by Rasheed Bustamam

How to use jQuery Selectors and CSS Selectors, and the basics of how they work

Conjure an army of HTML elements

The other day, I was interviewing someone who had finished the front-end certification of freeCodeCamp. He had also graduated from a rather prestigious bootcamp, where attendees coded from 8am to 8pm for six weeks straight. Yikes!

His programming ability was great, but I was surprised to see that his knowledge in CSS was lacking. And by lacking, I mean he didn’t know how to select a class to apply a style. This doesn’t necessarily reflect negatively on him. If anything, it highlights how a lot of programmers view CSS.

Many people think learning CSS is unimportant, since designers will usually be able to implement the CSS for you. While this is true, there are many times that you (as the programmer) will need to know some basic CSS in order to select an element and tell it to do something when something else happens.

For example, Green Sock Animation Platform (GSAP) is probably too program-y to be designer-centric. It requires a developer with a knowledge of CSS as well as programming.

I’m not saying that every developer needs to be a CSS master. But I think if you’re going to call yourself a full-stack developer, you should know the basics of CSS. And the basics start with the selector.

Disclaimer: jQuery selectors aren’t actually unique to jQuery — they’re actually CSS selectors. However, if you’re anything like me, you learned jQuery before you properly learned CSS, and therefore automatically associated selectors with jQuery. Though this article is about CSS selectors, it will help you if you need some clarifications on jQuery selectors as well.

The CSS Selector

I always find it helpful to play with code, so here’s a simple CodePen for playing with basic selectors.

In HTML, there are three ways to label or categorize elements. The first way is the broadest: by tag name. For example, you can select all divs on your page by using the simple selector div. Hey, that was easy!

The second way is probably the one you’ll use the most: the class attribute. You can select by class by using a dot (.) , so in the example above, to select the all elements with class section I use .section as a selector.

The third way is often overused, but still useful, and that’s the id attribute. IDs should identify your elements, just as your SSN (in the US) might identify you as a person. That means that IDs should be unique across the entire page. To select an element with a specific ID, you use the hashtag (#), or octothorpe as I like to call it. To select the element with the ID of other I use #other.

These are the most basic selectors. To recap:

  • select by tag name (no prefix)
  • select by class name (prefix of .)
  • select by ID (prefix of #)

These three selectors alone will allow you to select almost anything on your webpage.

Quiz 1

  1. How would you select all the paragraph tags on the page? (hint: paragraph tags are p)
  2. How would you select all items with class of button-text?
  3. How would you select the element with ID of form-userinput?

Feel free to share your answers in the comments!

Do all IDs need to be unique?

This is a small digression that I feel is very important. But if you’re only here to learn how to use selectors, feel free to skip to the next section

With a name like ID, you’d assume that every HTML ID must be unique or your HTML document will not work. After all, trying to have two const variables will make many editors yell at you, so wouldn’t HTML yell too?

The problem is, HTML will not yell at you. In fact, no one will even tell you anything is wrong. You may find a bug that stems from a non-unique ID. But you will drive yourself crazy trying to figure out the root cause of the bug, because it is a very subtle failure.

The example above demonstrates why having duplicate IDs might cause an issue on your webpage. First, there are actually two divs with the ID other. If you comment the styles for #other, you’ll see that both items actually get styled. This might make you think, “well hey, I can use IDs and classnames interchangeably!”

Not so fast. If you look in the JavaScript panel, you’ll see that I selected particular items based on their element label: tag name, class name, or ID. You’ll notice that document.getElementsByTagName and document.getElementsByClassName return a collection of all matching HTML elements. document.getElementById returns only the first HTML element it can find with the matching ID. You can verify this by uncommenting the getVanillaSelectors function and checking the console.

Using document.getElement(s)By[Type]

To further complicate things, if you use JavaScript’s querySelectorAll method (which takes in a CSS selector as input), you get a totally different result.

Using document.querySelectorAll

And just to mess with you, jQuery does something different despite having a similar syntax to querySelectorAll.

Using jQuery selector

I have no explanation for the different behavior. However, I can tell you how to avoid it. Here are my rules:

  1. Never use IDs. Use class attributes instead.
  2. If I need to use an ID, name-space it such that it will be unique even if a similar item exists on the page; for example menu-item-01

Sometimes forms, and their input fields, may need to have IDs. In that case, you can follow rule number 2. Here’s how I would namespace a form for user-signup:

<form id="user-signup">  <input id="user-signup-userid" label="user id" />  <input id="user-signup-password" label="password" /></form>

That way, if I have two forms on the same page (let’s say user-signup and user-signin), they are guaranteed to have unique IDs. Even if the userID fields are similar between the forms.

