My simple JS functions failing after adding DataTables()

I am NOT a JS expert, I just program well in other languages and do the same w/ what I know about JavaScript.

Some of this code is not even being accessed, but apparently scanned.
All have been in my 'library' file for months, some of it accessed at times, some never.  Even the `never` is burping.

The never-used code that burps:

// *************************************************************************************************

String.prototype.string_reverse = function () 
{ alert('this='+this);                                         // Object = Window!?
  return this.split('').reverse().join('');           // CRASHes
};

// usage:  str = str.string_reverse() ;
// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
function reverse(s) 
{
  return s.split('').reverse().join('');
}
// *************************************************************************************************
// *************************************************************************************************

String.prototype.replaceLast = function (what, replacement) 
{
  return what.string_reverse().replace(new RegExp(what.string_reverse()), replacement.string_reverse()).string_reverse();
};

// usage:  str = str.replaceLast('one', 'finish');

The is-used code that burps:

// *************************************************************************************************

String.prototype.strip_phone_format = function ()
{
  
  var phone=this.replace("(","") ;     // Error msg:  LibraryScripts.js:318 Uncaught TypeError: this.replace is not a function
  phone=phone.replace(" ","") ;
  phone=phone.replace(")","") ;
  phone=phone.replace("-","") ;
  
  return phone ;
} ;// *************************************************************************************************
// *************************************************************************************************

String.prototype.add_phone_format = function ()
{
  var just_numbers = this.strip_phone_format() ;
  
  return "("+just_numbers.substr(0,3)+") "+just_numbers.substr(3,3)+"-"+just_numbers.substr(6) ;
} ;

Two, three questions.

• Why is this just complaining now?
• Can I get help w/ the as-of-now unused code
• And the I-Need code?

Thanks!
Ralph

(DataTables() AJAX questions likely to follow.)