Object Properties
Properties are named data containers associated with an object. They
are defined by an object’s class and then set individually for
each object instance. Like variables, object properties can contain
any kind of data—strings, numbers, Booleans,
null
, undefined
, functions,
arrays, movie clips, or even other objects.
Referring to Properties
The familiar dot operator gives us access to an object’s properties. We separate the name of the property from the object it belongs to using a dot (a period), as follows:
objectName
.propertyName
where objectName
is the name of our object
and propertyName
must be a legal
identifier that matches the name of some property of
objectName
.
For example, if we have a ball
object instance
with a radius
property, we can access
radius
using:
ball.radius
Alternatively, we may refer to a property using the
[]
operator, as
follows:
objectName
[propertyName
]
The []
operator allows us to compose a property
name using any expression that resolves to a string. For example:
trace(ball["radius"]);
var prop = "radius";
trace(ball[prop]); // prop
resolves to "radius"
Built-in ActionScript properties are accessed in exactly the same way. Recall the syntax for retrieving the value of pi:
Math.PI
In that expression, we’re accessing the built-in
PI
property of the Math
object.
However, in pure OOP, we’ll nearly never access an
object’s properties directly; instead, we’ll use methods
to access property values. For example, to check the
volume
property of an instance ...
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