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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`

.