[zebra]

var calculator = {
  a: 0, 
  b: 0,
  sum: function() {
    return this.a + this.b;
  },

  mul: function() {
    return this.a * this.b;
  },

  readValues: function() {
    this.a = 90;
    this.b = 30;
  }
};

alert( "a" in calculator );