全角を2、半角を1と数えるコード

hanzencount.js

// copy from http://kihon-no-ki.com/javascript-count-multi-byte-characters-as-two-single-byte-one
var charcount = function (str) {
  len = 0;
  str = escape(str);
  for (i=0;i<str.length;i++,len++) {
    if (str.charAt(i) == "%") {
      if (str.charAt(++i) == "u") {
        i += 3;
        len++;
      }
      i++;
    }
  }
  return len;
}