this keyword is dynamically assigned and determined where it is called from.

There is 4 prioritization on the process of determining this context.

1. Is the function called by new keyword? - new binding

   var bar = new foo()

2. Is the function called by .apply or .call? - explicit binding

   var bar = foo.call( obj2 )

3. Is the function called as a method of object? - implicit binding

   var bar = obj1.foo()

4. Global on non strict mode and undefined on strict mode.

The other conditions are

1. ES6 Arrow functions, this retains the value of the enclosing lexical context's this. In global code, it will be set to the global object.
2. When a function is used as an event handler, its thisis set to the element the event fired from.