140bytes - romanize (arabic to roman) http://www.developpez.net/forums/d1362763/webmasters-developpement-web/javascript/ludique-defis-code-en-tweet/

romanize.js

// 140bytes - http://www.developpez.net/forums/d1362763/webmasters-developpement-web/javascript/ludique-defis-code-en-tweet/

/* 98b: */
function romanize(n,a,b,c,d){
for(a='',b=0,c=5;n;b++,c^=7)for(d=n%c,n=n/c^0;d--;a='IVXLCDM'[d>2?b+n-(n&=-2)+(d=1):b]+a);return a
}

/* Explain */
function romanize(n){
  for(a='',                            // final string
      b=0,                             // IVXLCDM[b]
      c=5;                             // we have to start to divide by 5
      n;                               // while n not falsy (equals 0), divided by 5, and 2, and 5, etc ...
      b++,        
      c^=7) {                          // c^=7 equals 2, and then, next cycle, c^=7 will be 5, etc
    console.log('d='+(n%c)+'/n='+(n/c^0));
    for(d=n%c,                         // store the rest. never be more than 4
        n=n/c^0;                       // divide by 5 or 2 and then round to floor => this is like n=Math.floor(n/c) or n=~~(n/c)
        d--;) {                        // while n not falsy (equals 0)
      console.log('n='+n+'/m='+(n&~1)+'/a='+a+'/b='+b+'/c='+c+'/d='+d+'/x='+(1+(d>2?b+n-(n&-2)+1:b)));
      a='IVXLCDM'[d>2?                 // for the 4's or 9's 
                    b+                 // handle the 5ish, 10ish, 50ish, etc
                    n-(n&=-2)          // add 1 or 0 and transform n to the even number just below
                                       // n-(n&=-2) equals 0 if n is even et 1 if not.
                                       // because if n equals 9 we have to handle the next 5ish : example IX
                                       // whereas if n equals 4 we are in the right 5ish : example IV
                                       // if n is event, (n&-2) === n. if not (n&-2) === n-1
                    +(d=1)             // one turn left (example : IV if 4, IX if 9, XC if 90)
                   :b]+a;              // for the others cases, repeat the letter until "d"
    }
  }
 
  return a;
}

romanizeTest.js

console.log( romanize(12) + ' / ' + (romanize(12) === "XII") );
console.log( romanize(49) + ' / ' + (romanize(49) === "XLIX") );
console.log( romanize(74) + ' / ' + (romanize(74) === "LXXIV") );
console.log( romanize(383) + ' / ' + (romanize(383) === "CCCLXXXIII"));
console.log( romanize(1000) + ' / ' + (romanize(1000) === "M"));
console.log( romanize(3888) + ' / ' + (romanize(3888) === "MMMDCCCLXXXVIII"));