en.javascript.info/1-js/04-object-basics/04-object-methods/3-why-this/solution.md

Here's the explanations.

1. That's a regular object method call.

2. The same, parentheses do not change the order of operations here, the dot is first anyway.

3. Here we have a more complex call (expression).method(). The call works as if it were split into two lines:

   f = obj.go; // calculate the expression
   f();        // call what we have
   

   Here f() is executed as a function, without this.

4. The similar thing as (3), to the left of the dot . we have an expression.

To explain the behavior of (3) and (4) we need to recall that property accessors (dot or square brackets) return a value of the Reference Type.

Any operation on it except a method call (like assignment = or ||) turns it into an ordinary value, which does not carry the information allowing to set this.