Constructors Considered Mildly Confusing

Table of Contents

Zeekat Softwareontwikkeling (Nederlands)Articles (English)

A problematic quote

Consider this quote from Flanagan 2006, page 111 that came up in a comp.lang.javascript question in january 2008:

In javascript, every object has a constructor property that refers to the constructor function that initializes the object.

Sounds nice: it makes constructors sound static like classes in Java. Even the new Constructor() syntax looks like it. And it seems true:

function MyConstructor() {}
var myobject = new MyConstructor();
myobject.constructor == MyConstructor;     // true

But life isn't that simple:

function MyConstructor() {}
MyConstructor.prototype = {};
var myobject = new MyConstructor();

myobject.constructor == MyConstructor;  // false

What's going on? Some definitions

Objects and methods

Javascript1 objects are simply bags of named properties that you can read and set. Javascript does not have classes.

Functions in javascript are first-class objects. Methods in javascript are just properties that are functions.


The prototype of an object is an internal property that I'll refer to here as {Prototype}. In other words, obj.prototype is in general not the obj's {Prototype}. The standard does not provide any way to retrieve the {Prototype} property from an object.

Property lookup

Javascript objects can delegate properties to their {Prototype} and their {Prototype} can do the same; all the way up to Object.prototype.

Whenever a property propname of an object is read, the system checks if that object has a property named propname. If that propery does not exist, the system checks the object's {Prototype} for that property, recursively.

This means that objects that share a {Protoype} also share the properties of that {Prototype}.

Setting properties

Whenever a property propname of an object is set, the property is inserted into that object, ignoring the {Prototype} chain of that object.

The {Prototype} property is set from the (public) prototype property of the constructor function when constructor function is called.

What's going on? Line by line.

This is what the relevant prototype and {Prototype} properties look like. The ellipses are objects, the arrows are properties that reference other objects. The {Prototype} chain(s) are in green.

#1: Define constructor function

function MyConstructor() {}


Fairly simple. MyConstructor.prototype is an object that's automatically created which in turn has a constructor property pointing back at MyConstructor. Remember that: the only objects that in fact have a constructor property by default are the automatically created prototype properties of functions.

The rest isn't really relevant but may confuse and enlighten (and hopefully in that order):

MyConstructor's {Prototype} is Function.prototype, not MyConstructor.prototype. Also note that the {Prototype} chain for each object ends up at Object.prototype.

Object.prototype's {Prototype} is actually null indicating that it's the end of the chain. 2

For the next steps I'm leaving out the {Prototype} chain of MyConstructor for clarity, since it doesn't change and it's not relevant.

#2: Assign new prototype property

MyConstructor.prototype = {}


We've now done away with the predefined MyConstructor.protoype object and replaced it with an anonymous object, shown here as {}. This object does not have a constructor property,

#3: Call constructor to create new object

var myobject = new MyConstructor();


From this graph, following the L</property lookup> rules, we can now see that myobject.constructor is delegated to Object.prototype.constructor, which points to Object. In other words:

function MyConstructor() {}
MyConstructor.prototype = {};
var myobject = new MyConstructor();

myobject.constructor == Object // true

What about instanceof ?

Javascript provides the instanceof operator that's intended to check the prototype chain of the object you're dealing with. From the above you might think that the following would return false:

function MyConstructor() {}
MyConstructor.prototype = {};
var myobject = new MyConstructor();

myobject instanceof MyConstructor  // true

But in fact it works. It also notices that myobject delegates to Object.prototype:

function MyConstructor() {}
MyConstructor.prototype = {};
var myobject = new MyConstructor();

myobject instanceof Object  // true

When instanceof is called it checks the prototype property of the given constructor and checks it agains the {Prototype} chain of the given object. In other words, it's not dependent on the constructor property.

All nice and dandy, but you can still break it if you try hard enough:

function MyConstructor() {}
var myobject = new MyConstructor();
MyConstructor.prototype = {};

[ myobject instanceof MyConstructor,     // false !
  myobject.constructor == MyConstructor, // true !
  myobject instanceof Object ]           // true

This is what the prototype chains look like after running that:


Constructors are not classes

In a class-based object system, typically classes inherit from each other, and objects are instances of those classes. Methods and properties that are shared between instances are (at least conceptually) properties of a class. Properties (and for some languages, methods) that should not be shared are properties of the objects themselves.

Javascript's constructors do nothing like this: in fact constructors have their own {Prototype} chain completely separate from the {Prototype} chain of objects they initialize.

Constructors do not work like class-based initializers

A constructor call associates a new object with a {Prototype} the constructor function may set additional properties on the object. Constructor calls do not call "inherited" constructors, and they shouldn't because the object's {Prototype} (the constructor's prototype) is assumed to be shared and (probably) already initialized.

Constructors are just functions

Any user-defined function in javascript automatically gets a prototype property which in turn has a constructor property that refers back to the function.

Any user-defined function in javascript can be called as a constructor by prepending new to the call. This will pass a new this object to the function and its {Prototype} property will be set to the prototype property of the function.


A comp.lang.javascript question

Subject: "x.constructor == Foo" vs "x instanceof Foo".
Message-ID: <fniu6a$2cn$>>

Flanagan 2006

JavaScript: The Definitive Guide, Fifth Edition. ISBN 10: 0-596-10199-6 | ISBN 13:9780596101992


2008/02/11 - initial version

2008/02/13 - added footnote 2

2008/02/17 - rewrote footnotes

2013/05/06 - Reformatted to org-mode, remove some cruft.

Author & copyright

© 2008 - 2013 Joost Diepenmaat, Zeekat Softwareontwikkeling.




"Javascript" in this text refers to implementations of Ecma-262.


John G Harris wrote in on comp.lang.javascript to say that this isn't quite true. Theoretically, the host system may swap the Object.prototype property with something else. As far as Javascript code is concerned, Object.prototype should be read-only. In the only browser I've tested (firefox) you can assign a new value to Object.prototype without raising an error, but the assignment is ignored.