Accessing parent function from JavaScript object

As this in Child will refer to Child object not Parent thus Store the reference of this of Parent in a variable which can be used later in childFunction.

var Parent = function() {
  var _self = this; //Store the reference
  this.Child = {
    childFunction: function() {
      _self.parentFunction(); //Use here
    }
  };
  this.parentFunction = function() {
    alert('In parentFunction');
  }
};
var parentObj = new Parent();
parentObj.Child.childFunction();

var Parent = function() {
 this.Child = {
    childFunction: function() {
      this.parentFunction();
    }
  };
  this.parentFunction = function() {

  }
};

Now inside this.Child childFunction this will refer to Child object and not the Parent.

So you must use self/that to refer to the parent.

 var Parent = function() {
     var that = this;
     this.Child = {

        childFunction: function() {
          that.parentFunction();
        }
      };
      this.parentFunction = function() {

      }
    };

You can just use Parent.parentFunction() as well.

 var Parent = function() {
     this.Child = {

        childFunction: function() {
          Parent.parentFunction();
        }
      };
      this.parentFunction = function() {

      }
    };

The problem is that this in your childFunction refers to the current object which is Child.

The common solution would be to access the parent using a variable that is available through a closure. For example:

var Parent = function() {
    var _this = this;

    this.Child = {
       childFunction: function() {
          _this.parentFunction();
       }
     };

    this.parentFunction = function() {
        // ...
    }
};

Alternatively, a more cumbersome approach would be to use bind to 'tie' the function context of the childFunction to the current this (which is the parent):

childFunction: function() {
    this.parentFunction();
}.bind(this);

See MDN