countByte.js

function countByte (str) {
  return decodeURI(
      encodeURI(str)  
    ).length          //URL-encode use UTF-8
  }                   //Note: this fails for input that contains lone surrogates.
}

Use http://mths.be/utf8js if you need something more robust.

function countByte (str) {
  return str.length * 2  //JavaScript String use UTF-16
}