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New JScript .NET Data Types

January 7, 2002

JScript has a limited set of data types: Number, String, Boolean, and Object. The JScript interpreter works behind the scenes to coerce, or convert, data stored in variables into one of its native types using a standard set of rules (see section nine of the ECMAScript Language Specification). For example, the listing below uses the typeof operator to demonstrate that one variable can manage several data types (you can view the sample here)

var a;
a = 5; // typeof(a) returns 'number'
a = "five"; // typeof(a) returns 'string'
a = false; // typeof(a) returns 'boolean'
a = new String("");// typeof(a) returns 'object'

The key benefit the JScript interpreter provides is that is makes code easier to write since it performs data type conversions for you. The key drawback is that the JScript interpreter can end up working against the developer in situations like the one I demonstrated last week (where the interpreter seems to unexpectedly change a data type from one type to another) - here's the code again:

a = 1;
b = "1";
a += b; // what is the value of a?

If you guessed that the value of a should be two, you're wrong. Based on the data type conversion rules in the ECMAScript standard, JScript evaluates both operands (a and b) and determines that one of them, b, is a String (based on the value that's assigned to b). Since there's a String involved in the operation (+ in this example), JScript converts a into a String and concatenates b to it. The result is that a is 11 - it's not new math, the result is the String "11".

JScript .NET addresses this type of problem in several ways:

  • JScript .NET introduces many new and useful data types
  • JScript .NET allows you to declare a variable and assign a type to it
  • JScript .NET does not perform type conversions for you when you work with variables you have previously declared
  • JScript .NET checks for type mismatch errors at compile time, making debugging easier
  • JScript .NET enforces data type constraints by throwing an exception at run time when you attempt to assign inappropriate values to certain data types

This article describes each feature and demonstrates how you can create your own data types.

Understanding JScript .NET's new data types

A key element of the .NET Framework is the Common Type System, a component of the Common Language Runtime (see Figure 1).

Click here for larger image

Figure 1 - How the Common Type System relates to the .NET Framework

The Common Type System is a set of data types that are available to all .NET programming languages, making it easy to share data across applications written in different programming languages. JScript .NET makes it easy to use Common Type System data types by transparently mapping its native data types to Common Type System data types. Table 1 lists the new JScript .NET data types, what Common Type System Type each maps to, and a brief description of each data type.

Table 1 - JScript .NET data types

Data Type CTS Type Description/Range
Boolean System.Boolean Boolean values - true and false.
byte System.Byte 0 to +255
char System.Char 0 to +65 535
decimal System.Decimal -79.2e27 to +79.2e27
double System.Double -1.7e308 to +1.7e308
float System.Single -3.4e38 to +3.4e38
int System.Int32 -2.147e9 to +2.147e9
long System.Int64 -9.223e18 to +9.223e18
sbyte System.Sbyte -128 to +127
short System.Int16 -32 768 to +32 767
String System.String String data type; represents an immutable String - use the .NET StringBuilder object for mutable strings
uint System.UInt32 Unsigned integer ranging from 0 to +4e9
ulong System.Uint64 Unsigned long integer ranging from 0 to +18e18
ushort System.UInt16 Unsigned short integer ranging from 0 to +65 535

The next section describes how to declare variables and use some of the new data types.

Declaring variables along with their data types

JScript .NET uses the familiar var statement to allow developers to declare variables before using them. Developers already familiar with JScript or JavaScript should find it easy to declare variables. Unlike the simple var statement that JScript and JavaScript support, JScript .NET allows you to define the variable's data type as shown in the following listing:

var a : int; // declare an integer variable...
var b : String; // ...a string varible
var c : ushort; // ...and an unsigned short integer

Declare a variable using the var statement, followed by a colon (:), which is followed by the data type of variable you want to declare, as shown above.

Understanding automatic type conversions

While JScript .NET encourages developers to declare variables and assign specific data types to them, JScript .NET remains backwards compatible with JScript and JavaScript. For example, you can successfully compile and run the second snippet of code shown in this article (with a minor modification), as shown below:

// save this as test01.js
// compile using the command: jsc test01.js
a = 1;
b = "1";
a += b; 
print (a); // prints 11 - the string "11"

If you have trouble compiling the application using jsc, the JScript .NET compiler, start a Visual Studio .NET Command Prompt and try again (from the Start menu, select: Start - Programs - Visual Studio .NET - Visual Studio .NET Tools - Visual Studio .NET Command Prompt).

What's happening in the above listing is exactly what happens when you execute the code in the second snippet in Internet Explorer or Windows Scripting Host: JScript converts the value of a into a String and concatenates the value of b to it. If you modify the listing by adding the following statement to the beginning of the listing, the program will still successfully compile and run:

var b : String;

The reason it works is that JScript .NET is using the same rules that JScript and JavaScript use to convert data types at run time. JScript .NET applies JScript's and JavaScript's type conversion rules only for variables that you don't declare or declare a type for. In addition, you'll run into problems when you start using language features like package and class, since they require you to declare all variables.