Combining Selectors

Sometimes a single selector just won’t cut it. Sometimes, you need to get every div that has a class name of section . Other times, you need every element with a class name of section that is a child element to a div with ID user-signup . There are many possible other selector combinations.

In this section, you will learn three ways of combining selectors, and I am confident that these will suit 90% of your needs. If you find that more than 11% of your needs are unfulfilled, come complain to me and I’ll edit that to say 89% of your needs :).

As before, let’s start with a CodePen:

Combining selectors for a single element

OK, first let’s cover combining selectors for a single element. This means selecting the element that has tag name of x, AND class name of y, AND ID of z. Of course, you don’t need all three, but you can combine all three.

Let’s say we want to select all divs with class of item. To do that, we combine the two: div.item. It goes from most general to most specific from left to right. That selects all div tags that ALSO have the class name of item. It’s important to note that there is no space between div and .item. Adding a space changes the selector completely, as I will go over in the next section.

If you uncomment the corresponding CSS, you’ll see that the section with the class name of item did not turn red. That’s because it’s not a div tag.

You can do this same pattern with class names and IDs. But if you have an element’s ID, you may as well just use the ID. There’s no reason to select an ID with a specific class name, because if you followed the ID rules above, you only have one item with that ID anyway.

But, just for parity, here’s an example of selecting the div with class of item and ID of other: div.item#other. Again, going from left to right, it goes most general to more specific.

Most likely you’ll use this syntax to select an element that has multiple classes. To do this, just separate all the classes with periods. To select all elements that have BOTH classes item and section, you’d use .item.section. The order doesn’t matter when you’re doing it this way, so .section.item will work too.

This one trick will allow you to be more specific in your selectors.

The “child” selector

The second way you can combine selectors is by using the “child” selector, as I like to call it. There’s two ways of doing this, so I’ll start with the most general.

First, you can select any child of a certain element by adding a space. For example, to select all child items of the #other element, it’d be #other .item. Notice the space between the selectors.

Second, you can select immediate children of a certain element by using the &gt;. An “immediate child” of an element is one that is only one level deep. In the example, there are two .item elements contained in the #other element, but one of the .item elements is wrapped up in a .wrapper element, so that one is not an immediate child.

To give you a visual, if you collapse everything underneath the #other element, you’d see this:

Collapsed children

Those two are the only direct children of the #other element. To select only the direct child .item of #other, you’d use #other > .item, which would select the direct child, but not the one underneath .wrapper. Nifty, huh?

Quiz 2

  1. How would you select all the paragraph tags that belong to section elements? (hint: paragraph tags are p)
  2. How would you select all items with class of button-text, that are descendants to items with class of button?
  3. How would you select the element with class of form-input that is a direct child of form elements?
  4. Bonus question: explain what this selector selects: header.title > form.user-signup button.button-danger

As before, feel free to share your answers in the comments!

Putting it all together — literally

You can combine combined selectors. Really. The bonus question above shows an example of it, but I’ve added some combinations at the end of the example codepen.

For example, you can choose all .items that are children of divs with class of parent-item by using div.parent-item .item. Whoa!

You can choose direct descendants as well. For example, to select all divs with class of item that are direct descendants to divs with class of parent-item: div.parent-item > div.item.

And finally, just to mess with you, you can walk down the entire DOM tree: div.parent-item .coolest-item .item. item class that is child to coolest-item class that is child to div with class of parent-item.

Note that it’s generally not a good idea to go beyond two or three levels of depth when nesting selectors. Then you get into weird realms of specificity that you can solve more effectively by naming your CSS classes better. But that’s out of the scope of this article. If you want to know more, let me know and I’ll write about it.

Bonus: Using Chrome DevTools to get a selector

You can use Chrome DevTools to get the selector of any element you can select on the DOM. Here’s how:

  1. Open Chrome DevTools. Since you’re selecting an element, go ahead and right click the element you want to select and click on “Inspect”:
Yep, it’s a picture of me writing this article

2. Right click the DOM element you want to select and hover over “Copy,” then click on “Copy Selector:”

I like the Dark Theme.

3. That’s it! The selector I copied, by the way, is #editor_93 > section > div.section-content > div:nth-child(3) > p.graf.graf — p.graf-after — figure.graf — trailing.is-selected. You can copy the selector into document.querySelector or jQuery and get the element.

This is the selector for the image above.

Hopefully this writeup was helpful! If you enjoyed it, please give me some claps so more people see it. Thanks!