can variables be declared directly with this keyword ??

can variables be declared directly with this keyword ??

guys i a few questions about the code in the below pluggin : 

Scrolly.js : its basically a parallax plugin . 

now if you check line 15 to 25 . see how the variables are declared : 

  1.  this.element = element;
  2.         this.$element = $(this.element);

  3.         this.options = $.extend( {}, defaults, options) ;
  4.         
  5.         this._defaults = defaults;
  6.         this._name = pluginName;
  7.         
  8.         this.init();

I taught in JS/jquery u had to declare a variable before you could access it with "this" keyword. ? or am i wrong ? also what is the author trying to achieve with this piece of code : 
  1.  this.element = element;
  2. this.$element = $(this.element);
i mean the value passed to the function Plugin
is stored in this.element in the 1st line , now why on the 2nd line is the author adding another variable this.$element and passing the value of $(this.element) to it . ? what's the purpose ?