Final conversion details

app.js

function checkDef( val ) {
    if( val === undefined ) {
        throw new Error( 'Number out of range' );
    }
}
function _doConvert( arabic, single, five, ten ) {
    var i = parseInt( arabic, 10 );
    switch( i ) {
        case 1: return single;
        case 2: return single + single;
        case 3: return single + single + single;
        case 4: checkDef( five ); return single + five;
        case 5: checkDef( five ); return five;
        case 6: checkDef( five ); return five + single;
        case 7: checkDef( five ); return five + single + single;
        case 8: checkDef( five ); return five + single + single + single;
        case 9: checkDef( ten ); return single + ten;
        case 0: return "";
    }
    if( isNaN(i) || i+"" != arabic+"" )
    {
        throw new Error( 'Cannot convert' );
    }
    throw new Error( 'Number out of range' );
}
var App = {
  ...  
  convertArabicToRoman = function(arabic){
    try{
      var thousands = _doConvert( Math.floor(arabic / 1000), "M" );
      arabic = arabic % 1000;
      var hundreds = _doConvert( Math.floor(arabic / 100), "C", "D", "M" );
      arabic = arabic % 100;
      var tens = _doConvert( Math.floor(arabic / 10), "X", "L", "C" );
      arabic = arabic % 10;
      return thousands + hundreds + tens + _doConvert( arabic, "I", "V", "X" );
    } catch( e ) {
        return e.message;
    }
  }
  ...
}