This is just pocket calculator simulator.

It tries to resemble behavior of real pocket calculator with all its quirks, like doing operations with omitted operands, effects of "=" being pressed multiple times, ignoring precedence and so on. While it's simulating pocket calculator workflow, its computational core has plain JS behaviour. Also thanks to http://files.righto.com/calculator/TI_calculator_simulator.html, as it gave a lot of clues about calculator internals and was used as one of references, when making this simulator.