scope of "this" in getters and setters of regular function

getter-setter.js

  function sum() {
    return this.a + this.b + this.c;
  }
  
  var o = {
    a: 1,
    b: 2,
    c: 3,
    get average() {
      return (this.a + this.b + this.c) / 3;
    }
  };
  
  Object.defineProperty(o, 'sum', {
      get: sum, enumerable: true, configurable: true});
  
  console.log(o.average, o.sum); // 2, 6