The JScript Type System, Part Two: Prototypes and constructors

A number of readers made some good comments on my article on JScript typing that deserve to be called out in more detail.

First, I was being a little sloppy in my terminology — I casually conflated static typing with strong typing, and dynamic typing with weak typing. Thanks for calling me on that. Under the definitions proposed by the reader, JScript would be a dynamically typed language (because every variable can take a value of any type) and a strongly typed language (because every object knows what type it is.) By contrast, C++ is a statically typed language (because every variable must have a type, which the compiler enforces) but also a weakly typed language (because the reinterpret cast allows one to turn pointers into integers, and so on.)

Second, a reader notes that one of the shortcomings of JScript is that though it is a strongly typed language (in our new sense) that it is a royal pain to actually determine the runtime type an object. The typeof operator has a number of problems:

  • null is listed as being of the object type, though technically it is a member of the Null type.
  • primitives (strings, numbers, Booleans) wrapped in objects are listed as being of the object type rather than their underlying type.
  • JScript, unlike VBScript, does not interrogate COM objects to determine the class name.
  • If JScript is passed a variant from the outside world that it cannot make sense of then typeof returns “unknown”.

Perhaps there is some other way. Prototype inheritance affords a kind of type checking, for example.

Prototype inheritance works like this: every JScript object has an object (or possibly null) called its prototype object.  Suppose an object foo has prototype object bar, and bar has prototype object baz, and baz has prototype object null. If you call a method on foo then JScript will search foo, bar and baz for that method, and call the first one it finds.

The idea is that one object is a prototypical object, and then other objects specialize it. This allows for code re-use without losing the ability to dynamically customize behaviour of individual objects.

Prototypes are usually done something like this:

var Animal = new Object();
// omitted: set up Animal object
function Giraffe()
{
  // omitted: initialize giraffe object.
}
Giraffe.prototype = Animal;
var Jerry = new Giraffe();

Now Jerry has all the properties and methods of an individual Giraffe object AND all the properties and methods of Animal.  You can use IsPrototypeOf to see if a given object has Animal on its prototype chain. Since prototype chains are immutable once created, this gives you a pretty reliable sort of type checking.

Note that Giraffe is not a prototype of Jerry. Note also that Animal is not the prototype of Giraffe! The object which is assigned to the prototype property of the constructor is the prototype of the instance.

Now, you all are not the first people to point out to me that determining types is tricky. A few years ago someone asked me what the differences are amongst

if (func.prototype.IsPrototypeOf(instance))

and

if (instance.constructor == func)

and

if (instance instanceof func)

The obvious difference is that the first one looks at the whole prototype chain, whereas the second two look at the constructor, right? Or is that true? Is there a semantic difference between the last two?

There is. Let’s look at some examples, starting with one that seems to show that there is no difference:

function Car() { }
var honda = new Car();
print(honda instanceof Car); // true
print(honda.constructor == Car);  // true

It appears that instance instanceof func and instance.constructor == func have the same semantics. They do not. Here’s a more complicated example that demonstrates the difference:

var Animal = new Object();
function Reptile() { }
Reptile.prototype = Animal;
var lizard = new Reptile();
print(lizard instanceof Reptile); // true
print(lizard.constructor == Reptile); // false

In fact lizard.constructor is equal to Object, not Reptile.

Let me repeat what I said above, because no one understands this the first time — I didn’t, and I’ve found plenty of Javascript books that get it wrong. When we say

Reptile.prototype = Animal;

this does not mean “the prototype of Reptile is Animal“.  It cannot mean that because (obviously!) the prototype of Reptile, a function object, is Function.prototype.  No, this means “the prototype of any instance of Reptile is Animal“.  There is no way to directly manipulate or read the prototype chain of an existing object in JScript.

Now that we’ve got that out of the way, the simple one first:

instance instanceof func means “is the prototype property of func equal to any object on instance’s prototype chain?”  So in our second example, the prototype property of Reptile is Animal and Animal is on lizard‘s prototype chain.

But what about our first example where there was no explicit assignment to the Car prototype?

The compiler creates a function object called Car.  It also creates a default prototype object and assigns it to Car.prototype.  So again, when we way

print(honda instanceof Car);

the instanceof operator gets Car.prototype and compares it to the prototype chain of honda. Since honda was constructed by Car it gets Car.prototype on its prototype chain.

To sum up the story so far,  instance instanceof func is actually a syntactic sugar for func.prototype.IsPrototypeOf(instance) This explains why lizard instanceof Reptile returns trueReptile.prototype is a prototype of lizard.

So what the heck is going on with the constructor property then?  How is it possible that we can say lizard = new Reptile(); and at the same time lizard.constructor == Reptile is false???

Let’s go back to our simple first example.  I said above that since Car has no prototype assigned to it, we create a default prototype.  During the creation of the default prototype, the interpreter assigns Car to Car.prototype.constructor.  That might be a little confusing, so let’s look at some pseudocode.  This:

function Car(){}

logically does the same thing as

var Car = new Function();
Car.prototype = new Object();
Car.prototype.constructor = Car;

Now we say

var honda = new Car();
print(honda.constructor == Car );

and what happens? honda has no constructor property, so it looks on the prototype chain for any object with a constructor property. In this case Car.prototype is on the prototype chain and it has a constructor property equal to Car, so the comparison is true.  Remember, any property of an object’s prototype object is treated as a property of the object itself – that’s what “prototype” means.

Now let’s look at our second example:

var Animal = new Object();
function Reptile(){ }
Reptile.prototype = Animal;

Logically this does the same thing as

var Animal = new Object();
var Reptile = new Function();
Reptile.prototype = new Object();
Reptile.prototype.constructor = Reptile;
Reptile.prototype = Animal;

Whoops. The default prototype has been thrown away.  Now when we say

print(lizard.constructor == Reptile );

what happens? lizard does not have a constructor property, so we look at the prototype chain and find Animal.  But Animal also does not have a constructor property either! So we look on Animal‘s prototype chain. Animal was constructed via new Object() so therefore it has Object.prototype on its prototype chain, and Object.prototype has a constructor property.  As you might expect from our previous discussion of how the constructor property is initialized, Object.prototype.constructor is set to Object.

Therefore lizard.constructor is equal to Object, not Reptile, even though lizard is an instance of Reptile and was constructed by the Reptile function object!

You would think that the script engine would automatically assign the constructor property to the object when it was constructed, but it does not. It assigns the property to the prototype and relies on prototype inheritance. I was not a member of the ECMAScript committee when this decision was made, so I don’t know why we standardized this rather bizarre behaviour, but we’re stuck with it now!


Notes from 2020:
This article is one I referred back to so many times over the years I worked on JScript; the prototype mechanism can be very confusing.

There were a few interesting comments when I first wrote this. A couple highlights:

  • In some implementations of ECMAScript you can manipulate the proto chain directly with the __proto__ property. Make sure you understand what you’re doing before you change it!
  • Using “new String” to make a string can lead to an object that has different properties than just making the “primitive” string. This is by design — bad design, but design nevertheless. This was the subject of part three of this series.

1 thought on “The JScript Type System, Part Two: Prototypes and constructors

  1. Pingback: The JScript Type System, Part Three: If It Walks Like A Duck… | Fabulous adventures in coding

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