Pythonic array filtering in JS

filter.js

function filter(callback, array){
    // Given a callback function taking one argument and an array,
    // return a new array of items passing the callback check.
    // The callback function is called once per item in the array and must return true/false.
    var ra = new Array();
    for(var i in array){
        if (callback(array[i]))
            ra.push(array[i]);
    }
    return ra;
}

var names = ["Chris", "Kate", "Steve", "Steph", "Alice", "Theodore"];

alert(
    filter(
        function(item){ // callback
            // Starts with 'S'
            return item.toString()[0] == 'S';
        },
        names // array
    )
)