Evaluate triangle by its side

My question is about using Javascript to do triangle evaluation by its sides. The following code is very initial version even it works. I'd like to know whether it could be more simplifying or there is other way to achieve the same result.

Thanks!

let a = Number(prompt('Please input the the first side (a)'))
let b = Number(prompt('Please input the the second side (b)'))
let c = Number(prompt('Please input the the third side (c)'))

if (a + b <= c || b + c <= a || c + a <= b || Number.isNaN(a) || Number.isNaN(b) || Number.isNaN(c) || a == "" || b == "" || c == ""){
  console.log("invalid")
}
else if ((a > 0 && b >0 && c >0 ) && (a == b && b == c && c == a)){
  console.log("equilateral triangle")
}
else if ((a > 0 && b >0 && c >0 ) && (a == b || b == c || c == a)){
  console.log("isosceles triangle")
}
else {
  console.log("scalene triangle")
}