How to avoid using promise catch to control program?

Question

A piece of codes:

// file defined here.
return getFile.then(function _gotFile(file) { // getFile is a promise, resolved OR rejected
  // A
  // do something on file
  return file;
})
  .catch(function() {
    // B
    // do other things on file, 
    return file;
  })

In the above piece of codes, I am using promise catch to control program flow: if getFile promise resolved, do A, otherwise, do B.

Is it good practice to write codes like this?

How to rewrite the above piece of codes to avoid this? Thanks

where are you returning the file in `then` and `catch` functions — QaMar ALi, Mar 07 '18 at 18:53

Bergi · Accepted Answer · 2018-03-07T19:38:42.423

If getFile promise resolved, do A, otherwise, do B.

Actually, no. It also does B when A failed. If you really want "otherwise", then you need to use .then(…, …) instead of .then(…).catch(…). See also Correct Try...Catch Syntax Using Async/Await.

Is it good practice to write codes like this?

Yes, and no. It depends.

It is totally normal to make control flow decisions with exceptions. If something fails, it is definitely a good practice to handle the error.

On the other hand, you can overuse exceptions. Sometimes returning a boolean value or result type enum or something similar through the fulfillment case, and distinguishing it using if/else, can be much simpler.

Also, when you are using exceptions/rejections, you need to beware and use them right. JS only has catch-all, no conditional exceptions, and doing it manually is cumbersome. Still, it is usually a good practice to catch only specific kinds of errors that you actually can handle, and to rethrow others. Have a look here for an exacmple.

How to avoid using promise catch to control program?

1 Answers1