line: [ arguments.callee - JavaScript | MDN ]
Liitle before examples:
If the JavaScript interpreter cannot guarantee that all the provided arguments are numbers at the point that the call is made, it needs to either insert checks for all the arguments before the inlined code, or it cannot inline the function.
I think that “inline function” in this context is something that matters for the JS compiler/interpreter. It’s not some special kind of function from the developer’s point of view, it’s that the compiler can do this with it: https://en.wikipedia.org/wiki/Inline_expansion
As to why arguments.callee can break it, it’s above my head, I’m afraid 