var numbers = {

   numberA: 5,

   numberB: 10,

   sum: function() {

     console.log(this === numbers); // => true

     function calculate() {

       // this is window or undefined in strict mode

       console.log(this === numbers); // => false

       return this.numberA + this.numberB;

     }

     return calculate();

   }

};

console.log(numbers.sum()); // => NaN or throws TypeError in strict mode