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Data reactivity with vanilla JS

I’ve been writing a bit about Reef, my 2.5kb alternative to React and Vue. Today, I wanted to talk about data reactivity, and how Reef handles it under-the-hood.

State-based UI

Reef (and React, and Vue, and Angular) use something called state-based UI.

Let’s say you have a data object with some todo list items.

var data = {
	listName: 'Starting at Hogwarts',
	todos: []

This is often referred to as state in the framework world, because it represents the state of your data at a particular moment in time.

With state-based UI, you specify how the UI should look based on the state, or data.

If there’s no data.todos at all, you might display a message to create some. If there are todo items, you want to display them. Reef handles this by letting you specify a template function that accepts the data object as an argument.

var template = function (data) {

	// Create the list heading
	var html = '<h1>' + data.listName + '</h1>';

	// If there are no list items, show a message
	// Otherwise, display the list items
	if (data.todos.length < 1) {
		html += '<p><em>You do not have any items to complete yet. Please add some.</em></p>';
	} else {
		html += '<ul>' + (todo) {
			return '<li>' + todo + '</li>';
		}).join('') + '</ul>';

	// Return the template
	return html;


We can then inject the UI into the DOM. A simple approach might use innerHTML.

app.innerHTML = template(data);

Reef using DOM diffing. As we learned in this week’s two part series (if you missed it, here are part 1 and part 2), it renders the UI like this.

// Get DOM maps
var templateMap = createDOMMap(stringToHTML(template(data)));
var domMap = createDOMMap(app);

// Diff the DOM
diff(templateMap, domMap, app);

So… what’s data reactivity?

With a state-based UI, if the data object (or state) changes, we need to rerender the UI.

Let’s say you added todo items. You’d then need to update the UI to show them.

// Update the data
data.todos = [
	'Fix my wand',
	'Buy new robes',
	'Enroll in courses'

// Render a new UI
// (DOM diffing is better. I'm using this approach here for simplicity.)
app.innerHTML = template(data);

Ideally, though, you wouldn’t have to do this manually. The UI should just react to updates to your data.

That’s data reactivity.

(As you’ve probably already guessed, that’s also where React gets its name.)

Implementing data reactivity with vanilla JS

Data reactivity isn’t particularly hard. There are a few ways to do it, but the simplest is the way React does it: with a helper function.

Reef using a method called setData() to update the data object.

You pass in an object with the properties you want to update (and their new values). It loops through each one, updates the data object, and then triggers a fresh render of the UI for you.

 * Reactivity update the data object
 * @param {Object} obj The data to update
var setData = function (obj) {
	for (var key in obj) {
		if (obj.hasOwnProperty(key)) {
			data[key] = obj[key];
	app.innerHTML = template(data);

You don’t need to pass in all properties—just the ones you’re updating.

Using our todos from earlier, you would do something like this. It updates the data object and renders a fresh UI in one step.

setData({todos: [
	'Fix my wand',
	'Buy new robes',
	'Enroll in courses'

Try it yourself

Here’s a demo on CodePen that you can play with.