from http://dmitrysoshnikov.com/ecmascript/javascript-the-core/

callercalleefun.js

// the code of the "foo" function
// never changes, but the "this" value
// differs in every activation
 
function foo() {
  alert(this);
}
 
// caller activates "foo" (callee) and
// provides "this" for the callee
 
foo(); // global object
foo.prototype.constructor(); // foo.prototype
 
var bar = {
  baz: foo
};
 
bar.baz(); // bar
 
(bar.baz)(); // also bar
(bar.baz = bar.baz)(); // but here is global object
(bar.baz, bar.baz)(); // also global object
(false || bar.baz)(); // also global object
 
var otherFoo = bar.baz;
otherFoo(); // again global object