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Functions and Objects

JScript supports a few built-in functions and objects. In addition, JScript allows you to create your own functions.

User-Defined Functions

Functions combine several operations under one name. If the function terminates correctly, it can return a result. JScript supports both user-defined and built-in functions. User-defined functions have the following syntax:

function test(parameters)
    body with statements
    return [return value];

You declare a function using the keyword function followed by a function name and a parameter list set in parentheses. The statements in the function body must be enclosed in braces. The return statement terminates a function and, optionally, returns a value.

To call a function in a script, you insert the function name and the parameters (set in parentheses). If you want to use the return value of a function, the function call must be on the right side of an assignment statement; if you want to ignore the return value, the function call can appear by itself on a line. The following statement calls the function test and assigns the return value to the variable result:

result = test();

If the function doesn't require parameters, you must use empty parentheses. You use commas to separate multiple parameters.

Built-In Functions

JScript has a few built-in functions to handle expressions and special characters and to convert strings and numeric values.

The eval function executes JScript code in a string, which is passed as a parameter, and returns the resulting value, as in value = eval("14+15");. The parseFloat function receives a string parameter and tries to convert it to a floating-point value. If the string parameter contains an illegal character (a character that isn't +, _, 0 to 9, ., or e), the string is converted only from the beginning to the illegal character. If the parameter doesn't contain a number, the NaN value is returned. The function parseInt requires a string as the first parameter. The second parameter must contain the code for the base (10 = decimal, 8 = octal, 16 = hexadecimal, and so on). An illegal string causes the result NaN. The parseInt function always returns an integer value.

For more details on built-in functions in JScript, see the JScript Language Reference.


JScript also supports a few built-in objects for processing strings, executing mathematical operations, and manipulating date and time values. You use the String object if a string is assigned to a variable or a property (as in name = "Born";). The object has several methods for manipulating strings. The Math object offers methods and properties for mathematical operators. For example, value = Math.PI; assigns the property PI to the variable value. The Date object handles date and time values. For example, var Name = new Date(parameters); creates a new date object and today = new Date(); returns the current date.

In JScript, you can handle objects and arrays in the same way. You can also access objects and collections in a similar way. To access a method or property of an object, you use the object name followed by a dot and the name of the property or method. The following statement uses the Echo method of the WScript object to display text in a message box:


In JScript, you must submit parameters in parentheses (unlike in VBScript).

To access an object in a collection, you can use an index value. The following statements are equivalent:

Res = Object.width; 
Res = Object[3];   // [3] should be equivalent to index "width". 
Res = Object["width"];

The brackets are valid when you access the numeric index, but you must omit the dot if an index value is used. Thus, the following statement causes a syntax error:

Res = Object.3;

If an object contains another object as a property, the naming scheme must be extended as follows:

var x4 = toDoToday.shoppingList[3].substring(0, 1);

The object property is followed by a dot, which is followed by a subobject.


Arrays are variables that contain a collection of values. When you create and handle arrays in JScript, you must use the Array object, as in the following statement:

var cities = new Array (10);

This statement creates the variable cities, which can contain 10 array items. The array items use the indexes from 0 to 9. You can access the items in this array using the array name and an index, as shown here:

cities[0] = "Rome";
cities[1] = "New York";
cities[2] = "Detroit";
cities[3] = "Manila";

The number in brackets is the index value, which indicates an array item. You can estimate the number of array items using the following code:

number = name.length;

To declare an array and initialize all array elements, you can use the following statement:

var cdays = new Array("Sunday", "Monday", "Tuesday",
                      "Wednesday", "Thursday", "Friday", "Saturday");

This statement creates a new array and assigns the data in parentheses to the array elements.

I'll discuss additional JScript features in later chapters. Different JScript language engines are available. Microsoft Windows 98 and Internet Explorer 4 include version 3 of the JScript language engine. Internet Explorer 5 comes with version 5 of the JScript language engine. WSH 2 and Internet Explorer 5.1 use version 5.1.