Constructors Considered Mildly Confusing

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

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.

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:

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.

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.

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.

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.

© 2008 - 2013 Joost Diepenmaat, Zeekat Softwareontwikkeling.

URI: http://joost.zeekat.nl/constructors-considered-mildly-confusing.html