This Keyword

Generally speaking, this keyword represents an object from each the function has been called from. That is why it is not possible to say for sure which object this is bound to given only the body of the function.
Only the call time defines to which object this will be bound.
If there is no parameter supplied for this it will be bound to the global object.
Invocation of function with a keyword new will result in binding of this to a brand new object:

new object1.fn();

function fn() {
  log(this); // {} - irrelevantly to what object1 is.
}

Invocation by the mean of the specific object

There are several ways to invoke the function by the means of the specific object:

object.function() - left-to-the-dot rule - could be used if function is a defined property of object
object['function']() - same as the above
function.call(binding-for-this, first-arg, second-arg..) - using a call method on function object. In this way the first parameter that will be passed to invocation will be bound to this. Note that in the following example this will be bound to object2 due to the fact that call overrides method access rules:

object1.function.call(object2, ..)

Problem

Could be seen in the following code snippet:

setTimeout(r.fn, 100)

given the following way of implementing setTimeot (pseudocode):

function setTimeout(cb, interval) {
  wait(interval)
  cb()
}

Here the fn function will be called with this being bound to global.

Solution

Pass a completely new function object as a callback:

setTimeout(function() { r.fn() }, 100)