code: function length(value) {
    var prefix
  
    /* istanbul ignore if - Currently also tested for at implemention, but we
     * keep it here because that’s proper. */
    if (value.charAt(0) !== ampersand) {
      return 0
    }
  
    prefix = value.split(ampersand, 2).join(ampersand)
  
    return prefix.length - decode(prefix).length
  }
docs: Returns the length of HTML entity that is a prefix of the given string (excluding the ampersand), 0 if it does not start with an entity.