Designing JScript .NET

while back a reader asked for a rundown on some of the design decisions we made when
designing JScript .NET.  That's a huge
topic, but fortunately I started writing a book on the subject a few years ago that
never found a publisher.  "urn:schemas-microsoft-com:office:office" />Tell
you what -- whenever I can't think of something more interesting to post, I'll put
snippets of it up on my blog.


were four main design goals for JScript .NET:


1)      JScript
.NET should be an extension of JScript.
This means that whenever possible a legal JScript program should be a legal JScript
.NET program. JScript .NET must continue to
be usable as a dynamic scripting language
. Further, it should be easy to take
an existing JScript Active Server Page and transform it into a JScript .NET page for


2)      JScript
.NET should afford the creation of highly
performant programs
. In particular, JScript .NET should work well with ASP.NET
because performance is often critical in server-side applications. Also, JScript .NET
should warn programmers when they use a language feature which could adversely impact


3)      JScript
.NET should work well with the .NET Common
Language Runtime (CLR).
The CLR provides interoperability with other languages.
It also ensures that JScript .NET works well with the .NET Frameworks, a huge library
of useful packages.


4)      JScript
.NET should make programming in the large
. It should support programming styles that make it easy to reason about
large programs. It should also provide programming constructs which make it easy to
divide up large projects among team members.


I'll talk a bit about the second point -- how can we make JScript .NET faster than
JScript?  (I know, I said a while back
that I'd give you my rant about performance and script languages.  Maybe
if I have time tomorrow.)


.NET takes a three-pronged approach towards improving performance. Individually each
would help somewhat, but as is often the case, the whole is greater than the sum of
its parts. These three prongs work together to afford the creation of more performant


1)      JScript
.NET has a type annotation system.

2)      JScript
.NET discourages and/or restricts certain programming idioms which cause egregious
performance problems.

3)      JScript
.NET uses the Common Language Runtime.


type annotation system can be complex when you look at the details -- I'll probably
talk more about it in detail later.  But
essentially the type annotation system is quite simple: the programmer attaches annotations
to variable declarations, function argument lists and function return values. These
annotations restrict the kinds of data which may be stored, passed or returned respectively.


the simple example of adding two and two:


var Alpha
= 2, Beta = 2;

// [
many lines of code omitted ]

var Gamma
= Alpha + Beta;


Though Alpha and Beta starts
off their lives as numbers who knows what they are by the time
Gamma is
declared? Some code may have stored a string or a date or any other kind of data into
Alpha or Beta.


adding two and two thereby becomes a complicated mess. The JScript engine must determine
at run time whether a string has been assigned to either
Alpha or Beta (or
both). If so then they must be both converted to strings and concatenated. If neither
is a string then both must be converted to numbers and added. This sounds simple enough
but glosses over a number of details. For instance, the conversions are themselves
possibly extremely complicated depending on what data have been stored in
Alpha and Beta.
Suffice to say that a seemingly simple operation like addition can take thousands
of machine cycles, entire microseconds in some cases.


is unfortunate. If there is one thing that computers are good at it's adding numbers
blindingly fast. If the JScript .NET compiler could somehow
know that only numbers could possibly be stored
Alpha and Beta then there
would be no need to determine the types at run time
. The compiler could simply
emit instructions optimized for the case of adding two numbers. This operation could
take nanoseconds instead of microseconds.


JScript .NET you annotate the type of a variable by appending a colon and the type
after the declaration.


var Alpha :
= 2, Beta :
= 2;

// [
many lines of code omitted ]

var Gamma :
= Alpha + Beta;


code generated for this addition should now be much more efficient.


annotation also allows function calls to be much more efficient. Consider a simple
function call on a string:


var Delta
= 123;

// [
many lines of code omitted ]

var Zeta
= Delta.toString();


Delta starts
off as a number it could be anything by the time
toString is
called. This is what we mean by a late bound scenario. The type of
Delta is
unknown, so at run time the object must be searched to see if it has a method named
This search is potentially expensive. It could be far more efficient if the compiler
knew ahead of time that
Delta should
be converted to a string the same way that all numbers are converted to strings.


var Delta :
= 123;

// [
many lines of code omitted ]

var Zeta :
= Delta.toString();


that the JScript .NET compiler has an automatic
type inference engine
. If the compiler
sees that a local variable in a function only ever has, say, strings assigned to it
then the compiler will treat the variable as though it was annotated with the
String type. However,
it is a poor programming practice to rely upon the type inference engine. Certain
programming practices (especially the use of the
eval function)
prevent the inferrer from making valid inferences. Also the inferrer can only make
inferences on local variables; the types of
unannotated global variables are not inferred.


are also other good reasons to add annotations rather than relying on the inferencer
to do it for you, but those will have to wait for another entry.

Comments (5)

  1. Anu says:

    I have to tell you, the fact that Javascript protoyped objects are not first class dot net objects is very annoying. This ability would have been very welcome.

  2. dave sanderman says:

    i left msft for a few years and recently went back. as far as i’m concerned, the worst thing that happened while i was gone was the proliferation of the use of the non-word "performant". makes me cringe every time.

    otherwise, great article. 🙂

  3. Eric Lippert says:

    Dave: "performant" is a perfectly good word with a solid back-formation etymology, clear and obvious meaning, and has no suitable existing synonym, and is therefore a welcome addition to the language. Languages evolve. Do you also cringe at "process" as a verb? Because a hundred years ago, it wasn’t. What about "contact" as a verb? Ditto. So you might as well complain about those while you’re at it for all the good it will do you.

    Anu: I’m not sure what exactly you mean by "not first class .NET objects". JScript prototype objects are not CLR compilant, but they are certainly .NET objects. Can you clarify?

  4. Anu says:

    I was looking to convert some code written for the Rhino Javascript engine with a java host program.

    Thus I would have liked to be able to access/call the JScript prototype objects from the equivilent CSharp host replacement.
    I think the only way I could do it would be turn things around and have the JScript code act as the host, and reference the CSharp code.

    Is it possible? What is the class of a JScript prototype object?

    like x.js
    import System;

    function test() {
    this.main = function() {

    class fred {
    function getTest() {
    return new test();

    with y.js
    using System;

    public class CSTest {
    public static void main(String [] args) {
    fred f = new fred();

  5. Eric Lippert says:

    I’m still not 100% sure that I understand what you’re asking but I think I get the gist. If you’re asking whether a JScript function declared in global scope that can be used as a constructor for a prototype object can be exposed as a class to a C# program — no, we don’t support that scenario. The strongly typed CLR class model and the dynamic prototype model do not work well together.

    That’s one reason why we added traditional inheritance-hierarchy classes to JScript .NET.

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