# undefined

#### Availability

Flash Player 5.

#### Usage

    undefined

#### Parameters

None.

#### Returns

Nothing.

#### Description

A special value, usually used to indicate that a variable has not yet been assigned a value. A reference to an undefined value returns the special value `undefined`. The ActionScript code `typeof(undefined)` returns the string `"undefined"`. The only value of type `undefined` is `undefined`.

When `undefined` is converted to a string, it converts to an empty string.

Note: In the ECMA-262 specification, `undefined` converts to the string `"undefined"`, not the empty string. This is a difference between ActionScript and the ECMA-262 specification.

The value `undefined` is similar to the special value `null`. In fact, when `null` and `undefined` are compared with the equality operator, they compare as equal.

#### Example

In this example, the variable `x` has not been declared and therefore has the value `undefined`. In the first section of code, the equality operator (`==`) compares the value of `x` to the value `undefined` and the appropriate result is sent to the Output window. In the second section of code, the equality operator compares the values `null` and `undefined`.

    // x has not been declared
    trace ("The value of x is " + x);
    if (x == undefined) {
      trace ("x is undefined");
    } else {
      trace ("x is not undefined");
    }

    trace ("typeof (x) is " + typeof (x));
    if (null == undefined) {
      trace ("null and undefined are equal");
    } else {
      trace ("null and undefined are not equal");
    }

The following result is displayed in the Output window:

    The value of x is x is undefined
    typeof (x) is undefined
    null and undefined are equal