Convert a number into a Roman numeral in JavaScript

Question

What is the best way to convert integers to roman numerals?

function romanNumeralGenerator (int) {

}

Consider the following sample inputs and outputs:

1 = "I"
5 = "V"
10 = "X"
20 = "XX"
3999 = "MMMCMXCIX"

Only numbers between 1 and 3999 are supported.