阿拉伯数字(1到4000)转化为罗马数字的javascript实现

function convert(num) {
  var a=[["","I","II","III","IV","V","VI","VII","VIII","IX"],  
                          ["","X","XX","XXX","XL","L","LX","LXX","LXXX","XC"],  
                          ["","C","CC","CCC","CD","D","DC","DCC","DCCC","CM"],
                           ["","M","MM","MMM"]];  
  var i=a[3][Math.floor(num/1000)];
  var j=a[2][Math.floor(num%1000/100)];
  var k=a[1][Math.floor(num%100/10)];
  var l=a[0][num%10];
  return  i+j+k+l;
   
}