JavaScript null reference error

Question

I have a piece of code shown below that is supposed to select the first object in a list of image object. When it runs, about half of the time I am getting an error:

TypeError: Cannot read property of mylist

I realize that this is happening because it is trying to reference an object that has not yet been given a reference to an object. Basically, the page will load, and I want to call this function in JavaScript, however if this code runs before the list is referenced, it errors out. Here is my code snippet:

window.onload = function () {
       try {
            var tnl = $find('mylist');


            tnl.selectFirstItem();
       }
       catch (exception) {
            alert(exception);
       }
}

The

tnl.selectFirstItem()

line is the one throwing the error to the catch block.

So, my question is, is there a way to force it to wait until this object "tnl" has been referenced?

Thanks!

Kolby · Accepted Answer · 2014-10-10 19:47:09Z

2

var tnl = $find('mylist');

should be

var tnl = $('#mylist');

assuming mylist is an ID.

You're using find wrong.

First you get your element, then you use it to find a child element. So you could do this:

var container = $('.containerElement');
container.find('#mylist');

answered Oct 10, 2014 at 19:47

Kolby

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8 Comments

Matt Jones Over a year ago

No, I am referencing an object created in HTML with ID="mylist" that's not the issue.

Matt Jones Over a year ago

It works about half the time as it is. Trust me it is not the issue.

Kolby Over a year ago

then you have other issues.Actually, it looks like you're not providing the right code. The error is saying the variable mylist is null. Where is that variable being set at?

Kolby Over a year ago

What is selectFirstItem()? I answered prematurely because you haven't provided us the right information on your project. What kind of javascript library are you using?

Matt Jones Over a year ago

Basically, that is all irrelevant. I guess I should have worded my question better. Really what I need to know if how can I force my Javascript to loop at the line tnl.selectFirstItem(); until the tnl has been initialized?

|

Igor · Accepted Answer · 2014-10-10 20:09:07Z

1

Here is the "loop" you are asking for. But this is still wrong. If tnl is not assigned after $find('mylist'); returns, it is not going to become assigned later. You likely have error thrown by $find call.

window.onload = function () {
  try {
    var tnl = $find('mylist');

    var timer = setInterval(function() {
      if (tnl) {
        clearInterval(timer); 
        tnl.selectFirstItem();
      }
    }, 1000);

  } catch (exception) {
    alert(exception);
  }
}

answered Oct 10, 2014 at 20:09

Igor

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2 Comments

Igor Over a year ago

@MattJones - beats me

Matt Jones Over a year ago

Read comment I posted on other answer provided.

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