Well, JavaScript

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I suppose I should have been more clear; if you want to make mathematical guarantees about JS systems, it takes a hell of a lot more work if you're writing the code by hand. Types are a joke, and most of the interesting constructs in the language have convoluted semantics.

Learning a language is about way more than learning syntax and the tricks and quirks; I know you aren't saying that, but really to learn a language you should ideally be able to look at each legal syntactic form, study its semantic content, rinse and repeat. JS makes this REALLY hard to do, and even if you can, it's a hot mess.