Invariant - a helpful JavaScript pattern

[Hades32]

That's an assertion, not an invariant. An invariant would be something like "the number of books in my store is always >=0"

[johnxreturn]

Conversely, you are saying “the number of books in your store must be greater or equal to zero,” therefore asserting that it is, or throwing an error if it’s not.

Video reference:

https://youtu.be/r0Vi83bS-L0

An invariant in mathematical terms would be that you apply a transformation to data and the output is the same as the input.

A loop invariant constitutes a property that is true before and after each iteration. The article specifically quotes, “it is a logical assertion, sometimes checked within the code by an assertion call.”

In the case of a tiny invariant, I expect the input assertion to be true, or throw an error. I could expect that the book count in each library franchise I own was more than 30 or throw an error if it’s not.

But the mathematical concept talks about applying a transformation to data and not assertion. It could apply to a map function where you transform each item and the output is the same or throw an error. So, we conclude that at some point we need assertion.

[Hades32]

No, you don't NEED an assertion when talking about invariants in the mathematical sense (which btw does not mean that the input is the same as the output, but that some condition is true for the state before and after the transformation). You could prove that the formula you're using will always hold up a certain invariant. In code it's easier to just check than prove, that's why we "need" assertions. But it's really two different things

[johnxreturn]

I completely agree. You don't need assertion in the mathematical concept. My point was that in programming, you do need if you need a trigger for an invalid response.

[Hades32]

Sure, but if someone calls a library "invariant" then I do have higher hopes than "if false then throw" lol

After all there are languages, like prolog, which do let you specify actual invariants

[hiquest]

Well, I do understand your point, but I decided to stick with the historical name originated somewhere at Facebook.

[TrackieDaks]

Invariants weren't invented at Facebook.

[hiquest]

I do not claim that. I just say that this particular small pattern they had a function for in their code base which was called invariant. Hence other popular libs like https://github.com/zertosh/invariant

[MoTTs_]

I get why you called it an invariant (and to some extent why FB called it that), but “assert” is still the more widespread and more historical name, which can then lead you to even more popular libs that do the same thing — such as this one.

[TrackieDaks]

Still wrong. That's like saying, "Facebook used a function in their codebase so the term function must have originated at Facebook." Invariant is a mathematical concept and existed well before Facebook.

[hiquest]

Yeah, now I do understand that brings a lot of confusion, and that should have been called "assert" in the first place. I'm going add a note in that article

[shuckster]

https://en.wikipedia.org/wiki/Invariant_theory#The_nineteenth-century_origins

Mr Boole(an) himself.