function subString(string, start, end) {
  return _.isString(string) ? string.substring(start, end) : undefined;
}
This method extracts the characters in a string between "start" and "end", not including "end" itself.
If "start" is greater than "end", this method will swap the two arguments, meaning _.subString(string, 1, 4) == _.subString(string, 4, 1).
If either "start" or "stop" is less than 0, it is treated as if it were 0.

@static
@memberOf _
@param {string} string - The string to extract the characters.
@param {number} [start=0] - The position where to start the extraction. First character is at index 0.
@param {number} [end=string.length] - The position where to start the extraction. First character is at index 0.
@returns {string} A new String containing the extracted characters.