Node.js: Extend JSON object

I would like to extend the native JSON object of node.js. In my example, I was able to extend it to add a merge(json1, json2) function, which merges 2 JSONs:

JSON.merge = function(target) {
    var sources = [].slice.call(arguments, 1);
    sources.forEach(function (source) {
        for (var prop in source) {
            target[prop] = source[prop];
        }
    });
    return target;
}

This script is larglely inspired from this question: Combine or merge JSON on node.js without jQuery

To have a cleaner code, I put this script in a library folder, and require it in my main script. This is what I did...

My lib/JSON.js file:

extends.merge = function(target){
    // The same script as above.
}

and in my app.js file:

JSON = require('./lib/JSON')

It works well, but now, native JSON methods don't work (like .stringify). Then I digged a little bit, and tried to extend JSON using .prototype, like this (in lib/JSON.js):

extends.prototype.merge = ...

But get the following error:

TypeError: Cannot set property 'merge' of undefined

I really don't know what to do (and what I'm doing) since solving a problems causes another problem.

Please, need help.

Thanks.