How to use `throw` properly in try...catch block when other unexpected errors might occur?

Question

The title may not be the best.

Code sample:

 a = 1; try { if (a == 1) { throw ('My fake error.'); } } catch (err) { console.log(err.message); }

Assume I have more complex code in the try block. If a statement in that try block throws an error I can get the text of the error by looking at err.message . But, if I throw an error myself I can only see the text if I look at err because err.message is undefined.

How do I catch both unexpected errors, and errors I throw in the same catch block without writing more complex code to see if err.message is null for instance? I would expect both to return a similar structure, but they do not.

Answer 1

A good approach to this sort of issue is to always throw Error objects, and not literals .

It is considered good practice to only throw the Error object itself or an object using the Error object as base objects for user-defined exceptions. The fundamental benefit of Error objects is that they automatically keep track of where they were built and originated.

If you throw a plain non-error value, that value will be the expression in the catch section - which will (probably) not have the .message (or .stack ) property, and be less than useful as a result.

So, replace throw ('My fake error.'); with throw new Error('My fake error.'); - and then, in catch , doing console.log(err.message) may log either My fake error. , or it may log whatever other unexpected error occurred, if the rest of the code around there is well structured enough to always throw Error objects. Just enabling and following that no-throw-literal ESLint rule referenced above would be sufficient.