How does Object.create not allow multiple inheritance in Javascript?

Question

The MDN gives this explanation of inheritance in Javascipt (with the comments showing the prototype chain):

var a = {a: 1}; 
// a ---> Object.prototype ---> null

var b = Object.create(a);
// b ---> a ---> Object.prototype ---> null
console.log(b.a); // 1 (inherited)

var c = Object.create(b);
// c ---> b ---> a ---> Object.prototype ---> null

var d = Object.create(null);
// d ---> null
console.log(d.hasOwnProperty); 
// undefined, because d doesn't inherit from Object.prototype

Here it looks to me like c is inheriting from multiple classes. How is this not multiple inheritance?

doldt · Accepted Answer · 2015-04-16 11:30:48Z

8

Multiple inheritance is when the parents are on the same level in the hierarchy:

c ---> b ---> Object.prototype ---> null
 \---> a ---> Object.prototype ---> null

In this case, it's simple inheritance from a class b which inherits from an other class a:

c ---> b ---> a ---> Object.prototype ---> null

Addendum: While the effects might seem similar (attributes of b will be also "found" in c via lookup in the prototype chain), do note the difference that multiple inheritance would allow a and b to have entirely different inheritance chains (in fact, inheritance "trees"), which is clearly not the case in your example.

edited Apr 16, 2015 at 11:30

answered Apr 16, 2015 at 7:55

doldt

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4 Comments

doldt Over a year ago

Thanks for the edits @zerkms, I was just about to add something like that myself.

zerkms Over a year ago

I hope my ASCII art isn't too ugly though :-)

Startec Over a year ago

I see. So just to be completely clear, in the question's example, the object is inheriting from other objects higher in the chain, not on the same level?

doldt Over a year ago

@Startec - I'm not sure I follow - that chain is THE inheritance chain, there's no inheriting from outside it by definition, and there's no elements on the same level, because it's a one-by-one chain.

Ondrej Tucny · Accepted Answer · 2015-04-16 08:00:27Z

1

Multiple inheritance means inheriting in parallel from two or more classes, i.e. having two or more direct ancestors. They form a tree.

In your case there's only one direct ancestor: b, and a is b's direct ancestor, but indirect of c. They form a linear chain.

answered Apr 16, 2015 at 8:00

Ondrej Tucny

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