Build a JS Calculator

(I know I may have been asking for a bit too much help lately but…)

Once more, I’m almost finished with a front-end certification project, but I’m stuck at test 13.

Basically, instead of calculating 5 * -5 properly, the thing thinks the calculation is
5 * 0 - 5. I’ve tried to fix it a few times, but I’m afraid I might break the whole thing if I do something wrong.

Pen link is below.