It all depends on what you see as the spirit of the challenge: if it’s just to design an interactive calculator web UI, then you may as well build up an expression string and use eval(). If you see it as the larger challenge of modeling a state machine, then eval is absolutely the last thing you want. In the end, it’s a toy project: have fun with it (that’s my theme of the day and the year) but move on when you’re satisfied with it, and if that means using eval, so be it.