Why is my if...else statement not returning false?

Question

I have a some code where If I type in a string, it should return false, yet it returns true, then runs the piece of code its not supposed to run.

var f1
var f2
var d
var nth
var answer

function start() {
  f1 = prompt("First Term")
  f1 = parseInt(f1)
  f2 = prompt("Second Term")
  f2 = parseInt(f2)
  d = f2 - f1
  nth = prompt("Nth Term")
  nth = parseInt(nth)
  if (f1 != NaN || f2 != NaN || nth != NaN) {
    answer = f1 + d * (nth - 1)
    alert("Terms:\n1st Term: " + f1 + "\nSecond Term: " + f2 + "\nNth Term: " + nth + "\n\nCalculating Common Difference:\n d = " + f2 + " - " + f1 + ", or d = " + d + "\n\nUsing the explicit formula:\n" + f1 + " + " + d + "(" + nth + " - 1) = " + answer)
  } else {
    alert("Please type in numbers.")
  }
}

start();

Anyone know why its running the code, even after putting in a string?

https://stackoverflow.com/questions/2652319/how-do-you-check-that-a-number-is-nan-in-javascript — epascarello, Aug 24 '21 at 16:11
Should I be using AND operators? I wanna make sure if any of the variables are a string, it returns false — WILLIAM MESSER, Aug 24 '21 at 16:12
if n1 is a number or n2 is a number or nth is a number than do this. You are not saying if they all are numbers do this. So yes, you need to use and, not or. OR you chnage your logic to throw the error in the if and do the calculation in the else. — epascarello, Aug 24 '21 at 16:13

Nina Scholz · Accepted Answer · 2021-08-24T16:15:51.197

You can not compare NaN directly.

NaN !== NaN

Better use isNaN instead with negation (logical NOT !) and logical AND &&.

var f1
var f2
var d
var nth
var answer

function start() {
  f1 = prompt("First Term")
  f1 = parseInt(f1)
  f2 = prompt("Second Term")
  f2 = parseInt(f2)
  d = f2 - f1
  nth = prompt("Nth Term")
  nth = parseInt(nth)
  if (!isNaN(f1) && !isNaN(f2) && !isNaN(nth)) {
    answer = f1 + d * (nth - 1)
    alert("Terms:\n1st Term: " + f1 + "\nSecond Term: " + f2 + "\nNth Term: " + nth + "\n\nCalculating Common Difference:\n d = " + f2 + " - " + f1 + ", or d = " + d + "\n\nUsing the explicit formula:\n" + f1 + " + " + d + "(" + nth + " - 1) = " + answer)
  } else {
    alert("Please type in numbers.")
  }
}

start()

Why is my if...else statement not returning false?

1 Answers1