Adding custom function to JQuery

jquery-custom-fn.js

// definition
$.fn.customFunction = function(options) {
  var defaults = {
    width: 650,
    height: 450
  };
  var opts = $.extend(defaults, options);
  var self = $(this[0]);  // if working on first/single element

  // functionality here
  self.css('color','red');
}

// usage
$('#mydiv').customFunction({color:'green'})