FJCVTZS is "Floating-point Javascript Convert to Signed fixed-point, rounding toward Zero". It is supported in Arm v8.3-A chips and later. Which is odd, because you don't expect to see JavaScript so close to the bare metal.

I can find explanations of what the instruction does, but not why it exists. This thread says "it exists as a single instruction is because JS's lack of an integer type means certain use cases need this operation obscenely often for no good algorithmic reason.". That's plausible but I would like a more detailed understanding.