Incorrect explanation of catching errors with asynchronous JavaScript in Express Tutorial

[granteckels]

MDN URL

https://developer.mozilla.org/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/mongoose

What specific section or headline is this issue about?

Database APIs are asynchronous

What information was incorrect, unhelpful, or incomplete?

In this section, in the part where it explains how to catch errors from async functions, I believe it's wrong. It puts the try...catch block on the async function call instead of inside the function or by running catch() on the async function call. In the current code, the error will not be caught.

This is the code in question:

async function myFunction {
  // ...
  await someObject.methodThatReturnsPromise();
  // ...
  await aFunctionThatReturnsPromise();
  // ...
}

try {
  // ...
  myFunction();
  // ...
} catch (e) {
 // error handling code
}

What did you expect to see?

Something like this with a proper explanation:

async function myFunction {
  // ...
  try {
    await someObject.methodThatReturnsPromise();
  } catch(e) {
    // error handling code
  }
  // ...
  try {
    await aFunctionThatReturnsPromise();
  } catch(e) {
    // error handling code
  }
  // ...
}

myFunction();

or

async function myFunction {
  // ...
  await someObject.methodThatReturnsPromise();
  // ...
  await aFunctionThatReturnsPromise();
  // ...
}

myFunction().catch((e) => {
  // error handle error
});

Do you have any supporting links, references, or citations?

No response

Do you have anything more you want to share?

No response

MDN metadata

No response

[hamishwillee]

Have you tried it?

My understanding is that as long as the potentially throwing function(s) are awaited, which they are, then this construction should work. So would your first example. Not sure about the second.

@Josh-Cena Can you advise whether the current code is OK? I'm pretty sure I tested this, but perhaps not.

[Josh-Cena]

async function myFunction {
  // ...
  await someObject.methodThatReturnsPromise();
  // ...
  await aFunctionThatReturnsPromise();
  // ...
}

try {
  // ...
  myFunction();
  // ...
} catch (e) {
 // error handling code
}

Indeed, this would never catch anything from myFunction, because it never synchronously throws an error. Both of your suggestions look good to me.

[hamishwillee]

Thanks both. I had figured that since someObject.methodThatReturnsPromise() awaits in the myFunction that effectively made myFunction synchronous w.r.t. the try. I just don't get this stuff.

Anyhow, I guess we should fix https://github.com/mdn/express-locallibrary-tutorial first and then update the docs. @granteckels Did you want to do the updates?

[granteckels]

The actual repo doesn't contain this code. It's just used as an example when explaining async JavaScript in the docs. So only the docs need to be updated. And, yes, I can do this.

[hamishwillee]

Thank you very much!