ok. I am not used to writing kata. I will follow the instructions throughly and rewrite it again later.

You give basically the entire algorithm to the user in the question initialization

The description is extremely vague and it isn't immediately obvious what the function's output should be

The sample tests are incorrect.

The actual tests only go up to 23 and hence no numbers that would return Here is one! appears.

Also, there are no random tests.

nah this is more like a 7

in progress :)

oh, i didn't know about that, thanks for the information

Can you explain why if accepts a predicate value instead of a thunk, while for and while predicates accepts a thunk? This is not how typical languages handle if statements with side effects.

It was never specified that multiple values can be in _case. This only occurs in the test cases.

The question below should be an issue: const is only tested for direct assignment.

(Also, I don't agree that const variables can be deleted. It's const after all. In almost all languages you can't do this either.)

Related: You only gave primitive examples of the expected behaviour in terms of a giant code block. This is not acceptable as kata specification.

For a proper language you need to specify the syntax of the language (what goes after what), what goes into each argument, the expected order of evaluation, etc. Otherwise this is more of a toy project than an actual kata.

There are no random tests.

The test setup is incorrect: it blocks in Mocha are run in async parallel, so you cannot depend on each it block running one after another.

Each it block should create its own Language object from scratch and test from there.

(Test frameworks that uses describe/it structure in other languages also does this in general, btw)

I think it's possible, because I didn't think that far ahead, in this KATA I just want to make it simple, not too complicated

Can variables be redefined from mutable to immutable and back? example:

lang.const.x(10);

lang.x // 10

lang.const.x(20);

lang.x // 10

lang.let.x(15);

lang.x // 15 or should it still be 10? and vice versa from let to const

im aware of how the javascript language works, but with regards to this language we're creating in the kata, it isn't clear how this should behave

i enjoyed the finger workout i got from scrolling through this answer

only thing that would make this better is nested ternaries