Reactivity Patterns are Core to Web Development

We handle a lot with JavaScript in websites and web apps since the browser is an entirely asynchronous environment. We must respond to user inputs, communicate with servers, log, perform, etc. All these tasks involve updates to the UI, Ajax requests, browser URLs, and navigation changes, making cascading data changes a core aspect of web development.

As an industry, we associate reactivity with frameworks, but you can learn a lot by implementing reactivity in pure JavaScript. We can mix and match these patterns to wire behavior to data changes.

Learning core patterns with pure JavaScript will lead to less code and better performance in your web apps, no matter what tool or framework you use.

I love learning patterns because they apply to any language and system. Patterns can be combined to solve your app’s exact requirements, often leading to more performant and maintainable code.

Hopefully, you’ll learn new patterns to add to your toolbox, no matter what frameworks and libraries you use!

PubSub Pattern (Publish Subscriber)

PubSub is one of the most foundational patterns for reactivity. Firing an event out with publish() allows anyone to listen to that event subscribe() and do whatever they want in a decoupled from whatever fires that event.

const pubSub = {
  events: {},
  subscribe(event, callback) {
    if (!this.events[event]) this.events[event] = [];
    this.events[event].push(callback);
  },
  publish(event, data) {
    if (this.events[event]) this.events[event].forEach(callback => callback(data));
  }
};

pubSub.subscribe('update', data => console.log(data));
pubSub.publish('update', 'Some update'); // Some update

Note the publisher has no idea of what is listening to it, so there is no way to unsubscribe or clean up after itself with this simple implementation.

Custom Events: Native Browser API for PubSub

The browser has a JavaScript API for firing and subscribing to custom events. It allows you to send data along with the custom events using dispatchEvent.

const pizzaEvent = new CustomEvent("pizzaDelivery", {
  detail: {
    name: "supreme",
  },
});

window.addEventListener("pizzaDelivery", (e) => console.log(e.detail.name));
window.dispatchEvent(pizzaEvent);

You can scope these custom events to any DOM node. In the code example, we use the global window object, also known as a global event bus, so anything in our app can listen and do something with the event data.

<div id="pizza-store"></div>

const pizzaEvent = new CustomEvent("pizzaDelivery", {
  detail: {
    name: "supreme",
  },
});

const pizzaStore = document.querySelector('#pizza-store');
pizzaStore.addEventListener("pizzaDelivery", (e) => console.log(e.detail.name));
pizzaStore.dispatchEvent(pizzaEvent);

Class Instance Custom Events: Subclassing EventTarget

We can subclass EventTarget to send out events on a class instance for our app to bind to:

class PizzaStore extends EventTarget {
  constructor() {
    super();
  }
  addPizza(flavor) {
    // fire event directly on the class
    this.dispatchEvent(new CustomEvent("pizzaAdded", {
      detail: {
        pizza: flavor,
      },
    }));
  }
}

const Pizzas = new PizzaStore();
Pizzas.addEventListener("pizzaAdded", (e) => console.log('Added Pizza:', e.detail.pizza));
Pizzas.addPizza("supreme");

The cool thing about this is your events aren’t firing globally on the window. You can fire an event directly on a class; anything in your app can wire up event listeners directly to that class.

Observer Pattern

The observer pattern has the same basic premise as the PubSub pattern. It allows you to have behavior “subscribed” to a Subject. And when the Subject fires the notify method, it notifies everything subscribed.

class Subject {
  constructor() {
    this.observers = [];
  }
  addObserver(observer) {
    this.observers.push(observer);
  }
  removeObserver(observer) {
    const index = this.observers.indexOf(observer);
    if (index > -1) {
      this.observers.splice(index, 1);
    }
  }
  notify(data) {
    this.observers.forEach(observer => observer.update(data));
  }
}

class Observer {
  update(data) {
    console.log(data);
  }
}

const subject = new Subject();
const observer = new Observer();

subject.addObserver(observer);
subject.notify('Everyone gets pizzas!');

The main difference between this and PubSub is that the Subject knows about its observers and can remove them. They aren’t completely decoupled like in PubSub.

Reactive Object Properties with Proxies

Proxies in JavaScript can be the foundation for performing reactivity after setting or getting properties on an object.

const handler = {
  get: function(target, property) {
    console.log(`Getting property ${property}`);
    return target[property];
  },
  set: function(target, property, value) {
    console.log(`Setting property ${property} to ${value}`);
    target[property] = value;
    return true; // indicates that the setting has been done successfully
  }
};

const pizza = { name: 'Margherita', toppings: ['tomato sauce', 'mozzarella'] };
const proxiedPizza = new Proxy(pizza, handler);

console.log(proxiedPizza.name); // Outputs "Getting property name" and "Margherita"
proxiedPizza.name = 'Pepperoni'; // Outputs "Setting property name to Pepperoni"

When you access or modify a property on the proxiedPizza, it logs a message to the console. But you could imagine wiring any functionality to property access on an object.

