Prototypal inheritance and new keyword

Question

In backbone.js under the inherits method, the authors does this:

var ctor = function() {};
// some other code ...

var child;
// some other code ...
ctor.pro         


        

Answer 1

Remember that a prototype is an instance of the parent type. Using child.prototype = parent.prototype would set the child's prototype equal to the parent's prototype, rather than an prototypical instance of the parent.

Here's a huge problem that arises if you use child.prototype = parent.prototype: if you try to alter the child's prototype, you're also altering the parent's prototype, because they are the same object.

Child.prototype.childOnlyValue = 5;
// WARNING: Parent.prototype.childOnlyValue is now also 5,
//             because Parent.prototype === Child.prototype

Creating the new instance of the parent is absolutely necessary. Otherwise, you'll have a flat prototype chain with a single shared prototype, so you'll have problems like the one I've outlined above.