The primary (primitive) data types are:

• String

• Number

• Boolean

The composite (reference) data types are:

• Object

• Array

The special data types are:

• Null

• Undefined

A string value is a chain of zero or more Unicode characters (letters, digits, and punctuation marks) strung together. You use the string data type to represent text in JScript. String literals can be included in your scripts by enclosing them in matching pairs of single or double quotation marks. Double quotation marks can be contained within strings surrounded by single quotation marks, and single quotation marks can be contained within strings surrounded by double quotation marks. The following are examples of strings:

"Happy am I; from care I'm free!"
'"Avast, ye lubbers!" roared the technician.' 
"42"
'c'

Notice that JScript does not have a type to represent a single character. To represent a single character in JScript, you create a string that consists of only one character. A string that contains zero characters ("") is an empty (zero-length) string.

Escape Sequences

'The image path is C:\\webstuff\\mypage\\gifs\\garden.gif.'

'The caption reads, \"After the snow of \'97. Grandma\'s house is covered.\"'

In JScript, there is no distinction between integer and floating-point values; a JScript number can be either (internally, JScript represent all numbers as floating-point values).

Integer Values

Floating-point Values

Whereas the string and number data types can have a virtually unlimited number of different values, the Boolean data type can only have two. They are the literals true and false. A Boolean value is a truth-value — it expresses the validity of a condition (tells whether the condition is true or not).

Comparisons you make in your scripts always have a Boolean outcome. Consider the following line of JScript code.

y = (x == 2000);

Here, the value of the variable x is tested to see if it is equal to the number 2000. If it is, the result of the comparison is the Boolean value true, which is assigned to the variable y. If x is not equal to 2000, then the result of the comparison is the Boolean value false.

Boolean values are especially useful in control structures. Here, you combine a comparison that creates a Boolean value directly with a statement that uses it. Consider the following JScript code sample.

if (x == 2000)
    z = z + 1;
else
    x = x + 1;

The if/else statement in JScript performs one action if a Boolean value is true (in this case, z = z + 1), and an alternate action if the Boolean value is false (x = x + 1).

You can use any expression as a comparative expression. Any expression that evaluates to 0, null, undefined, or an empty string is interpreted as false. An expression that evaluates to any other value is interpreted as true. For example, you could use an expression such as:

if (x = y + z) // This may not do what you expect — see below!

Note that the above line does not check if x is equal to y + z, since only a single equal sign (assignment) is used. Instead, the code above assigns the value of y + z to the variable x, and then checks if the result of the entire expression (the value of x) is zero. To check if x is equal to y + z, use the following code.

if (x == y + z) // This is different to the code above!

For more information on comparisons, see Controlling Program Flow.

The null data type has only one value in JScript: null. The null keyword cannot be used as the name of a function or variable.

A variable that contains null contains "no value" or "no object." In other words, it holds no valid number, string, Boolean, array, or object. You can erase the contents of a variable (without deleting the variable) by assigning it the null value.

Notice that in JScript, null is not the same as 0 (as it is in C and C++). Also note that the typeof operator in JScript will report null values as being of type Object, not of type null. This potentially confusing behavior is for backwards compatibility.

The undefined value is returned when you use:

• an object property that does not exist,

• a variable that has been declared, but has never had a value assigned to it.

Notice that you cannot test to see if a variable exists by comparing it to undefined, although you can check if its type is "undefined". In the following code example, assume that the programmer is trying to test if the variable x has been declared:

// This method will not work
if (x == undefined)
    // do something

// This method also won't work - you must check for
// the string "undefined"
if (typeof(x) == undefined)
    // do something

// This method will work
if (typeof(x) == "undefined")
    // do something

Consider comparing the undefined value to null.

someObject.prop == null;

This comparison is true,

• if the property someObject.prop contains the value null,

• if the property someObject.prop does not exist.

To check if an object property exists, you can use the new in operator:

if ("prop" in someObject)
    // someObject has the property 'prop'