Can someone look over the code for my platformer?

[TheVividYoshi]

I was wondering if someone can look at my platformer code and see if there are improvements I can make to it. I wrote it from scratch, with very little knowledge about making a platformer in the first place, so please be kind! I have attached it below.

[schrodinger]

I dare to say there are always improvements to be made
but if you need help for something specific maybe you can get more focused/useful replies,
it's hard to give general advices without knowing what you would like/expect your code to do

[TheVividYoshi]

Well, I am creating a sort of Cave Story-like platforming engine, but I'm not sure how to do jumping and gravity. I got the left and right movement working so far, and jumping somewhat works, but it is nowhere near high enough.

[schrodinger]

If this is your first times experimenting, why not just using builtin movements?
Did they fail your needs in some way?
They are usually enough for many tasks.

Although many like going with their own codes for smoother/custom functions
(not being a platformer expert myself - but surely many in this forum are)

[TheVividYoshi]

I tried making my own engine because it is more customizable. For instance, I can have variables for running speed and jump height (which is not quite working out), and gimmicks (that will come once I can get the engine to a semi-usable state).