gistfile1.js

var input = [1, 2, 3, 4, 5, 6];

function isEven(x){
    return x % 2 == 0;
}

function filter(input, fn){
    var output = [];
    for(var i=0; i < input.length; i++){
        if(fn(input[i])){ output.push(input[i]); }
    }
    return output;
}

console.log(filter(input, isEven));
console.log(input);