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The button still doesn’t do anything, though: that’s where JavaScript comes in.

JavaScript is for behavior

JavaScript code defines the behavior of interactive or dynamic elements on a page. For example, the embedded CodePen examples in this article are powered by JavaScript.

Without JavaScript, to make the Add Item button in our example work would require us to use special HTML to make it submit data back to the server (, if you’re curious). Then the browser would discard the entire page and reload an updated version of the entire HTML file. If this shopping list was part of a larger page, anything else the user was doing would be lost. Scrolled down? You’re back at the top. Watching a video? It starts over. This is how all web applications worked for a long time: any time a user interacted with a webpage, it was as if they closed their web browser and opened it again. That’s not a big deal for this simple example, but for a large complex page which could take a while to load, it’s not efficient for either the browser or the server.

If we want anything to change on a webpage without reloading the entire page, we need JavaScript (not to be confused with Java, which is an entirely different language… don’t get me started). Let’s try adding some:

Now when we type some text in the box and click the “Add Item” button, our new item is added to the list and the item count at the top is updated! In a real app, we would also add some code to send the new item to the server in the background so that it will still show up the next time we load the page.

Separating JavaScript from the HTML and CSS makes sense in this simple example. Traditionally, even more complicated interactions would be added this way: HTML is loaded and displayed, and JavaScript runs afterwards to add things to it and change it. As things get more complex, however, we start needing to keep better track of things in our JavaScript.

If we were to keep building this shopping list app, next we’d probably add buttons for editing or removing items from the list. Let’s say we write the JavaScript for a button that removes an item, but we forget to add the code that updates the item total at the top of the page. Suddenly we have a bug: after a user removes an item, the total on the page won’t match the list! Once we notice the bug, we fix it by adding that same totalText.innerHTML line from our “Add Item” code to the “Remove Item” code. Now we have the same code duplicated in more than one place. Later on, let’s say we want to change that code so that instead of “(2 items)” at the top of the page it reads “Items: 2.” We’ll have to make sure we update it in all three places: in the HTML, in the JavaScript for the “Add Item” button, and in the JavaScript for the “Remove Item” button. If we don’t, we’ll have another bug where that text abruptly changes after a user interaction.

In this simple example, we can already see how quickly these things can get messy. There are ways to organize our JavaScript to make this kind of problem easier to deal with, but as things continue to get more complex, we’ll need to keep restructuring and rewriting things to keep up. As long as HTML and JavaScript are kept separate, a lot of effort can be required to make sure everything is kept in sync between them. That’s one of the reasons why new JavaScript frameworks, like React, have gained traction: they are designed to show the relationships between things like HTML and JavaScript. To understand how that works, we first need to understand just a teeny bit of computer science.

Two kinds of programming

The key concept to understand here involves the distinction between two common programming styles. (There are other programming styles, of course, but we’re only dealing with two of them here.) Most programming languages lend themselves to one or the other of these, and some can be used in both ways. It’s important to grasp both in order to understand the main benefit of HTML-in-JS from a JavaScript developer’s perspective.

• Imperative programming: The word “imperative” here implies commanding a computer to do something. A line of imperative code is a lot like an imperative sentence in English: it gives the computer a specific instruction to follow. In imperative programming, we must tell the computer exactly how to do every little thing we need it to do. In web development, this is starting to be considered “the old way” of doing things and it’s what you do with vanilla JavaScript, or libraries like jQuery. The JavaScript in my shopping list example above is imperative code.
  • Imperative: “Do X, then do Y, then do Z”.
  • Example: When the user selects this element, add the .selected class to it; and when the user de-selects it, remove the .selected class from it.
• Declarative programming: This is a more abstract layer above imperative programming. Instead of giving the computer instructions, we instead “declare” what we want the results to be after the computer does something. Our tools (e.g. React) figure out the how for us automatically. These tools are built with imperative code on the inside that we don’t have to pay attention to from the outside.
  • Declarative: “The result should be XYZ. Do whatever you need to do to make that happen.”
  • Example: This element has the .selected class if the user has selected it.

HTML is a declarative language

Forget about JavaScript for a moment. Here’s an important fact: HTML on its own is a declarative language. In an HTML file, you can declare something like:

<section>
  <h1>Hello</h1>
  <p>My name is Mike.</p>
</section>

When a web browser reads this HTML, it will figure out these imperative steps for you and execute them:

1. Create a section element
2. Create a heading element of level 1
3. Set the inner text of the heading element to “Hello”
4. Place the heading element into the section element
5. Create a paragraph element
6. Set the inner text of the paragraph element to “My name is Mike”
7. Place the paragraph element into the section element
8. Place the section element into the document
9. Display the document on the screen

As a web developer, the details of how a browser does these things is irrelevant; all that matters is that it does them. This is a perfect example of the difference between these two kinds of programming. In short, HTML is a declarative abstraction wrapped around a web browser’s imperative display engine. It takes care of the “how” so you only have to worry about the “what.” You can enjoy life writing declarative HTML because the fine people at Mozilla or Google or Apple wrote the imperative code for you when they built your web browser.

