> Why does this require JS (to the point of giving a blank page!) for something that can be trivially done without?

This is a slide deck (not a blog post or article) hence its dependence on JS.

(though even blog posts I write usually depend on JS to provide proper syntax highlighting and diagrams)

> Can someone explain to me why it has to assume that arr.length could change? It states it does, but I don't see why.

If you don't know where `throwConcurrentModificationError(arr)` goes then you don't know what it does, e.g. it can do something like this:

    H.throwConcurrentModificationError = function (arr) {
      arr.push(10);
    };

(contrary to its name - but JITs don't optimize based on names)

That's why JIT has to assume global side effects from the `throwConcurrentModificationError` call - unless of course it emits an unconditional deoptimization right before it which is what happens in the (0, o.f)() case.