I have function that scrolls to a anchor position in a list

   function snapToAnchor(anchor) {
    $('#divProductScroll').scrollTop(0);
    var offset = $(anchor).offset().top - $('#divProductScroll').offset().top;
    $('#divProductScroll').scrollTop(offset);
    //$('#divProductScroll').animate({ scrollTop: offset }, 250);
}

but this is giving me error some times

saying Cannot read property 'top' of null

I am not so good with JavaScript

Can any one help with this issue?

I found the Problem.

I have function that scrolls to a anchor position in a list

   function snapToAnchor(anchor) {
    $('#divProductScroll').scrollTop(0);
    var offset = $(anchor).offset().top - $('#divProductScroll').offset().top;
    $('#divProductScroll').scrollTop(offset);
    //$('#divProductScroll').animate({ scrollTop: offset }, 250);
}

but this is giving me error some times

saying Cannot read property 'top' of null

I am not so good with JavaScript

Can any one help with this issue?

I found the Problem.

• javascript
• nullpointerexception

• 2 I'm guessing you're not passing a legitimate tag name to the function in the anchor parameter. – Jonathan M Commented Oct 28, 2011 at 18:58
• one of those two .offset() calls is returning a null. Split up the substraction and assign each offset to a var, and see which one es out null. – Marc B Commented Oct 28, 2011 at 18:59
• could learn how to use firebug, then put a break inside the function, so you can see what's in the variable anchor – yitwail Commented Oct 28, 2011 at 19:03

3 Answers 3

The anchor variable does not contain a valid selector for you site. Calling offset on this will return null.

Example

var a = $('#non-existing-id'); //returns empty object
var b = a.offset(); // returns null
b.top; //throws a TypeError exception

You can simply debug your program by inserting alert(anchor) on line 3. This will show you the contents of the variable.

if anchor has id of the DOM element - then correct way to use it with jQuery:

$('#' + anchor).offset()

otherwise you'll get a null (and error).

$(anchor).offset().top

will usually output a string not a number... something like 100px. you need to extract the number from the string.

var topoffset = $(anchor).offset().top;
topoffset = topoffset.split('px', 1);