I have always found jQuery selectors very complex and have never quite understood how to use them properly.   I just found this article from the website Optimize your jQuery selectors for best performance.  It is a fantastic article and full credit goes to them.  The only reason I am copying it here is because I want to make sure that the information stays in my toolbox.   Please check out their page to see more information.

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• ID selector $(‘#elementID’)
• Tag selector $(‘p’)
• Class selector $(‘.CSSClass’)
• Attribute selector $(‘[type=”text”]‘)
• Pseudo selector $(‘:visible’)

Always use ID selector if possible

Accessing DOM is a very expensive operation, so it’s beneficial to minimize effort and time. As we all know, the ID attribute is unique for each HTML element on the page. in JavaScript document.getElementById() is the function that one would use to select the HTML element. It’s the fastest and the best way to select the element because it directly maps to the element. jQuery is a library written on top of JavaScript, which means that it internally calls the JavaScript functions to do the job. When you use ID as a selector in jQuery, it internally calls document.getElementById(). To select the HTML element with elm as ID, the jQuery code looks like this: $(“#elm”);
This method works well in all browsers, so it’s a good choice if you are using an older browser.

Cache your selector

Caching improves the performance of the application. You can cache data or objects for better performance. You can also cache your jQuery selectors for better performance using the ID as your selector (as mentioned in the previous tip). When you don’t cache the selector, jQuery must rescan the DOM to get the element. You may not feel the difference in small applications, but with large applications this becomes very critical. Let’s look at the process of caching objects in jQuery. The following simple line of code caches the selector and stores it in the $elm variable:

var $elm = $("#elm");

Now use the $elm variable instead of using the jQuery ID selector. Like this:

var $elm = $("#elm");
 $elm.addClass(‘dummy’);

Remember, the scope of a variable is limited to where it is defined. If it is defined as a global variable then you can use it anywhere in the jQuery code, but if it is inside a method the scope will be limited to that particular method only. Global caching is useful for elements which are frequently used in the code.

Define a context with jQuery selectors

By default, jQuery selectors perform their search within the DOM. But while defining the selector, you can pass the context, which limits the searching range for the jQuery selector. In other words, you are instructing jQuery to look inot the context rather than beginning the search from the document root. This helps in speeding up the searching process which will definitely improve the performance. Passing the context is optional, but you should use it whenever you can.
Enough theory! Let’s take a look at a practical example. Consider the following HTML code:

<div id="”parent1”">
<div class="”child”"></div>
<div class="”child”"></div>
<div class="”child”"></div>
</div>

If you want to select all the div element with child class you can use the following jQuery code:

var $elm = $(".child");

The above code will search for elements with child class starting from the document root. It can be optimized by passing via an alternate selector. Like,

var $parent = $("#parent1");
 var $elm = $(".child", $parent);

This limits the search range, so now searching is limited to the div with ID parent1. The context argument can be a DOM element, a document, or a jQuery object. If you pass context, then jQuery will internally use the find method to retrieve the elements. So the above code is internally converted to:

var $elm = $parent.find(".child");

jQuery first checks if the passed context is a jQuery object. If it is, it calls the find() method on the given context. If the given context is a DOM element, it first converts it to a jQuery object and then executes the find() method. As such, it is better to pass the object as context instead of the DOM element because it will reduce the conversion time.

Don’t repeat your selectors

As mentioned earlier, you can cache the jQuery selectors which prevents you from repeating your selector. jQuery also offers chaining, which allows you to chain multiple methods in a single call. Consider the following jQuery code:

$("div").css("color", "red");
 $("div").css("font-size", "14px");
 $("div").text("New Text!");

Below is the first optimized version using caching:

var $div = $("div");
 $div.css("color", "red");
 $div.css("font-size", "14px");
 $div.text("New text goes here!");

Next is the second optimized version using chaining:

$(“div”).css({“color”, “red”, “font-size”, “14px”}).text(“New text goes here!”);

Use class selector wisely

jQuery also provides a CSS class selector which allows you to select elements with a particular CSS class. This is again a very useful and popular selector as it allows you to select multiple elements at once. However, you have to be cautious while using class selector. The following jQuery code will select all the elements with “dummy” class applied to them:
$(“.dummy”)

In this case, jQuery has to scan the complete DOM to discover elements with dummy class. As mentioned earlier, traversing DOM is a very expensive process so it’s always better to minimize this effort. In this case, you can pass a tag name to reduce the searching scope. Like this:

$("div.dummy");

The above code tells jQuery to only give me the DIV elements with dummy CSS class applied. Though the class selector works quite well in modern browsers, older browsers have performance issues with class selector. That being said, it’s not always a good choice to add a tag name with class selector. Why?

In this example, jQuery will first search for all elements with class dummy and then filter records to only return those elements that are div elements. So when the dummy class is only meant for div elements, you need to specify the tag name in order to add an extra step of filtering. Keep in mind that this is only useful when the CSS class is applied to different HTML elements like span, paragraph and div.

Be very specific about selectors

Did you know that jQuery selectors are executed from right to left? In other words, the last selector will be executed first when there are multiple selectors. Consider the following example:

$("div.parent .child");

In this example, jQuery will first search for all elements with class child (last selector executed first) and then apply a filter to return only those elements which are div elements with a parent class applied. This is the optimized version:

$(".parent div.child");

Here we are more specific on the last selector, which helps to speed up performance!

Look for an alternative for the Pseudo selector

Pseudo class selectors are CSS selectors with a colon preceding them. They are also available with jQuery – :visible, :checked or :first. Pseudo selectors are useful for selecting elements with different states, or a particular element from a set of elements, but they are slower compared to other jQuery selectors. You should either find a way to replace the pseudo selector or be very specific in using it. Let’s take a look at examples of both. The following jQuery code selects all elements which are visible:

$(":visible");

You can be more specific here. Like,

$("input:visible");

or even better,

$("form").find("input:visible");

Pseudo selectors like :first, :last and :eq allow you to select a particular element from a set of elements. As an example, the following jQuery code will select the first table row using the :first pseudo selector.

$("tr:first")

The above code can be replaced with better performing code, but first you need to cache the selector (table row in this case).

var tRows=$('tr');

Since jQuery stores this as an array, we can take advantage of it. To get the first table row:

var firstRow=$(tRows[0]);

To get the last element (:last),

var lastRow = $(tRows[tRows.length - 1]);

Or to get any nth row (:eq(n)),

var nRow=$(tRows[n]);