In Javascript, What does isNaN function do? code example

Example 1: isnan in javascript

var s = userInput[0];
  if(isNaN(s) !== true)
  {
      console.log("yes");
  }
  else
  {
      console.log("no");
  }

Example 2: isNaN javascript

var j ="Hello World" // isNaN (j) returns true
  var n = 15; // isNaN (n) returns false

  if(isNaN(n) == true)   // NaN means >> Not a Number
  {
      console.log("yes, it's a string");
  }
  else
  {
      console.log("no, it's a number");
  }