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Don't share a wall of code. All we want is the problem area, the code related to your issue.
I've used this page from MDN: https://developer.mozilla.org/en-US/docs/Web/API/Window/prompt - more specifically, I scrolled down until I found "Notes", which gave information on how to handle integer values(the default value returned is a string, however the solution shown in the Notes section shows you how to cast the value to an integer).
I've assumed that you're looking for the user to be able to insert two separate numbers and then combine them together, which means that the equation variable, I decided to split into two separate variables.
I haven't dealt with the operation itself though(+, -, *, /, etc.) as I believe you'll need to know how to handle strings for that - I'm not experienced in JavaScript, so I cannot continue.
I hope something like this is what you're looking for.