this is not what we wanted to do, but...

The await keyword stops execution of an async function, but we have some code that should continue working at the time when await freezes the function, that means we can't place the code after await, only before.

As far as I understand, the code tahat is indicated as "The rest of the code here continues to execute" can be asynchronous too, so the resulting example will be as follows:

async function test() {
   console.log('Some synchronous code');   

   setImmediate(() => { 
       console.log('Some asynchronous code'); 
   });

   const users = await getUsersFromDB();
   // Code in here is not executed until promises returns
   console.log(users);
}

Upvotes: 0

Bergi
Bergi

Reputation: 664548

Using then is the simplest solution, but you can use an AIIFE:

function test() {
    (async () => {
         const users = await getUsersFromDB();
         console.log(users);
    })().catch(console.error);
    console.log('The rest of the code here continues to execute');
    [...]
    // Promise resolves and logs the users value
}

An alternative could only be async do expressions.

Upvotes: 1

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