js how to tell if variable is nan code example

Example 1: javascript check if is nan

function isNaN(x) {
   return x !== x;
};
isNaN(NaN);//true

Example 2: check if var is NaN

let b=1
let n=readline()
if(isNaN(b)){
  console.log("You typed 0!")
}else{
  console.log("You did not type 0!")
}