Should a Promise.reject message be wrapped in Error?

Question

Using the native (ES6) Promise. Should I reject with an Error:

Promise.reject(new Error('Something went wrong'));

Or should I just reject with a string:

Promise.reject('Something went wrong');

And what is the difference in browser behaviour?

Solution

Yes, it most definitely should. A string is not an error, when you have errors usually it means something went wrong which means you'd really enjoy a good stack trace. No error - no stack trace.

Just like with try/catch, if you add .catch to a thrown rejection, you want to be able to log the stack trace, throwing strings ruins that for you.

I'm on mobile so this answer is rather short but I really can't emphasize enough how important this is. In large (10K+ LoC) apps stack traces in rejections really made the difference between easy remote bug hunting and a long night in the office.

Comments

[Felix Kling]

I don't think there is a difference. You simply reject with a value. What that value is and how it is supposed to be processed depends on your application.

[CodingIntrigue]

@FelixKling MDN does say For debugging purposes, it is useful to make reason an instanceof Error - it doesn't however say why, any ideas?

[Felix Kling]

@RGraham: Where does it say that?

[CodingIntrigue]

Sorry, on the docs page for Promise.reject

[Felix Kling]

@RGraham: Probably for the reason Benjamin mentions in his answer, to get a stack trace. However, that's a characteristic of Error, not of passing an Error object to .reject(). The browser or JavaScript still doesn't care what you pass to .reject().

[Benjamin Gruenbaum]

Just like you can throw strings, but shouldn't.

[jib]

You should use new: Promise.reject(new Error('Something went wrong'));

[jib]

Also, in practice, just throw new Error('Something went wrong')); because inside promise-chains and Promise constructor executor functions, this turns into a rejection.

[Bergi]

Related: Throwing strings instead of Errors

[Dominic]

Thanks I am also working on large apps glad that someone has had some practical experience with it

[Benjamin Gruenbaum]

In all honesty? Consider Bluebird, it has better stack traces, typed catch, and unhabdled rejection tracking which are all killer features for debugging. ES6 promises work well so far only in Firefox in our experience.

[Benjamin Gruenbaum]

2022 update: Don't prefer bluebird, all the changes for debugging have been upstreamed to native promises. (Errors should 100% be wrapped in new Error())