Edit in JSFiddle

(function() {
    Array.prototype.antMethod = function() {
        return 'antMethod';
    }
    
    console.log('--- inside closure: ' + [].antMethod());
    
    for(var val in ['A', 'B']) {
        console.log('--- val in closure: ');
        console.log(val);
    }
})();

console.log('--- global: ' + [].antMethod());

for(var val in ['A', 'B']) {
    console.log('--- val global: ');
    console.log(val);
}