f([1, 2, 3, 4, 2, 3]); == 'ABCDBC'

gomi.js

function f(ary) {
  var i, n, s, r, m;

  s = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
  r = '';
  m = {};

  for (i = n = 0; i < ary.length; i++) {
    if (m[ary[i]] == null) {
      m[ary[i]] = s[n++];
    }

    r += m[ary[i]];
  }

  return r;
}