3.10 Doing Something with a Property of an Object

NN 2, IE 3

3.10.1 Problem

You want to examine (or modify) the values of properties belonging to an object, but the object and its properties may change from one examination to another.

3.10.2 Solution

Use a for/in loop to access every property of an object, regardless of the property's name. The following function assembles a list of properties and their values for any object passed as an argument to the function:

function listProperties(obj, objName) {
    var result = "";
    for (var i in obj) {
        result += objName + "." + i + "=" + obj[i] + "\n";
    }
    alert(result);
}

In this special type of loop, the variable (i in this example) is automatically assigned the name of each property (in string form) as the loop progresses through the list of available properties for the object. By using the string name as an index to the object (obj[i] in this example), the value of that property is returned.

3.10.3 Discussion

Figure 3-1 show what the alert dialog box generated by the function would display for one of the sales objects defined in Recipe 3.9.

Figure 3-1. Object property enumeration example

The type of property enumeration shown in the listProperties( ) function in the Solution is useful not only for custom objects but also for DOM objects. When using it with DOM objects, some browser-specific behaviors reveal themselves. For example, IE for Windows enumerates all of the event handler properties of the object. Netscape 6 and later enumerate properties and methods. Unfortunately, Opera does not handle DOM objects, but the for/in type of looping works with custom objects.

3.10.4 See Also

Recipe 3.4 for looping through all entries of an array.