1let value = 0 / 0; // undefined
2if (isNaN(value)) value = 0;

1// NaN:  Not a Number in javascript
2console.log(parseInt('1.23'));				// 1.23
3console.log(parseInt('something'));			// NaN

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

1var val = Number.NaN;