bad.js

function Parent () {
  this.blah = 'test';
}

function Child () {
}

Child.prototype = new Parent()

var instance = new Child()
console.log(instance.hasOwnProperty('blah'));
// false

good.js

function Parent () {
  this.blah = 'test';
}

function Child () {
  Parent.call(this);
}

Child.prototype = Parent.prototype;

var instance = new Child()
console.log(instance.hasOwnProperty('blah'));
// true