The Boolean data type comprises two values: true and false . No other values are valid for variables of Boolean type. The default value of a Boolean variable that has been declared but not initialized is false .

The int data type is stored internally as a 32-bit integer and comprises the set of integers from

-2,147,483,648 (-2³¹) to 2,147,483,647 (2³¹ - 1), inclusive. Previous versions of ActionScript offered only the Number data type, which was used for both integers and floating-point numbers. In ActionScript 3.0, you now have access to low-level machine types for 32-bit signed and unsigned integers. If your variable will not use floating-point numbers, using the int data type instead of the Number data type should be faster and more efficient.

For integer values outside the range of the minimum and maximum int values, use the Number data type, which can handle values between positive and negative 9,007,199,254,740,992 (53-bit integer values). The default value for variables that are of the data type int is 0.

The Null data type contains only one value, null . This is the default value for the String data type and all classes that define complex data types, including the Object class. None of the other primitive data types, such as Boolean, Number, int and uint, contain the value null . Flash Player and Adobe AIR will convert the value null to the appropriate default value if you attempt to assign null to variables of type Boolean, Number, int, or uint. You cannot use this data type as a type annotation.

In ActionScript 3.0, the Number data type can represent integers, unsigned integers, and floating-point numbers. However, to maximize performance, you should use the Number data type only for integer values larger than the 32-bit int and uint types can store or for floating-point numbers. To store a floating-point number, include a decimal point in the number. If you omit a decimal point, the number will be stored as an integer.

The Number data type uses the 64-bit double-precision format as specified by the IEEE Standard for Binary Floating-Point Arithmetic (IEEE-754). This standard dictates how floating-point numbers are stored using the 64 available bits. One bit is used to designate whether the number is positive or negative. Eleven bits are used for the exponent, which is stored as base 2. The remaining 52 bits are used to store the significand (also called the mantissa ), which is the number that is raised to the power indicated by the exponent.

By using some of its bits to store an exponent, the Number data type can store floating-point numbers significantly larger than if it used all of its bits for the significand. For example, if the Number data type used all 64 bits to store the significand, it could store a number as large as 2⁶⁵ - 1. By using 11 bits to store an exponent, the Number data type can raise its significand to a power of 2¹⁰²³.

The maximum and minimum values that the Number type can represent are stored in static properties of the Number class called Number.MAX_VALUE and Number.MIN_VALUE .

Number.MAX_VALUE == 1.79769313486231e+308 
Number.MIN_VALUE == 4.940656458412467e-324

Although this range of numbers is enormous, the cost of this range is precision. The Number data type uses 52 bits to store the significand, with the result that numbers that require more than 52 bits to represent precisely, such as the fraction 1/3, are only approximations. If your application requires absolute precision with decimal numbers, you need to use software that implements decimal floating-point arithmetic as opposed to binary floating-point arithmetic.

When you store integer values with the Number data type, only the 52 bits of the significand are used. The Number data type uses these 52 bits and a special hidden bit to represent integers from -9,007,199,254,740,992 (-2⁵³) to 9,007,199,254,740,992 (2⁵³).

Flash Player and Adobe AIR use the NaN value not only as the default value for variables of type Number , but also as the result of any operation that should return a number but does not. For example, if you attempt to calculate the square root of a negative number, the result will be NaN . Other special Number values include positive infinity and negative infinity .

The String data type represents a sequence of 16-bit characters. Strings are stored internally as Unicode characters, using the UTF-16 format. Strings are immutable values, just as they are in the Java programming language. An operation on a String value returns a new instance of the string. The default value for a variable declared with the String data type is null . The value null is not the same as the empty string ( "" ), even though they both represent the absence of any characters.

The uint data type is stored internally as a 32-bit unsigned integer and comprises the set of integers from 0 to 4,294,967,295 (2³² - 1), inclusive. Use the uint data type for special circumstances that call for non-negative integers. For example, you must use the uint data type to represent pixel color values, because the int data type has an internal sign bit that is not appropriate for handling color values. For integer values larger than the maximum uint value, use the Number data type, which can handle 53-bit integer values. The default value for variables that are of data type uint is 0.

The void data type contains only one value, undefined . In previous versions of ActionScript, undefined was the default value for instances of the Object class. In ActionScript 3.0, the default value for Object instances is null . If you attempt to assign the value undefined to an instance of the Object class, Flash Player or Adobe AIR will convert the value to null . You can only assign a value of undefined to variables that are untyped. Untyped variables are variables that either lack any type annotation, or use the asterisk ( * ) symbol for type annotation. You can use void only as a return type annotation.

The Object data type is defined by the Object class. The Object class serves as the base class for all class definitions in ActionScript. The ActionScript 3.0 version of the Object data type differs from that of previous versions in three ways. First, the Object data type is no longer the default data type assigned to variables with no type annotation. Second, the Object data type no longer includes the value undefined , which used to be the default value of Object instances. Third, in ActionScript 3.0, the default value for instances of the Object class is null .

In previous versions of ActionScript, a variable with no type annotation was automatically assigned the Object data type. This is no longer true in ActionScript 3.0, which now includes the idea of a truly untyped variable. Variables with no type annotation are now considered untyped. If you prefer to make it clear to readers of your code that your intention is to leave a variable untyped, you can use the new asterisk ( * ) symbol for the type annotation, which is equivalent to omitting a type annotation. The following example shows two equivalent statements, both of which declare an untyped variable x :

var x 
var x:*

Only untyped variables can hold the value undefined . If you attempt to assign the value undefined to a variable that has a data type, Flash Player or Adobe AIR will convert the value undefined to the default value of that data type. For instances of the Object data type, the default value is null , which means that Flash Player or Adobe AIR will convert the value undefined to null if you attempt to assign undefined to an Object instance.