Compatibility vs. Performance

Earlier
I mentioned that two of the design goals for JScript .NET were high
performance and compatibility
with JScript Classic"urn:schemas-microsoft-com:office:office" />.
Unfortunately these are somewhat contradictory goals! JScript Classic has many dynamic
features which make generation of efficient code difficult. Many of these features
are rarely used in real-world programs. Others are programming idioms which make programs
hard to follow, difficult to debug and slow.

JScript
.NET therefore has two modes: compatibility
mode and fast
mode. In compatibility mode there should be almost no JScript
program which is not a legal JScript .NET program. Fast mode restricts the use of
certain seldom-used features and thereby produces faster programs.

The
JSC.EXE command-line compiler and ASP.NET both use fast mode by default. To turn fast
mode off in JSC use the /fast- switch.

Fast
mode puts the following restrictions on JScript .NET programs:

* All
variables must be declared with the var keyword. As
I discussed earlier, in JScript Classic it is sometimes legal to use a variable without
declaring it. In those situations, the JScript Classic engine automatically creates
a new global variable but when in fast mode, JScript .NET does not. This is a good
thing -- not only is the code faster but the compiler can now catch spelling errors
in variable names.

* Functions
may not be redefined. In JScript Classic it is legal to have two or more identical
function definitions which do different things. Only the last definition
is actually used. This is not legal in JScript .NET in fast mode. This is also goodness,
as it eliminates a source of confusion and bugs.

* Built-in
objects are entirely read-only. In JScript Classic it is legal to add, modify
and (if you are perverse), delete some properties on the Math object,
the String prototype
and the other built-in objects.

* Attempting
to write to read-only properties now produces errors. In JScript Classic writing
to a read-only property fails silently, in keeping with the design principle I discussed
earlier: muddle on through.

* Functions
no longer have an arguments property. The
primary use of the arguments property
is to create functions which take a variable number of arguments. JScript .NET has
a specific syntax for creating such a function. This makes the arguments object
unnecessary. To create a JScript .NET function which takes any number of arguments
the syntax is:

function
MyFunction(... args : Object[] )

{

  //
now use args.length, args[0], etc.

}

Generally
speaking, unclear
code is slow code. If the compiler is unable to generate good
code it is usually because the restrictions on the objects described in the code are
so loose as to make optimization impossible. These few restrictions not only let JScript
.NET generate faster code, they also enforce good programming style without overly
damaging the "scripty" nature of the language. And if you must run code which has
undeclared variables, redefined functions, modified built-in objects or reflection
on the function arguments, then there is always compatibility mode to fall back upon.

JScript
.NET also provides warnings when programming idioms could potentially produce slow
code. For example, recall my earlier article on string concatenation. Using
the += operator
on strings now produces a warning which suggests using a StringBuilder instead.
JScript .NET also produces warnings when code is likely to be incorrect. For example,
using a variable before initializing it produces a warning. So
does branching out of a finally block
now produce warnings, and so on.