How to modify the message of a Javascript Error object?

Question

Is there any harm in updating the message of an Error object like this?

const err = new Error('bar');
...
err.message = `foo ${err.message}`;

My objective is to add some useful information to the error message when logging the error.

Dufour

Answer 1

It can be useful to add some extra information/breadcrumbs as an exception travels up through your application layers. That said, you are mutating an object, which can be hard to reason about in a large code-base; exceptions management normally being a cross-cutting concern in your application.

Also bear in mind that some libraries will extend the Error class and leave the message property without a setter, making it ready-only.

Strephon

Answer 2

I just noticed some harm :-(

On chrome 127.0.6533.17 (but not firefox 127.0.2), after your code, the error object is in a screwy state, in which err.message contains your additional annotation, but printing err does not show it:

const err = new Error('bar');
err.message = `foo ${err.message}`;
console.log("err.message = ",err.message);  // "err.message =  foo bar"
console.log("err = ",err);  // "err =\nError: bar\n    at <anonymous>:1:13"

The above is from chrome. On firefox, it shows "foo bar" in the final message instead.

Peddler