You guys might as well work together.

see https://github.com/overset/javascript-natural-sort

var stringAlphanumericalOne = [' ','1','a'];
var stringAlphanumericalTwo = ['img ','img1','imga', 'imgz'];
var stringAlphanumericalThree = ['img 99','img199','imga99', 'imgz99'];

console.log( stringAlphanumericalOne.sort(naturalSort) );
console.log( stringAlphanumericalTwo.sort(naturalSort) );
/* in the call below it fails! order should be ["img 99", "img199", "imga99", "imgz99"] */
console.log( stringAlphanumericalThree.sort(naturalSort) );

see live on http://jsbin.com/cipimosedoqe/1/edit?js,console

Storing state in a module of this nature strikes me as dangerous. It forces users to code defensively to avoid being affected by other consumers of the library. I suggest changing the API to work something like this:

var naturalSort = require('natural-sort');

var values = ['B', 'a', 'D', 'c'];

values.sort(naturalSort); // ['B', 'D', 'a', 'c']

values.sort(naturalSort({insensitive: true})); // ['a', 'B', 'c', 'D']

values.sort(naturalSort({desc: true})); // ['D', 'c', 'B', 'a']

Or, if you want to preserve the current API, let users modify an instance:

var NaturalSort = require('natural-sort');

var naturalSort = new NaturalSort();
naturalSort.insensitive = true;
// ...