In most cases you don't want to do that. If function relay on this in its body (which is equal to window unless function defined in object or "use strict"; is used) you probably will encounter some side effects :) It's safer to pass null and use bind if you need to keep function's current context.

Perhaps an example will make it clear:

var obj = {
  val: 0,
  fn: function func() {
    this.val++;
    return this.val;
  }
}
window.val = 100;
obj.fn.val = 1000;

obj.fn(); // 1 -> this === obj
obj.fn.apply(obj); // 2
obj.fn.apply(null); // 101
obj.fn.apply(obj.fn); // 1001

var fn = obj.fn;
// this now refer to window instance
fn(); // 102
fn.apply(null); // 103 -> same as fn()
fn.apply(fn); // 1002 -> don't expected behaviour
fn.apply(obj); // 3

var bindedFn = obj.fn.bind(obj);
// this now is always refer to obj
bindedFn.apply(obj); // 4
bindedFn.apply(null); // 5
bindedFn.apply(obj.fn); // 6

[].slice will create new object and Array.prototype.slice don't

if (-1 !== handler) will always run code

propKey goes to global