ActionScript for Flash MX: The Definitive Guide, Second Edition

This is the Title of the Book, eMatter Edition

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

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Chapter 5: Operators

When the operands have disparate datatypes, the interpreter performs a type conver-

sion before performing the comparison. Here are the rules the interpreter follows:

1. If both operands are of the same type, compare them and return the result. (If

null is compared to undefined, true is returned.)

2. If one operand is a number and the other operand is a string, convert the string

to a number and go back to step 1.

3. If one operand is a Boolean, convert the Boolean to a number (

true =1,false =

0) and go back to step 1.

4. If one operand is an object, invoke the valueOf() method of the object to con-

vert it to a primitive type. Return

false if this is not possible. Otherwise, go back

to step 1.

5. Return

false if the previous steps don’t obtain a valid result.

Note that if one operand is an object and the other is a Boolean, the Boolean will be

converted to a number and compared to the primitive value of the object. This

means that

someObject = = true is normally false, even if someObject exists, because

someObject is converted to a number or a string for the comparison, while true is

converted to the number 1. To force

someObject to be treated as a Boolean in a com-

parison, use the Boolean() function, like this:

Boolean(someObject) = = true // Returns true if someObject ...