JS async do stuff after return

Question

I am rewriting some JavaScript code that uses callbacks to async functions. Basically doing:

function doStuff(callbackFn){
    var result = foo();
    callbackFn(result);
}

//changing in

async function(){
    var result = foo();
    return result; 
}

//So I can change
doStuff(function(x){
     doStuffWith(x)
});

//into

doStuff().then(x => doStuffWith(x));

Now I encounter a function where I have

function doStuff(callbackFn){
    var result = foo();
    var bar = callbackFn(result);
    baz(bar); 
}

And I have no idea how to start on this one... Any suggestions?

Not all callbacks are there to do asynchronous tasks. Not all use of callbacks can be refactored into promises. This is one such case. — deceze, Jul 23 '19 at 06:33
@deceze is correct - this doesn't look like async code. It's just a sequence of actions that are supposed to be synchronous - call `foo()`, then pass the result to `callbackFn` and finally call `baz` with the result of that. Three steps that can only be run in sequence. — VLAZ, Jul 23 '19 at 06:36
The examples you gave are not exactly asynchronous callbacks. Could you show us your real code, please? What does `baz` do, and what is the `bar` value that the callback returns? — Bergi, Jul 23 '19 at 07:40

score 3 · Accepted Answer · answered Jul 23 '19 at 06:49

Callback functions are used (mainly) for two different purposes:

Coordinate asynchronous code.
Pass executable code snippets around as opposed to only values.

Your first case looks like #1, doStuff does something, possibly asynchronously, and to enable other code to continue executing if and when doStuff is done, it takes a callback which it will call at the appropriate time.

The other case looks more like #2, where doStuff takes a piece of executable code to get a value that it depends on for its own execution. The classical example of that is Array.prototype.sort:

[5, 1, 7, 3].sort((a, b) => b - a)

This callback fills in a part of the algorithm of sort. It has nothing to do with asynchronous execution and cannot be converted to async/await/.then, and neither does it need to.

score -2 · Answer 2 · answered Jul 23 '19 at 06:41

-2

First, your callbackFn must ALSO be converted to async. I'm assuming you know how to wrap synchronous functions inside a Promise for conversion to async.

Having that done, you would just do this:

async function doStuff(callbackFn) {
  const result = foo();
  const bar = await callbackFn(result); //callbackFn must be async
  await baz(bar); //baz function must be async

  return someResponse;
}

answered Jul 23 '19 at 06:41

Keith Harris

1,118
3
13
25

Why convert it to async when all you do is `await` it? The only difference between this and OP's code is adding superfluous things to add more superfluous syntax on top of it. If `callbackFn` is synchronous *that's fine* - why would it need to be async? – VLAZ Jul 23 '19 at 06:43
The question here was "how to convert a function to async" not whether this function warrants async conversion. The "await" keyword tells the program to hand over processing of the function to a thread so that it won't keep the user waiting for fulfillment. "Await" can only be used on async functions, therefore, you must convert to async. – Keith Harris Jul 23 '19 at 06:52
2

But that's *useless*. If the conversion is *really* needed (and I'm not convinced it is), then the entirety of the work you need to do is make `doStyff` asynchronous. That's it. Having *every* single function in your code base async for the sake of it being async is terrible design for more reasons I can count. Yet it all comes back to - it's useless. It's misguided at best or a cargo cult otherwise. – VLAZ Jul 23 '19 at 06:54
The concept of async/await existed in C# since the beginning. Only recently did Javascript implement, and I'm so glad they did! It makes coding awesome, like REALLY AWESOME! But with that comes necessary changes. That's why version 8 of Node.js includes a utility called util.promisify() that enables developers to easily convert legacy SYNCHRONOUS code into ones that return a promise and be used with async and await. Not all functions need it, only the blocking ones do. – Keith Harris Jul 23 '19 at 07:09

JS async do stuff after return

2 Answers2