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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`.