JavaScript Setters don't effect the expression result as expected

file.js

var obj = {
  _fooValue: 1,

  _fooGetterShouldReturnThis: false,  

  get foo() {
    var value =  this._fooGetterShouldReturnThis ? this : this._fooValue;
    this._fooGetterShouldReturnThis = false;
    return value;
  },

  set foo(value) {
    this._fooGetterShouldReturnThis = true;
    this._fooValue = value + 1;
  }
}


var foo = (obj.foo = 10);

console.log(foo);     // Output `10` instead of `Object { ··· }` or even `11`
console.log(obj.foo); // Output `Object { ··· }` which is `obj`
console.log(obj.foo); // Output `11` which is `obj._fooValue`