You can use parseInt
to set the values as integers.
var lol=prompt("Please enter which 2d polygon you would like this awesome calculator to calculate.")
if(lol==="trapezoid"){
var tr1=Number(prompt("Enter the top base."))
var tr2=Number(prompt("Enter the bottom base."))
var tr3=Number(prompt("Now enter the height."))
confirm((tr1+tr2)*(tr3)/2)
}
Here's a JSFiddle with Benjamine's Number
point