Reactive Individual Properties: Object.defineProperty

You can do an identical thing for a specific property using Object.defineProperty. You can define getters and setters for properties and run code when a property is accessed or modified.

const pizza = {
  _name: 'Margherita', // Internal property
};

Object.defineProperty(pizza, 'name', {
  get: function() {
    console.log(`Getting property name`);
    return this._name;
  },
  set: function(value) {
    console.log(`Setting property name to ${value}`);
    this._name = value;
  }
});

// Example usage:
console.log(pizza.name); // Outputs "Getting property name" and "Margherita"
pizza.name = 'Pepperoni'; // Outputs "Setting property name to Pepperoni"

Here, we’re using Object.defineProperty to define a getter and setter for the name property of the pizza object. The actual value is stored in a private _name property, and the getter and setter provide access to that value while logging messages to the console.

Object.defineProperty is more verbose than using a Proxy, especially if you want to apply the same behavior to many properties. But it’s a powerful and flexible way to define custom behavior for individual properties.

Asynchronous Reactive Data with Promises

Let’s make using the observers asynchronous! This way we can update the data and have multiple observers run asynchronously.

class AsyncData {
  constructor(initialData) {
    this.data = initialData;
    this.subscribers = [];
  }

  // Subscribe to changes in the data
  subscribe(callback) {
    if (typeof callback !== 'function') {
      throw new Error('Callback must be a function');
    }
    this.subscribers.push(callback);
  }

  // Update the data and wait for all updates to complete
  async set(key, value) {
    this.data[key] = value;

    // Call the subscribed function and wait for it to resolve
    const updates = this.subscribers.map(async (callback) => {
      await callback(key, value);
    });

    await Promise.allSettled(updates);
  }
}

Here’s a class that wraps a data object and triggers an update when the data changes.

Awaiting Our Async Observers

Let’s say we want to wait until all subscriptions to our asynchronous reactive data are processed:

const data = new AsyncData({ pizza: 'Pepperoni' });

data.subscribe(async (key, value) => {
  await new Promise(resolve => setTimeout(resolve, 500));
  console.log(`Updated UI for ${key}: ${value}`);
});

data.subscribe(async (key, value) => {
  await new Promise(resolve => setTimeout(resolve, 1000));
  console.log(`Logged change for ${key}: ${value}`);
});

// function to update data and wait for all updates to complete
async function updateData() {
  await data.set('pizza', 'Supreme'); // This will call the subscribed functions and wait for their promises to resolve
  console.log('All updates complete.');
}

updateData();

Our updateData function is now async, so we can await all the subscribed functions to resolve before continuing our program. This pattern allows juggling asynchronous reactivity a bit simpler.

Reactive Systems

Many more complex reactive systems are at the foundations of popular libraries and frameworks: hooks in React, Signals in Solid, Observables in Rx.js, and more. They usually have the same basic premise of when data changes, re-render the components or associated DOM fragments.

Observables (Pattern of Rx.js)

Observables and Observer Pattern are not the same despite being nearly the same word, lol.

Observables allow you to define a way to produce a sequence of values over time. Here is a simple Observable primitive that provides a way to emit a sequence of values to subscribers, allowing them to react as those values are produced.

class Observable {
  constructor(producer) {
    this.producer = producer;
  }

  // Method to allow a subscriber to subscribe to the observable
  subscribe(observer) {
    // Ensure the observer has the necessary functions
    if (typeof observer !== 'object' || observer === null) {
      throw new Error('Observer must be an object with next, error, and complete methods');
    }

    if (typeof observer.next !== 'function') {
      throw new Error('Observer must have a next method');
    }

    if (typeof observer.error !== 'function') {
      throw new Error('Observer must have an error method');
    }

    if (typeof observer.complete !== 'function') {
      throw new Error('Observer must have a complete method');
    }

    const unsubscribe = this.producer(observer);

    // Return an object with an unsubscribe method
    return {
      unsubscribe: () => {
        if (unsubscribe && typeof unsubscribe === 'function') {
          unsubscribe();
        }
      },
    };
  }
}

Here’s how you would use them:

// Create a new observable that emits three values and then completes
const observable = new Observable(observer => {
  observer.next(1);
  observer.next(2);
  observer.next(3);
  observer.complete();

  // Optional: Return a function to handle any cleanup if the observer unsubscribes
  return () => {
    console.log('Observer unsubscribed');
  };
});

// Define an observer with next, error, and complete methods
const observer = {
  next: value => console.log('Received value:', value),
  error: err => console.log('Error:', err),
  complete: () => console.log('Completed'),
};

// Subscribe to the observable
const subscription = observable.subscribe(observer);

// Optionally, you can later unsubscribe to stop receiving values
subscription.unsubscribe();

The critical component of an Observable is the next() method, which sends data to the observers. A complete() method for when the Observable stream closes. And an error() method when something goes wrong. Also, there has to be a way to subscribe() to listen for changes and unsubscribe() to stop receiving data from the stream.

