function-context-simple-call-use-call-and-apply.js

// An object can be passed as the first argument to call or apply and this will be bound to it.
var obj = {a: 'Custom'};

// This property is set on the global object
var a = 'Global';

function whatsThis(arg) {
  return this.a;  // The value of this is dependent on how the function is called
}

whatsThis();          // 'Global'
whatsThis.call(obj);  // 'Custom'
whatsThis.apply(obj); // 'Custom'