this variable depends (resolves) based on invocation/execution context (and not where it is defined).
this refers to object which has called the method.

6 ways of invoking a function/method

1. Regular function (without Object qualifier)
   • In browser, default object would window if strict mode is off and undefined if strict mode is on.
   • In node, default object would be global object.
     e.g.

   fun(); // Its equivalent to window.fun()

2. Constructor function with new keyword e.g.

   var obj = new fun();

3. Object method Association of the function stays even after invocation

   var val = 1;
   var myObj = {
   val : 2,
   someFunction : function() {
   	console.log(this.val);
   }
   };
   myObj.someFunction();

4. IIFE (Immediately Invoked Function Expression) / Self Invoking function value of this is always equal to window object. e.g.

   (function fun(){ /* body */ }()); 

   IIFE is self invoked, it has not been called by any object. Hence, the value of this inside IIFE is Window object.
5. Use call() function with invocation target and params values comma separated.
   e.g. fn.call(object, varargs….) No long term association

   sum.call(emp, 1, 2);

6. Use apply() function with invocation target and param values in an array.
   e.g. fn.apply(object, array of values) No long term association
   

   sum.apply(emp, [1,2]);

Variables are passed

1. this - Invocation context, who calls this
2. arguments
3. length
4. name
5. caller

Functions are provided by javascript

1. apply
2. call
3. bind
4. toString
5. Symbol

https://hackernoon.com/this-in-javascript-8e8d4cd3930