Keep it hidden. You shouldn't pollute the global scope unless necessary. 

(function(){
    var your_script;
})();

Using the same 'immediately invoked function' you can get a hold of an undefined variable which is `undefined` 

(function(undefined){
    var your_script;
    // now you compare for `undefined` using:
    if (your_script === undefined){
        // ...
    }
})();

Map and Reduce 

var l = [21,49,80,43,59];

var doubled = l.map(function(d){
    return d*2;
});

var max_value = doubled.reduce(function(a,b){
    return a > b ? a : b;
}, 0);

Filter 

var l = [
    {id: 5, username: 'bill', age: 32},
    {id: 6, username: 'john', age: 24},
    {id: 7, username: 'brad', age: 17}
];

var adults = l.filter(function(o){
    return o.age >= 18;
});

Augmenting an array of elements with forEach 

var l = [
    {id: 3, first_name: 'john', last_name: 'doe'},
    {id: 4, first_name: 'bill', last_name: 'masset'}
];

l.forEach(function(o){
    o.full_name = o.first_name + ' ' + o.last_name;
});

Objects, the right way 

var object_factory = function(params){
    var hidden_property = 42;
    var built_with_params,
        etc;

    var hidden_func = function(m,n){
        return m > n;
    };

    return {
        public_method: function(){
            return hidden_property;
        },
        another: function(m,n){
            hidden_func(m,n);
        }
    }
};

Arrays are objects, too 

var l = [1,2,3,4,5];

l.total = l.reduce(function(a,b){ return a + b; }, 0);

So comparing equality also acts like objects, not primitive lists 

> [] == []
false
> ['a'] == ['a']
false