Question

I’m trying to highlight the table cell that is in row, col.

var $tr = $('tr:eq(' + row + ')');
$($tr:nth-child(col)).addClass('highlight');

Answer 1

var $tr = $('tr:eq(' + row + ')');
$tr.find(':nth-child(' + col + ')').addClass('highlight');

Using .find(). See demo.

Alternatively, if you don’t need a reference to the $tr, you can do it all via one selector:

$('tr:eq(' + row + ') :nth-child(' + col + ')').addClass('highlight');

(Demo)

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As for your question

How to use a jQuery variable in a selector

The result of (all) jQuery traversal / selector methods is of type jQuery. This makes it possible to do advanced chaining. Also, it is on this type that the jQuery methods is defined. This means that you need to perform filtering etc. on the resulting object. Thus, the following is invalid (: is not a JavaScript operator, nth-child() is (probably) not defined as a JavaScript function):

$($tr:nth-child(1)).addClass('highlight');

But can be rewritten as:

$tr.find(':nth-child(1)').addClass('highlight');

Here we’re applying the selector :nth-child(1) on the jQuery object assigned to $tr. This object can hold a reference to any number of DOM elements, even zero!