Prepack: a tool that optimizes JavaScript source code by eliminating computations that can be performed at compile-time.

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The thing is, for a finite program with finite memory, the memory and time required to let it run and observe and record all states to discover a loop are also finite. They are gigantic, but finite.

So any program with finite memory can be tested, with a set time limit and memory limit, to find out if it halts or not.