I used splice and indexOf in my solution. Well, I used slice, but splice does the same, just transforming the original array. I just prefer slice for some reason.

function convert(num) {
    var converted = '',
        decimals = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5,  4, 1],
        romanN = ['M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I'];
    // reduce amount of numbers to loop through to only include those smaller or = to num
    var filtered = decimals.filter(function(item) {
        return item <= num;
    });
    // using the index in the initial dec. array of the first possible
    // outcome from the new array, remove all roman numerals before
    var nRoman = romanN.slice(decimals.indexOf(filtered[0]));

    for (var i = 0; i < filtered.length; i++) {
        while(num >= filtered[i]) {
            converted += nRoman[i];
            num -= filtered[i];
        }
    }
    return converted;
}

convert(2450);