If you declare variable a as an int, the program will not compile. The JScript .NET compiler will complain about the types being mismatched ("Type mismatch" error) and will not produce an executable version of your program. This brings us to the next benefit of using variables: compile time checking.

Understanding compile time checking

As I described in the previous article, a compiler produces two outputs: a list of errors and warnings, and an executable program. JScript .NET is no different from other compilers: as it compiles your code, it keeps track of errors and warnings, and if it encounters any critical errors it will not produce an executable version of your code. This feature is useful a variety of scenarios, since it makes it easy to detect and locate common programming mistakes like assigning inappropriate values to variables.

Consider the following listing:

var a : sbyte;

The code won't compile because the JScript .NET compiler knows that an sbyte type can manage values between -128 and +127, and assigning 200 to it is clearly an error. The JScript .NET compiler is able to clearly determine the value of a since it does not change in the above listing (there isn't any code that changes the value of a at runtime); as a result, the compiler generates a "Type mismatch" error and does not produce an executable version of the code.

Compile time checks can also ensure that you assign types that are compatible with each other. For example, consider the following listing:

var a : char; // range is 0 to 65,535
var b : byte; // range is 0 to 255 

a = b; // Ok

This is legal JScript .NET code since the byte data type's range falls within the char data type. This obviously wouldn't work if you attempt to assign a to b (since the range of a byte is smaller than a char data type).

When the compiler cannot definitively determine the value of a variable at compile time, the compiler produces an executable image and relies on runtime checks to detect errors.

Understanding data type constraints

Consider the following listing:

import System;

var a : sbyte;

  a = 100;
  a += 100;
catch ( e : Exception) 
  print( e.Message );
  // prints: "Exception of type System.OverflowException was thrown"

The code declares an sbyte variable, assigns the value 100 to it, and attempts to add 100 to it using the += operator. The above code successfully compiles since the JScript .NET compiler cannot establish what the value of a will be unless it executes the code. Since the compiler's job is not to execute code, it creates an executable image, allowing you to execute the program. The code encapsulates the attempt to add 100 to a within a try/catch block since the program must rely on run time checks to detect assignment errors. When the code executes, the .NET Class Library throws an Overflow exception because the result, 200, exceeds the capacity of the sbyte data type. The .NET Class Library throws the exception because the JScript .NET sbyte type transparently maps to the Common Type System System.Sbyte data type, as shown in Table 1.

The Common Type System enforces data type constraints at runtime using exceptions. In fact, the.NET Class Library makes extensive use of exceptions to report errors or exceptional conditions. As a result, you should become exception-aware since seemingly simple operations can cause your applications to unexpectedly terminate. A future article in this series is dedicated to errors, exceptions and becoming exception-aware.

Creating your own data types

There are scenarios when isolated variables are too simple to express your designs. For example, suppose that you're creating an application that collects information about visitors to your site. The information that your site collects includes a visitor's first and last name, and their email address. This information is useful throughout your application and simple string variables quickly become awkward. A solution to this problem would be to encapsulate all of a visitor's details within an object, making it easier to manage and easier to add methods that, for example, validate the user's email address's format.

JScript and JavaScript provide some support for creating your own objects, as shown in the following listing:

function Person()
  this.firstName = "";
  this.lastName = ""; = "";

function Person_Create(first,last,emailAddress)
function Person_toString()
  return this.lastName + ", " + this.firstName + " : " +
    + "\nEmail address " + 
    (this.isValidEmail() ? "is" : "is not") +  " valid";

var  aPerson;
aPerson = new Person;

The listing defines a Person object and associates a create, isValidEmail (not shown - refer to the sample code), and toString method with it. This code appears in a sample page, which you can experiment with (use your borwser's "view source" feature to review the source code). The sample page is shown in Figure 2 and the output is in Figure 3.

Figure 2 - Creating a custom data type using JScript in Internet Explorer

Figure 3 - Output of custom data type page

While the solution seems to work, it is messy since the Person object really doesn't exist as a first class object. The Person object seems to exist because it takes advantage of JScript's late binding features - there's no way of binding the Person_Create function to the Person type, nor is there any means of controlling access to a Person's variables.

Creating a custom data type using JScript .NET

JScript .NET addresses the above issues by allowing you to create your own data types using the class keyword. Consider the following code:

class Person
  public var firstName : String;
  public var lastName : String;
  private var emailAddress : String;
  // rest of class follows later in this article...

The listing defines a new person class with three member variables, just as the HTML page did in the previous example. The differences here are that each of the variables is bound to a specific type (String), and two of the three variables are public while one is private. public variables are just like regular variables - you can directly access and modify them. A private variable, in contrast, is not accessible to any code that resides outside of the class that defines the variable. The class in this example makes the emailAddress variable private since it controls access to it through special functions, called accessors. Accessor functions behave like variables but are actually functions that reside within a class, allowing developers a high degree of control over what actions other code can take on a class's variables.

