Compute how many bytes you'd need to represent a Javascript string in UTF-8. This is great if you're doing naughty things like forcing utf-8 encoded text into latin1 database columns, and want to avoid silently truncating text.

string_byteSize.js

/* Returns the number of bytes needed to represent the given
 * Javascript string in UTF-8. */
String.prototype.byteSize = function () {
  var bytes = 0;
  for(i = 0; i < this.length; i++) {
    var charCode = this.charCodeAt(i);
    if (charCode <= 0x7F)
      bytes += 1;
    else if (charCode <= 0x7FF)
      bytes += 2;
    else if (charCode <= 0xFFFF)
      bytes += 3;
    else
      bytes += 4;
  }
  return bytes;
}