JavaScript is an imperative language

We’ve already looked at a simple example of imperative JavaScript in the shopping list example above, and I mentioned how the complexity of an app’s features has ripple effects on the effort required to implement them and the potential for bugs in that implementation. Now let’s look at a slightly more complex feature and see how it can be simplified by using a declarative approach.

Imagine a webpage that contains the following:

• A list of labelled checkboxes, each row of which changes to a different color when it is selected
• Text at the bottom like “1 of 4 selected” that should update when the checkboxes change
• A “Select All” button which should be disabled if all checkboxes are already selected
• A “Select None” button which should be disabled if no checkboxes are selected

Here’s an implementation of this in plain HTML, CSS and imperative JavaScript:

There isn’t much CSS code here because I’m using the wonderful PatternFly design system, which provides most of the CSS for my example. I imported their CSS file in the CodePen settings.

All the small things

In order to implement this feature with imperative JavaScript, we need to give the browser several granular instructions. This is the English-language equivalent to the code in my example above:

• In our HTML, we declare the initial structure of the page:
  • There are four row elements, each containing a checkbox. The third box is checked.
  • There is some summary text which reads “1 of 4 selected.”
  • There is a “Select All” button which is enabled.
  • There is a “Select None” button which is disabled.
• In our JavaScript, we write instructions for what to change when each of these events occurs:
  • When a checkbox changes from unchecked to checked:
    • Find the row element containing the checkbox and add the .selected CSS class to it.
    • Find all the checkbox elements in the list and count how many are checked and how many are not checked.
    • Find the summary text element and update it with the checked number and the total number.
    • Find the “Select None” button element and enable it if it was disabled.
    • If all checkboxes are now checked, find the “Select All” button element and disable it.
  • When a checkbox changes from checked to unchecked:
    • Find the row element containing the checkbox and remove the .selected class from it.
    • Find all the checkbox elements in the list and count how many are checked and not checked.
    • Find the summary text element and update it with the checked number and the total number.
    • Find the “Select All” button element and enable it if it was disabled.
    • If all checkboxes are now unchecked, find the “Select None” button element and disable it.
  • When the “Select All” button is clicked:
    • Find all the checkbox elements in the list and check them all.
    • Find all the row elements in the list and add the .selected class to them.
    • Find the summary text element and update it.
    • Find the “Select All” button and disable it.
    • Find the “Select None” button and enable it.
  • When the “Select None” button is clicked:
    • Find all the checkbox elements in the list and uncheck them all.
    • Find all the row elements in the list and remove the .selected class from them.
    • Find the summary text element and update it.
    • Find the “Select All” button and enable it.
    • Find the “Select None” button and disable it.

Wow. That’s a lot, right? Well, we better remember to write code for each and every one of those things. If we forget or screw up any of those instructions, we will end up with a bug where the totals don’t match the checkboxes, or a button is enabled that doesn’t do anything when you click it, or a row ends up with the wrong color, or something else we didn’t think of and won’t find out about until a user complains.

The big problem here is that there is no single source of truth for the state of our app, which in this case is “which checkboxes are checked?” The checkboxes know whether or not they are checked, of course, but, the row styles also have to know, the summary text has to know, and each button has to know. Five copies of this information are stored separately all around the HTML, and when it changes in any of those places the JavaScript developer needs to catch that and write imperative code to keep the others in sync.

This is still only a simple example of one small component of a page. If that sounds like a headache, imagine how complex and fragile an application becomes when you need to write the whole thing this way. For many complex modern web applications, it’s not a scalable solution.

Moving towards a single source of truth

Tools, like React, allow us to use JavaScript in a declarative way. Just as HTML is a declarative abstraction wrapped around the web browser’s display instructions, React is a declarative abstraction wrapped around JavaScript.

Remember how HTML let us focus on the structure of a page and not the details of how the browser displays that structure? Well, when we use React, we can focus on the structure again by defining it based on data stored in a single place. When that source of truth changes, React will update the structure of the page for us automatically. It will take care of the imperative steps behind the scenes, just like the web browser does for HTML. (Although React is used as an example here, this concept is not unique to React and is used by other frameworks, such as Vue.)

