When you're the only script that should work on a page.

Highlander.js

var HighlanderJS = (function (immortals) {
    var highlander = {};
    for (var immortal in immortals) {
        highlander[immortal] = immortals[immortal];
        delete immortals[immortal];
    }
    
    immortals.HighlanderJS = function (onlyOne) {
        onlyOne.apply(highlander);
    };
    
    return immortals.HighlanderJS;
})(window);

HighlanderJS(function () { alert("lolz"); });