Javascript - Error creating objects on the fly

Question

I am trying to simulate data to a function that would usually receive a JSON parsed data structure. When running this I get an error TypeError: can't convert undefined to object here: data.targets[i] = {

What am I doing wrong?

function SendFakeTargets(maxTargets, interval) {
    var data = {};
    data.timestamp = +new Date;

    var time = data.timestamp * 0.0005;
    var x = Math.sin(time) * 192 + 256;
    var y = Math.cos(time * 0.9) * 192 + 256;

    console.log(x, y);

    for (var i = 0; i < maxTargets; i++) {
        console.log(i);

        data.targets[i] = { //error is here
            id: i,
            x: x + (i * 10),
            y: y + (i * 10)
        };
    }

    HandleTargetData(data);

    setTimeout("SendFakeTargets("+maxTargets+", "+interval+")", interval);
}

please don't use a string as the callback parameter to setTimeout ... — Alnitak
– Alnitak, Commented Oct 4, 2012 at 13:35

Fabrizio Calderan · Accepted Answer · 2012-10-04 13:34:13Z

4

you should previously declare

data.targets = [];

before using data.targets[i] inside the loop, otherwise data.targets is undefined. In a shorter way you could write

var data = {
    timestamp : +new Date,
    targets   : []
};

And as a side note, never use strings in setTimeout/Interval. Do instead

 setTimeout(function() {
      SendFakeTargets(maxTargets, interval);
 }, interval);

edited Oct 4, 2012 at 13:34

answered Oct 4, 2012 at 13:28

Fabrizio Calderan

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2 Comments

Chris Over a year ago

Thanks, what is the reasoning behind using a function call as opposed to a string? Is it solely to avoid an eval or is there another reason?

Fabrizio Calderan Over a year ago

yep, it is to mainly avoid an eval (and for better readability - especially of arguments - imho)

Grahame A · Accepted Answer · 2012-10-04 13:30:40Z

1

I think you need to initialize the targets array before using it, as it is undefined. You are defining data as

var data = {}

which is declaring it as an empty object, anything else that you're doing with it is added on the fly - arrays need to be initialized before you can call any index in them. I believe what you need to do is:

var data = { targets: [] }

answered Oct 4, 2012 at 13:30

Grahame A

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Comments

Waterboy · Accepted Answer · 2012-10-04 13:29:17Z

0

You never declared "data.targets" as a an object so javascript doesn't know how to assign anything to it.

At the top of your code just define "targets":

data.timestamp = +new Date;
data.targets = {} or [];
...

answered Oct 4, 2012 at 13:29

Waterboy

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