Let’s go back to our list of checkboxes from the example above. In this case, the truth we care about is simple: which checkboxes are checked? The other details on the page (e.g. what the summary says, the color of the rows, whether or not the buttons are enabled) are effects derived from that same truth. So, why should they need to have their own copy of this information? They should just use the single source of truth for reference, and everything on the page should “just know” which checkboxes are checked and conduct themselves accordingly. You might say that the row elements, summary text, and buttons should all be able to automatically react to a checkbox being checked or unchecked. (See what I did there?)

Tell me what you want (what you really, really want)

In order to implement this page with React, we can replace the list with a few simple declarations of facts:

• There is a list of true/false values called checkboxValues that represents which boxes are checked.
  • Example: checkboxValues = [false, false, true, false]
  • This list represents the truth that we have four checkboxes, and that the third one is checked.
• For each value in checkboxValues, there is a row element which:
  • has a CSS class called .selected if the value is true, and
  • contains a checkbox, which is checked if the value is true.
• There is a summary text element that contains the text “{x} of {y} selected” where {x} is the number of true values in checkboxValues and {y} is the total number of values in checkboxValues.
• There is a “Select All” button that is enabled if there are any false values in checkboxValues.
• There is a “Select None” button that is enabled if there are any true values in checkboxValues.
• When a checkbox is clicked, its corresponding value changes in checkboxValues.
• When the “Select All” button is clicked, it sets all values in checkboxValues to true.
• When the “Select None” button is clicked, it sets all values in checkboxValues to false.

You’ll notice that the last three items are still imperative instructions (“When this happens, do that”), but that’s the only imperative code we need to write. It’s three lines of code, and they all update the single source of truth. The rest of those bullets are declarations (“there is a…”) which are now built right into the definition of the page’s structure. In order to do this, we write our elements in a special JavaScript syntax provided by React called JSX, which resembles HTML but can contain JavaScript logic. That gives us the ability to mix logic like “if” and “for each” with the HTML structure, so the structure can be different depending on the contents of checkboxValues at any given time.

Here’s the same shopping list example as above, this time implemented with React:

JSX is definitely weird. When I first encountered it, it just felt wrong. My initial reaction was, “What the heck is this? HTML doesn’t belong in JavaScript!” I wasn’t alone. That said, it’s not HTML, but rather JavaScript dressed up to look like HTML. It is also quite powerful.

Remember that list of 20 imperative instructions above? Now we have three. For the price of defining our HTML inside our JavaScript, the rest of them come for free. React just does them for us whenever checkboxValues changes.

With this code, it is now impossible for the summary to not match the checkboxes, or for the color of a row to be wrong, or for a button to be enabled when it should be disabled. There is an entire category of bugs which are now impossible for us to have in our app: sources of truth being out of sync. Everything flows down from the single source of truth, and we developers can write less code and sleep better at night. Well, JavaScript developers can, at least…

It’s a trade-off

As web applications become more complex, maintaining the classic separation of concerns between HTML and JavaScript comes at an increasingly painful cost. HTML was originally designed for static documents, and in order to add more complex interactive functionality to those documents, imperative JavaScript has to keep track of more things and become more confusing and fragile.

The upside: predictability, reusability and composition

The ability to use a single source of truth is the most important benefit of this pattern, but the trade-off gives us other benefits, too. Defining elements of our page as JavaScript code means that we can turn chunks of it into reusable components, preventing us from copying and pasting the same HTML in multiple places. If we need to change a component, we can make that change in one place and it will update everywhere in our application (or in many applications, if we’re publishing reusable components to other teams).

We can take these simple components and compose them together like LEGO bricks, creating more complex and useful components, without making them too confusing to work with. And if we’re using components built by others, we can easily update them when they release improvements or fix bugs without having to rewrite our code.

The downside: it’s JavaScript all the way down

All of those benefits do come at a cost. There are good reasons people value keeping HTML and JavaScript separate, and to get these other benefits, we need to combine them into one. As I mentioned before, moving away from simple HTML files complicates the workflow of someone who didn’t need to worry about JavaScript before. It may mean that someone who previously could make changes to an application on their own must now learn additional complex skills to maintain that autonomy.

There can also be technical downsides. For example, some tools like linters and parsers expect regular HTML, and some third-party imperative JavaScript plugins can become harder to work with. Also, JavaScript isn’t the best-designed language; it’s just what we happen to have in our web browsers. Newer tools and features are making it better, but it still has some pitfalls you need to learn about before you can be productive with it.

Another potential problem is that when the semantic structure of a page is broken up into abstract components, it can become easy for developers to stop thinking about what actual HTML elements are being generated at the end. Specific HTML tags like