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NaN

NaN is a value representing Not-A-Number. Numeric operations or functions return NaN if they cannot calculate a value. We had seen this with parseInt. It returns NaN, if it can't parse a number. Another example is the calculation of the square root from a negative number:
var age = parseInt('Babylon');
var length = Math.sqrt(-1);
Both age and length get the value NaN.

If you want to test for NaN, you unfortunately can't use strict equality. NaN is the only value in JavaScript that when compared to itself returns false. That is, NaN === NaN results in false. To check if a value is NaN use the Number.isNaN function:
var input = 'Babylon';
var message;
var age = parseInt(input);
if ( Number.isNaN(age) ) {
  message = 'Input is not a number!';
}

Exercise

Write a function parseFirstInt that takes a string and returns the first integer present in the string. If the string does not contain an integer, you should get NaN. parseFirstInt('No. 10') should return 10 and parseFirstInt('Babylon') should return NaN.