ActionScript for Flash MX: The Definitive Guide, Second Edition

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Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

Null

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109

Null

Intellectually, the null type is nearly identical to the undefined type. Like the unde-

fined datatype, the null datatype is used to represent a lack of data and has only one

legal value, the primitive value

null. However, the null value is not assigned by the

interpreter automatically; it must be assigned by us deliberately.

We assign

null to a variable, array element, or object property to indicate that the

specified data container does not contain a legal number, string, Boolean, array, or

object value. For example, we might assign an initial value of

null to an object prop-

erty to indicate that it exists but has not yet been assigned a useful value.

To remove a variable or object property, you should use the delete operator rather

than assigning the value

null to the variable or property.

Note that

null compares equal only to itself and undefined:

null = = undefined; // true

null = = null; // true

To guarantee that a variable or property contains null (rather than undefined), check

its datatype using the typeof operator:

var x = null;

if (typeof x = = "null") {

trace("x is null");

}

Alternatively, as of Flash Player 6, we can use the strict equality operator (===), as

follows:

if (x = = = null) {

trace("x is null");

}

Though null and undefined are similar, their roles in ActionScript ...