ActionScript: The Definitive Guide

The for-in Loop

A for-in statement is a specialized loop used to list the properties of an object. New programmers may want to skip this section for now and return to it after reading Chapter 12.

Rather than repeating a series of statements until a given test expression yields the value false, a for-in loop iterates once for each property in the specified object. Therefore, for-in statements do not need an explicit update statement because the number of loop iterations is determined by the number of properties in the object being inspected. The syntax of a for-in loop looks like this:

for (var thisProp in object) {
  substatements;  // Statements typically use thisProp in some way
}

The substatements are executed once for each property of object; object is the name of any valid object; thisProp is any variable name or identifier name. During each loop iteration, the thisProp variable temporarily holds a string that is the name of the object property currently being enumerated. That string value can be used during each iteration to access and manipulate the current property. The simplest example of a for-in loop is a script that lists the properties of an object. Here we create an object and then itemize its properties with a for-in loop:

var ball = new Object( );
ball.radius = 12;
ball.color = "red";
ball.style = "beach";

for (var prop in ball) {
  trace("ball has the property " + prop);
}

Because prop stores the names of the properties of ball as strings, we can use prop with the [] operator ...