How to add global variable in custom js file
You can do something like

(function ($, Drupal, undefined) {
    var my_global;

    Drupal.behaviors.exampleModule = {
        attach: function (context, settings) {
            my_global = 1234;
        }
    }
})(jQuery, Drupal)


and the Javascript scope-chain rules will make my_global visible to your whole closure.  If you need a true global, move the declaration of my_global:

var my_global;

(function ($, Drupal, undefined) {
    Drupal.behaviors.exampleModule = {
        attach: function (context, settings) {
            my_global = 1234;
        }
    }
})(jQuery, Drupal)


Personally, my JS files for Drupal start out as

var MYMODULE = (function(me, $, Drupal, undefined) {
    me.name = "MYMODULE";

    function attach (context, settings) {
        console.log(me.name + ".attach()");
    }

    Drupal.behaviors[me.name] = {
        attach: attach
    };

    return me;
}(MYMODULE || {}, jQuery, Drupal));


and I then add locals for module-wide variables, or directly to the module if other modules need them.  The article JavaScript Module Pattern: In-Depth explains this very well.