Javascript: using magic object keys

Question

I just wrote a function and I'm not sure if it is a good idea.

The motivation is, I'd like to store some internal bookkeeping data along with my business data in one big happy javascript object. When that object is consumed, I would like to strip the internal variables. As a convention, my internal variables take the form _internalNotes whereas my business variables don't start with an underscore.

Anyway, this is the function. It works as intended, but is this a terrible pattern?

export const stripInternals = (obj) => {
  const props = {}
  Object.keys(obj).forEach(key => {
    if (key.slice(0,1) !== '_') props[key] = obj[key]
  })
  return props
}

Answer 1

First, I'll state my opinion. This is a bad idea. Many languages don't support adding/removing fields like this, and they get along just fine. There are also nasty performance consequences to doing this. Really, though, for me this is just very hacky and relies on implicit rules which, if violated (and they can easily be violated), will result in completely bizarre behavior.

So what should you do instead?

You want some "internal" meta-data around some business data. So just do that: simply "convert" the business data into an object { internals: ..., business_data: ... } (or you can have the "internal" fields directly). Your stripInternals function can then simply be replaced by (obj) => obj.business_data. This will be much more explicit and simple and will likely perform better. If you handle all the wrapping/unwrapping yourself, this will never cause surprises, i.e. this will work correctly no matter what the "business data" is.

Answer 2

When applied to a framework, the pattern you're using is generally called Convention over Configuration. It is commonly accepted to be a useful approach to simplifying the interaction between the framework code and the user code (although there are some dissenters).

The question you should be asking, though, is whether your task is really difficult enough that writing framework-style code like this is actually appropriate. Is there not a simpler thing you could be doing that would work instead? Why do you need this code?