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.

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.

• javascript
• ecmascript-6
• error-handling

Share Improve this question Follow edited Jan 23, 2019 at 13:46 p7adams 66233 gold badges99 silver badges2626 bronze badges asked Jan 23, 2019 at 13:38 JoeTideeJoeTidee 26.2k2929 gold badges113113 silver badges163163 bronze badges

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2 Answers 2

Sorted by: Reset to default 2

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.

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.