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Internal and external prototype

ECMAScript has two different prototype concepts which can be confusing:

  • Internal prototype: present for all objects, controls actual property and method lookups.

  • External prototype: the .prototype property of a constructor function, used to initialize the internal prototype of objects created using constructor calls (new MyConstructor()).

"Internal prototype" and "external prototype" are not standard terms, but used in Duktape documentation for clarity.

See also:

Internal prototype

The internal prototype, also referred to simply as "prototype", is specified as the internal property [[Prototype]] in the ECMAScript specification:

It affects actual property lookups such as obj.prop. To simplify a bit, a property lookup:

  • Checks if the property is found in the target object directly as a so-called "own property".

  • If the property is found, the lookup finishes.

  • If the property is not found, the internal prototype of the object is looked up. If the internal prototype is null, the lookup fails. Otherwise the property is looked up from the prototype object recursively. The sequence of objects looked up is called the "prototype chain".

Being an internal property, the [[Prototype]] property is not directly accessible, but it can be interacted with using:

  • obj.__proto__: an accessor property inherited from Object.prototype which allows the internal prototype to be read/written for objects which inherit from Object.prototype (not all objects do!). Specified in ES2015, also non-standard implementations before ES2015.

  • Object.getPrototypeOf() and Object.setPrototypeOf(): explicit methods added in ES2015 to read/write the internal prototype.

  • duk_get_prototype() and duk_set_prototype(): Duktape C API calls to read/write the internal prototype.

The ECMAScript APIs (__proto__ and Obj.setPrototypeOf) prevent creation of prototype loops; an attempt to create one fails with a TypeError:

duk> var obj1 = {}; var obj2 = {};
= undefined
duk> Object.setPrototypeOf(obj1, obj2); Object.setPrototypeOf(obj2, obj1);
TypeError: type error (rc -105)
	setPrototypeOf  native strict preventsyield
	global input:1 preventsyield

However, the Duktape C API is lower level and allows you to create a prototype loop (which should generally be avoided). Duktape has sanity limits to terminate lookups from looped prototype chains.

External prototype

The external prototype is the .prototype property present in most ECMAScript functions. It's present in all functions by default, but can be removed manually (also some built-ins don't have the property). The .prototype property only has an effect when a function is called as a constructor, i.e.:

var obj = new MyConstructor();

When the constructor call happens (see http://www.ecma-international.org/ecma-262/5.1/#sec-13.2.2 for details):

  • Duktape will create a new empty object (the "default instance") whose internal prototype is initialized to the current value of the constructor's .prototype property. If the .prototype value is not an object or is missing, the internal prototype is initialized to Object.prototype.

  • The constructor function is called with the this binding pointing to the newly created default instance. The constructor can then add properties to the default instance e.g. as this.name = "my object";.

  • Normally the constructor function has no return value (or returns undefined explicitly) in which case the default instance is returned as the value of new MyConstructor().

  • However, if the constructor function explicitly returns an object value, e.g. using var ret = { foo: 123 }; return ret;, the default instance will be ignored and the returned object will become the value of the new MyConstructor() call. This allows a constructor to return a function or a Proxy object, for example.

The only point where the external prototype has an effect is in initializing the default instance. In particular, if you ignore the default instance and explicitly return an object value from the constructor, the external prototype has no effect on the objects created.

Default external prototype object of ECMAScript functions

When you declare a function in ECMAScript code the ECMAScript semantics provide an automatic external .prototype object. For example, when declaring:

function MyConstructor() {
    // ...
}

the following objects are automatically created:

  • MyConstructor: the constructor function itself.

  • MyConstructor.prototype: points to a newly created prototype object.

The prototype object is empty except for a .constructor property which points back to the constructor function, so that:

MyConstructor.prototype.constructor === MyConstructor

The internal prototype of MyConstructor.prototype is Object.prototype so that when properties are looked up from objects created using new MyConstructor(), the prototype chain is by default:

  • The object itself

  • MyConstructor.prototype

  • Object.prototype

For simple objects a common idiom is for inherited methods to be added to MyConstructor.prototype.

There are many ways to control the eventual prototype chain of instance objects:

  • You can edit or replace MyConstructor.prototype to point to a different object. This will only affects objects created after the change.

  • You can change the internal prototype of MyConstructor.prototype so that another object (perhaps an ancestor) will be included in the prototype chain: Object.setPrototypeOf(MyConstructor.prototype, MyAncestor.prototype). This will affect also objects already created.

  • You can replace the "default instance" given to the constructor call with a different object, which gives full control over the prototype chain of the instance. This will obviously affect only new instances created.

  • Finally, you can edit object instances after they've been created.

No default external prototype object for Duktape/C functions

Duktape/C functions behave identically to ECMAScript functions when called as constructors: a default instance is created based on the .prototype property of the Duktape/C function, etc.

However, to minimize memory usage, Duktape/C functions don't have an external prototype (.prototype property) by default. This means that the default instance will inherit from Object.prototype unless you:

  • Add a .prototype property for the Duktape/C function explicitly.

  • Ignore the "default instance", creating and returning the instance object explicitly in the Duktape/C function.

  • Editi the internal prototype of an instance after its creation.

See: How to write a native constructor function.