Javascript - cross-file - function - access

Hello.
I organized my javascript functions in separate files.
Let´s say I have a file fileA.js that contains the following function:

var fileA = (function fileA() {

    "use strict";

    function a1() {
        return 'resulttext_a1';
    }

     function a2() {
          return 'resulttext_a2';

    return {
          a1:a1,
          a2:a2
    };
}());

I can call a1 or a2 functions from php inside the script tags e.g. by using fileA.a1.
That works fine.
But let´s imagine I have fileB with functions b1 and b2.
How can I "cross-refer" to e.g. fileA.1 inside of e.g. b1?

I tried inside b1 using this.fileB.b1, but that did not work.
I added both files to the javascript libraries, but that does not work.
Also the using of "this" is difficult because of "use strict"....

Can anybody help please? Thanks a lot.