Question

From the javascript console in Chrome:

> function Person(name){this.name=name;}
undefined

At this point, Person.prototype should be an empty Object according to the Javascript specs.
Let’s assign it:

> p=Person.prototype
  > Person

Note that that > Person is clickable and it expands to:

constructor: function Person(name){this.name=name;}
__proto__: Object

But… wasn’t it meant to be an empty object? What is all the extra stuff?
If you do an alert:

alert(p)

You get [object Object]. Why, when you type it in the Chrome console, does it come out with > Person which expands? Wasn’t it meant to be an empty object?

Thank you!

Answer 1

No, the prototype always has the constructor property which points to the function it is the prototype of. And of course it inherits from an object too, that is the internal __proto__ property.

It is defined in ECMAScript 5 Section 13.2, Creating Function Objects:

(…)

16. Let proto be the result of creating a new object as would be constructed by the expression new Object() where Object is the standard built-in constructor with that name.

17. Call the [[DefineOwnProperty]] internal method of proto with arguments "constructor", Property Descriptor {[[Value]]: F, { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}, and false.

18. Call the [[DefineOwnProperty]] internal method of F with arguments "prototype", Property Descriptor {[[Value]]: proto, { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false}, and false.

(…)

This means nothing else than:

Create a new empty object called proto (16). Define the property constructor on that object and set the value to F (the function itself) (17). Then define the property prototype on the function F and set its value to proto.

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If you alert an object, then the object is converted to a string. The default behaviour is to convert an object to the [object Object] string, unless the "special" toString method is overridden.

The Chrome console lists these properties because it is meant for debugging, so you need information. [object Object] is not very informative.

FWIW, an empty object looks like this:

You can also see the internal __proto__ property here. An empty object always inherits some default properties, but it does not have own properties.