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The JScript Type System, Part Two: Prototypes and constructors

  • Artigo

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. So 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 guys 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? Actually, 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.

 

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 the prototype property (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,

Comments

  • Anonymous
    November 06, 2003
    Just a side note. The prototype object is usually called proto ( [[Protoype]] in the spec) and almost all ECMAScript engines (except the Microsoft ones) allows read write of this private property.
  • Anonymous
    November 06, 2003
    Can you explain the logic behind the following madness (in JScript Classic), which appears to mean that a string is not always a String but a regexp is always a RegExp?Conversly, what is the recommended way of determining if a value is a string?slit instanceof String => falsesobj instanceof String => trueslit instanceof Object => falsesobj instanceof Object => truetypeof( slit ) => stringtypeof( sobj ) => objecttypeof( slit ) == typeof( sobj ) => falserelit instanceof RegExp => truereobj instanceof RegExp => truerelit instanceof Object => truereobj instanceof Object => truetypeof( relit ) => objecttypeof( reobj ) => objecttypeof( relit ) == typeof( reobj ) => trueAs generated by:function println(s) { WScript.echo(s) /System.Console.Out.WriteLine(s)/ }function evalprintln( s ){ if( !s ) { println(""); return } var out = s + " => "; try{ out += eval(s) } catch( e ) { out += "Error: " + e.description } println(out);}var slit = "Literal String";var sobj = new String( "String object" );var relit = /literal regexp/ivar reobj = new RegExp( "RegExp object" );var tests = [ "slit instanceof String", "sobj instanceof String", "slit instanceof Object", "sobj instanceof Object", "typeof( slit )", "typeof( sobj )", "typeof( slit ) == typeof( sobj )", null, "relit instanceof RegExp", "reobj instanceof RegExp", "relit instanceof Object", "reobj instanceof Object", "typeof( relit )", "typeof( reobj )", "typeof( relit ) == typeof( reobj )" ]; for( var ix in tests ) { evalprintln( tests[ix] ) }
  • Anonymous
    November 06, 2003
    Erik -- IIRC, Netscape removed the support for proto from their engine some years ago because it was a bad idea to let people mess with the prototype chain this way. The ECMA standard does not require the property to be exposed, and it is dangerous to do so.
  • Anonymous
    November 06, 2003
    Jay, if you read the ECMA spec you will see that there are string primitives and string objects, just as there are number/boolean primitives and number/boolean objects. Basically you should never use "new String(...)" because it is a waste of time and energy.var nlit = 42var nobj = new Number(42)print(nlit instanceof Number)print(nlit instanceof Object)print(nobj instanceof Number)print(nobj instanceof Object)
  • Anonymous
    November 06, 2003
    >JScript, unlike VBScript, does not interrogate COM objects to determine >the class name. This gets the award for the most unrelated error message:TypeName(myObject)throws, when the type information is unavailable:Out Of string space: 'TypeName'
  • Anonymous
    November 07, 2003
    Can I see a repro for that? If the type name is unavailable, it should return "Unknown" or, if it is an IDispatch, "Object". TypeName does return "Out of string space", but only when the attempt to allocate the string for the type name fails.
  • Anonymous
    November 07, 2003
    Peter: I know both the Mozilla engines (C and Java) and the Macromedia engine has support for this. I was pretty sure that Opera supported this as well but I just verifed that they do not.I can understand why it is considered dangerous but usage of proto can be very useful. For example it allows you to skip creating an instance that is used as the prototype.fucntion SubClass( args ){ SuperClass.call( args );}SubClass.prototype.proto = SuperClass.prototype;Well, well... now that we have ECMAScript v4 around the corner there is no real need for this.
  • Anonymous
    November 07, 2003
    > Well, well... now that we have ECMAScript v4 around the corner there is no real need for this.That's news to me. Could you more precisely define "around the corner"?
  • Anonymous
    November 07, 2003
    > conflated static typing with strong typing"static typing" unhelpfully confuses when checks are made with the kind of checks that are made.Simpler to understand that C++ is a statically-checked weakly-typed language (and also that some things are dynamically checked in C++).Simpler to understand that JScript is a dynamically-checked untyped (safe?) language. I think you demonstrated that it's pretty hard to figure out what the type of a value is in JScript - so why insist on calling it a typed language?
  • Anonymous
    November 07, 2003
    [quote]Can I see a repro for that? If the type name is unavailable, it should return "Unknown" or, if it is an IDispatch, "Object". [/quote]I got this when trying it on a COM object that uses run-time generated type info (ie. CreateDispTypeInfo). This only provides a minimal implementation of ITypeInfo. This is not common(or recommended) so I wouldn't worry too much. If it helps the COM object is out of process.Also see this:http://groups.google.com/groups?q=typeName+%22Out+of+string+space%22
  • Anonymous
    November 08, 2003
    The comment has been removed
  • Anonymous
    November 08, 2003
    The link I posted was just something I found while searching on the error. My situation relates to using run time type info. Have you tested TypeName with run time type info? It is obsolete and I only use it for testing so this discussion is essentially accademic.My guess is that something similar is happening, ITypeInfo must be returning bogus data. Assuming that you check all error results this could even be a bug in the implementation of ITypeInfo for CreateDispTypeInfo. As mentioned, this is obsolete and probably hasn't been looked at for a long time.I think my object was based on the code found here, so if you want to copy and paste:http://docs.rinet.ru:8083/VidimyyC/vcu28fi.htm#I25
  • Anonymous
    May 01, 2004
    "That's news to me. Could you more precisely define "around the corner"?"

    I guess I was wrong :'( This has been in the works for way too long...

    Known implemantions of ECMAScript v4:

    ActionScript
    QTScript
    JScript 7

    One possible reason for JS2 taking so long might be that ECMA moved all their resources to C# instead?
  • Anonymous
    May 02, 2004
    A more likely reason is that Waldemar Horwat, who was the primary driving force behind the E4 spec, no longer works for AOL-Time-Warner-Netscape and hence is no longer being paid to drive the spec process forward. (Rumour has it that he's at google now.)

    Microsoft is still involved in the E4 process, but it is slow going.
  • Anonymous
    January 18, 2009
    PingBack from http://www.keyongtech.com/4947719-intercept-instanceof