javascript inheritance why use Object.create? [duplicate]

Question

I saw this in StackOverflow: https://stackoverflow.com/a/7786090/289246

The answer tells to do:

DynamicBody.prototype = Object.create( PhysicsBody.prototype );

What is the reason to use Object.create?
Why can't we just use:

DynamicBody.prototype = PhysicsBody.prototype;

?

Answer 1

You can (technically), but then any change you make to DynamicBody.prototype will also be made on PhysicsBody.prototype, i.e. it will affect all PhysicsBody instances, and that is usually not what you want.

Example:

function Foo() {};
Foo.prototype.say = function() {
   alert('Foo');
};
var foo = new Foo();

function Bar() {};
Bar.prototype = Foo.prototype;
Bar.prototype.say = function() {
    alert('Bar');
};

foo.say(); // alerts 'Bar'

Object.create adds one level of indirection.