1/**
2* Number.isNaN is almost identical to ES5 global isNaN method. 
3* Number.isNaN returns whether the provided value equals NaN. 
4* This is a very different question from “is this not a number?”.
5*/
6
7Number.isNaN({}); // <- false, {} is not NaN
8Number.isNaN('ponyfoo') // <- false, 'ponyfoo' is not NaN
9Number.isNaN(NaN) // <- true, NaN is NaN
10Number.isNaN('pony'/'foo') // <- true, 'pony'/'foo' is NaN, NaN is NaN
11
12isNaN({}); // <- true, {} is not a number
13isNaN('ponyfoo'); // <- true, 'ponyfoo' is not a number
14isNaN(NaN); // <- true, NaN is not a number
15isNaN('pony'/'foo'); // <- true, 'pony'/'foo' is NaN, NaN is not a number

1function isNaN(x) {
2   return x !== x;
3};
4isNaN(NaN);//true

1// Use isNaN() 
2// In javascript compare NaN direct alweys return false
3let num1 = Number("Vishal");
4
5// This code never work
6if(num1 == NaN){  // Direct compare NaN alweys return false so use isNaN() function
7  ....... Your Code .......
8}  
9
10// This code work
11if(isNaN(num1){
12  .........Your Code .......
13}
14

1  var s = userInput[0];
2  if(isNaN(s) !== true)
3  {
4      console.log("yes");
5  }
6  else
7  {
8      console.log("no");
9  }

1  var j ="Hello World" // isNaN (j) returns true
2  var n = 15; // isNaN (n) returns false
3
4  if(isNaN(n) == true)   // NaN means >> Not a Number
5  {
6      console.log("yes, it's a string");
7  }
8  else
9  {
10      console.log("no, it's a number");
11  }