In Javascript: Losing type of object when passed from Array?

Question

I'm passing an object of a certain type to an array. When I print the object from an outside reference I can see all the fields. When I print the same object from the array reference, I see almost none.

To Explain, here is the code that illustrates the problem:

var test = new OpenLayers.Layer.Vector();

var array = [test];

console.log("******* Printing Test directly ***************");
printObjectProperties(test);

console.log("****** Printing Test from array **************");
for (obj in array){
    printObjectProperties(obj);     
}

The printObjectProperties() function:

function printObjectProperties(eventData) {
        for (var prop in eventData) {
            if (!(typeof (eventData[prop]) == "undefined")) {
                console.log(prop + " = " + eventData[prop]);
            }
        };
}

The complete code (complete html file, you can run and see for youself in the browser console log): (live example)

<html>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js" type="text/javascript"></script>
<script src="http://www.openlayers.org/api/OpenLayers.js"></script>
<script>

    $(document).ready(function(){

        var test = new OpenLayers.Layer.Vector();

        var array = [test];

        console.log("******* Printing Test directly ***************");
        printObjectProperties(test);

        console.log("****** Printing Test from array **************");
        for (obj in array){
            printObjectProperties(obj);     
        }

    });


function printObjectProperties(eventData) {
        for (var prop in eventData) {
            if (!(typeof (eventData[prop]) == "undefined")) {
                console.log(prop + " = " + eventData[prop]);
            }
        };
}

</script>
</html>

It is more helpful if you create a jsFiddle demo. I made one: jsfiddle.net/tu6er — Felix Kling
– Felix Kling, Commented Aug 8, 2011 at 11:23
Thanks. I actually created a demo, search for "live example" — Ben
– Ben, Commented Aug 8, 2011 at 11:24
Why don't you just pass the object/array to console.log instead of trying to format it yourself? Try this and you'll see all the properties: jsbin.com/evazip/3/edit#javascript,html — Matthew
– Matthew, Commented Aug 8, 2011 at 11:30
@Ben: for...in is used to loop over properties of objects. It is much slower than a for loop. For more reasons to not use it for arrays, have a look at developer.mozilla.org/en/JavaScript/Reference/Statements/… — Felix Kling
– Felix Kling, Commented Aug 8, 2011 at 14:13

pimvdb · Accepted Answer · 2011-08-08 11:24:58Z

3

In a for in loop, the first variable (obj) is the key. In an array that's the index.

So basically you're iterating over a number in printObjectProperties. Hence the 0 = 0, because the first index of your array (you have one element) is 0.

Use this instead:

printObjectProperties(array[obj]);

which would evaluate to:

printObjectProperties(array[0]);

or:

printObjectProperties(test);

answered Aug 8, 2011 at 11:24

pimvdb

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Comments

Evan Trimboli · Accepted Answer · 2011-08-08 11:25:20Z

1

You don't iterate over an array using for in:

for (var i = 0, len = array.length; i < len; ++i) {
    console.log(array[i]);
}

answered Aug 8, 2011 at 11:25

Evan Trimboli

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