The emailAddress variable is important since its contents must conform to a specific format; as a result, the class restricts access to it through functions that validate the format of the variable's content, as shown in the following listing:

// This function verifies the format of the email
// address on assignment
function set eMail(theAddress:String)
    throw(new Exception("Invalid Email address format"));
// This function does not attempt to verify
// the format of the email address since it 
// simply returns the current value
function get eMail() : String 
  return emailAddress;

// This is a read-only class property (since
// only the get function has been defined) that
// returns true if the current email address is
// valid. This function is used in one of  the class's
// constructors and in the set eMail function.
function get isEmailValid() : Boolean
  var emailReg = "^[\\w-_\.]*[\\w-_\.]\@[\\w]\.+[\\w]+[\\w]$";
  var regex = new RegExp(emailReg);

  if (!regex.test(this.emailAddress)) 
    return false;
  return true;

The listing demonstrates three accessor functions: set and get eMail and get isEmailValid. The comments in the listing provide details on the role of each function. The class verifies the email address's format using a regular expression that returns true for email addresses like tom@somedomain.tld, and essam.ahmed@my.domain.tld.

The class makes it easy to create new Person objects since it has two constructors. A constructor is analogous to the Person_Create function in the previous example: a constructor usually sets a class's initial state. The Person class has two constructors: a default constructor that initializes the class to an empty state, and a constructor that allows developers to create a Person initialized with a first and last name, and email address. The listing that follows demonstrates how to create both types of constructors:

// Default constructor
function Person()
  firstName=new String("");
  lastName=new String("");
  emailAddress=new String("");

// Constructor that initializes the Person based on
// names and email address
function Person(fName:String,lName:String,eMail:String)
  firstName=new String(fName);
  lastName=new String(lName);
  emailAddress=new String();

The sample Windows Forms application demonstrates how to use the Person class. A form similar to the one shown in Figure 4 appears when you start the application; the form looks very similar to the one in Figure 2.

Figure 4 - Windows Forms application that demonstrates how to use custom data types in JScript .NET

When you click on the Ok button, the application creates a new instance of the Person class using the following code:

var aPerson : Person;
aPerson = new Person(firstName.Text,lastName.Text,emailAddress.Text);

Each parameter that the Person constructor receives is based on the values that appear in the form - the Text property of each is similar to the HTML input element's value property.

The class allows developers to check the validity of the email address through the isEmailValid property (the get isEmailValid accessor function). There are cases when it is more appropriate to check the validity of the email address when the Person object initializes, as opposed to requiring that developers check the value of a class's property. The sample code also demonstrates how to validate the email address in the Person class constructor - the constructor reports an invalid email address using an exception. Although it is possible to throw exceptions from a constructor, it is generally not a good practice to follow and I provide it here for demonstration only. Use the directions that follow at the end of this section to build compile the application to report email validation exceptions when the Person object is created.

Regardless of what the state of the email address is, if you're using the application's default build, the application pops up a message box that echoes the values in the fields and an assessment of the email address's validity, as shown in Figure 5.

Figure 5 - Output of the sample Windows Forms application

The advantages that a JScript .NET class provides over a JScript object are many. The include:

  • The ability to specify what code can access member variables and functions through the public, protected, and private keywords
  • The ability to directly associate member functions with a specific, user-defined data type
  • A high degree of control over what other code can do with your class and its members
  • A high degree of encapsulation, offering the potential for better reuse across applications, potentially reducing the effort you spend on creating new applications.

Directions for downloading and building the sample application

These directions assume that the systems you use these directions on have version Beta 2 of the .NET Framework installed.

  1. Download the sample using this link
  2. Extract the contents of the ZIP file into a new folder
  3. Start a Visual Studio .NET Command Prompt as described earlier in this article and type the command buildPerson to compile the sample code. If the sample compiles successfully, you'll see a form on the screen that asks you to fill in a name and email address. If the sample does not compile successfully ensure that you can execute jsc, the JScript .NET compiler, from the command prompt.

Compile the alternate version of the application, described in the article, using the command:

buildPerson alt

When you run the application, change the value of the email address field to an invalid email address format and click on the Ok button. You should see an error about the email address being invalid - refer to the sample code for details.

Note that there seems to be a bug in the version of the .NET Framework I have installed on my system that inconsistently reports an email address as valid when it is actually not valid. If this happens on your system, change the value of the email address field and try again - it usually works after one or two tries (even if you remove a single character each try).


This article described the new JScript .NET data types, how they map to Common Type System data types, and how to use data types to perform compile time and run time checks to assist in your debugging efforts and improve the quality of your code. This article also demonstrated how to create your own data types using JScript .NET in the context of comparing the process with JScript. The next article in this series goes into more detail about the class statement in addition to the const, enum, package and import statements.

Essam Ahmed is the author of "JScript .NET Programming" (ISBN 0764548689, Published by Hungry Minds September 2001), many articles (including some at and book reviews (also available at Contact Essam at, or at his Web site

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