What is Reactivity?

December 14, 2023

1,274 words

Post contents

This article is intended for newcomers to HTML and JavaScript programming. However, it's suggested that you read this article explaining what the DOM is first.

As an experienced frontend engineer, I'm often asked:

"Why would you want to use a modern frontend framework like React, Angular, or Vue?"

While I have a whole (free) book on the topic, my short answer is typically "Reactivity". The follow-up response I usually get from this is:

"What is reactivity?"

In short, Reactivity is the ability to reflect what's in your JavaScript application's memory on the DOM as HTML.

See, when you're building a website using only static HTML, the output to the DOM is straightforward.

html
<!-- index.html -->
<main id="a">
<ul id="b">
<li id="c">Item 1</li>
<li id="d">Item 2</li>
</ul>
<p id="e">Text here</p>
</main>
A chart showing the document object model layout of the above code. It shows that the 'main' tag is the parent to a 'ul' tag, and so on

The problems start when we want to introduce interactivity into our output.

Let's build a small-scale application that:

  • Has a button with a counter inside of it
  • Start the counter at 0
  • Every time the button is clicked, add one to the counter
A 'main' tag that has a button child. This button updates a JavaScript count when pressed and when that value is updated, will change the text of the button

To do this, let's start with some HTML:

html
<main>
<button id="add-button">Count: 0</button>
</main>

Then we can add in the required JavaScript to make the button functional:

html
<script>
let count = 0;
const addBtn = document.querySelector('#add-button');
addBtn.addEventListener('click', () => {
count++;
addBtn.innerText = `Count: ${count}`;
});
</script>

Adding a List

Not too bad, let's increase the difficulty a bit by:

  • Adding an unordered list (<ul>)
  • Every time count is increased, add a new <li> with a unique string inside
The main has a button and an unordered list. This button updates the JavaScript count value, which in turns updates the button text and adds a list to the ul element

That might look something like this:

html
<main>
<button id="add-button">Count: 0</button>
<ul id="list"></ul>
</main>
<script>
let count = 0;
const listEl = document.querySelector('#list');
function makeListItem(innerText) {
const li = document.createElement('li');
li.innerText = innerText;
listEl.append(li);
}
const addBtn = document.querySelector('#add-button');
addBtn.addEventListener('click', () => {
count++;
addBtn.innerText = `Count: ${count}`;
makeListItem(`List item: ${count}`);
});
</script>

Removing items from the list

Okay! Things are heating up! For one last exercise, let's:

  • Add a button that removes 1 from count
  • When this button is pressed, remove the last element from the list
There are two buttons now, each change the JavaScript count value. This count value is then reflected in the text nodes for each button, and the list respectively

Notice how complex our logic tree is getting?

html
<main>
<button id="add-button">Add one to: 0</button>
<button id="remove-button">Remove one from: 0</button>
<ul id="list"></ul>
</main>
<script>
let count = 0;
const listEl = document.querySelector('#list');
function makeListItem(innerText) {
const li = document.createElement('li');
li.innerText = innerText;
listEl.append(li);
}
function removeListItem() {
listEl.lastChild.remove();
}
const addBtn = document.querySelector('#add-button');
const removeBtn = document.querySelector('#remove-button');
function updateBtnTexts() {
addBtn.innerText = `Add one to: ${count}`;
removeBtn.innerText = `Remove one from: ${count}`;
}
addBtn.addEventListener('click', () => {
count++;
updateBtnTexts();
makeListItem(`List item: ${count}`);
});
removeBtn.addEventListener('click', () => {
count--;
updateBtnTexts();
removeListItem();
});
</script>

Wow! That got complex, quick, didn't it?!

Exactly... That leads me to the question:

Shouldn't it be simpler?

Notice how each time we added another item that depended on count, our data didn't change. Instead, we had to add ever increasing levels of complexity to our codebase to glue our JavaScript state to the DOM representation of said state.

If we strip away all of this glue, we're left with a drastically simplified codebase:

html
<main>
<button id="add-button">Add one to: 0</button>
<button id="remove-button">Remove one from: 0</button>
<ul id="list"></ul>
</main>
<script>
// Magical land where `count` changes auto-update the DOM
let count = 0;
addBtn.addEventListener('click', () => {
count++;
});
removeBtn.addEventListener('click', () => {
count--;
});
</script>
The two buttons update the count, but that's all there is. Everything else is handled invisibly to you

Look at how many lines disappeared!

Not only is this nicer method of writing code theoretically possible, it's widely adopted by millions of developers via a frontend framework.

Some examples of frontend frameworks include:

These frameworks allow you to write code that focused on the data in JavaScript, rather than how it will be bound to the DOM:

jsx
const App = () => {
const [count, setCount] = useState(0);
return (
<div>
<button onClick={() => setCount(count + 1)}>Add one to: {count}</button>
<button onClick={() => setCount(count - 1)}>
Remove one from: {count}
</button>
<ul>
{Array.from({ length: count }).map((_, i) => (
<li>List item {i}</li>
))}
</ul>
</div>
);
};
typescript
@Component({
selector: "app-root",
standalone: true,
imports: [NgFor],
template: `
<button (click)="count = count + 1">Add one to: {{ count }}</button>
<button (click)="count = count - 1">Remove one from: {{ count }}</button>
<ul>
<li *ngFor="let item of [].constructor(count); let i = index">
List item {{ i }}
</li>
</ul>
`,
})
export class AppComponent {
count = 0;
}
vue
<script setup>
import { ref } from "vue";
const count = ref(0);
</script>
<template>
<button @click="count++">Add one to: {{ count }}</button>
<button @click="count--">Remove one from: {{ count }}</button>
<ul id="list">
<li v-for="(_, i) of [].constructor(count)">List item {{ i }}</li>
</ul>
</template>

This, dear reader, is the core idea behind reactivity: Allowing us to focus on how we want to change the state stored in JavaScript and allowing some other mechanism to abstract away how it shows up on-screen.

These mechanisms can have wildly different methods to them, too!

For example, here's what each of the frameworks utilize under-the-hood:

FrameworkReactivity MethodRendering Method
ReactExplicit Function CallsVDOM
AngularZone.jsIncremental DOM
VueProxiesVDOM

This is real nerd hours, don't feel bad if this just looks like gibberish to you right now.

Conclusion

This has been a look at what reactivity is and why you might want to use a modern frontend framework to utilize it in your apps today.

Next time, we'll talk about what "Reconciliation" is and how it impacts most React and Vue frontend applications today.

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