Repeating instead of adding

Question

Why are you repeating the number instead of adding?

var valor = prompt("calculo")
var valor1 = prompt("adição")

if (valor + 0 || valor1 + 0) {
  document.write(valor += valor1)
} else {
  document.write("valor incorreto")
}

Answer 1

Every return of the prompt is a string, in which case you are adding 2 string, where concatenation will occur.

You can use parseint() to fix the problem.

var valor = parseInt(prompt("calculo"))
var valor1 = parseInt(prompt("adição"))

if (valor + 0 || valor1 + 0) {
  document.write(valor += valor1)
} else {
  document.write("valor incorreto")
}