The most popular libraries that use this pattern are Rx.js and MobX.

“Signals” (Pattern of SolidJS)

Hat tip Ryan Carniato’s Reactivity with SolidJS course.

const context = [];

export function createSignal(value) {
    const subscriptions = new Set();

    const read = () => {
        const observer = context[context.length - 1]
        if (observer) subscriptions.add(observer);
        return value;
    }
    const write = (newValue) => {
        value = newValue;
        for (const observer of subscriptions) {
            observer.execute()
        }
    }

    return [read, write];
}

export function createEffect(fn) {
    const effect = {
        execute() {
            context.push(effect);
            fn();
            context.pop();
        }
    }

    effect.execute();
}

Using the reactive system:

import { createSignal, createEffect } from "./reactive";

const [count, setCount] = createSignal(0);

createEffect(() => {
  console.log(count());
}); // 0

setCount(10); // 10

Here’s the complete code for his vanilla reactivity system with a code sample that Ryan writes in his course.

“Observable-ish” Values (Frontend Masters)

Our Frontend Masters video player has many configurations that could change anytime to modify video playback. Kai on our team created “Observable-ish” Values (many years ago now, but we just published it for this article’s sake), which is another take on a reactive system in vanilla JavaScript.

It’s less than 100 lines of code and has stood the test of time! For 7+ years, this tiny bit of code has underpinned delivering millions of hours of video. It’s a mix of PubSub with the ability to have computed values by adding the results of multiple publishers together.

Here’s how you use the “Observable-ish” values. Publish changes to subscriber functions when values change:

const fn = function(current, previous) {}

const obsValue = ov('initial');
obsValue.subscribe(fn);      // subscribe to changes
obsValue();                  // 'initial'
obsValue('initial');         // identical value, no change
obsValue('new');             // fn('new', 'initial')
obsValue.value = 'silent';   // silent update

Modifying arrays and objects will not publish, but replacing them will.

const obsArray = ov([1, 2, 3]);
obsArray.subscribe(fn);
obsArray().push(4);          // silent update
obsArray.publish();          // fn([1, 2, 3, 4]);
obsArray([4, 5]);            // fn([4, 5], [1, 2, 3]);

Passing a function caches the result as the value. Any extra arguments will be passed to the function. Any observables called within the function will be subscribed to, and updates to those observables will recompute the value.

Child observables must be called; mere references are ignored. If the function returns a Promise, the value is assigned async after resolution.

const a = ov(1);
const b = ov(2);
const computed = ov(arg => { a() + b() + arg }, 3);
computed.subscribe(fn);
computed();                  // fn(6)
a(2);                        // fn(7, 6)

Reactive Rendering of UI

Here are some patterns for writing and reading from the DOM and CSS.

Render Data to HTML String Literals

Here’s a simple example of rendering some pizza UI based on data.

function PizzaRecipe(pizza) {
  return `<div class="pizza-recipe">
    <h1>${pizza.name}</h1>
    <h3>Toppings: ${pizza.toppings.join(', ')}</h3>
    <p>${pizza.description}</p>
  </div>`;
}

function PizzaRecipeList(pizzas) {
  return `<div class="pizza-recipe-list">
    ${pizzas.map(PizzaRecipe).join('')}
  </div>`;
}

var allPizzas = [
  {
    name: 'Margherita',
    toppings: ['tomato sauce', 'mozzarella'],
    description: 'A classic pizza with fresh ingredients.'
  },
  {
    name: 'Pepperoni',
    toppings: ['tomato sauce', 'mozzarella', 'pepperoni'],
    description: 'A favorite among many, topped with delicious pepperoni.'
  },
  {
    name: 'Veggie Supreme',
    toppings: ['tomato sauce', 'mozzarella', 'bell peppers', 'onions', 'mushrooms'],
    description: 'A delightful vegetable-packed pizza.'
  }
];

// Render the list of pizzas
function renderPizzas() {
  document.querySelector('body').innerHTML = PizzaRecipeList(allPizzas);
}

renderPizzas(); // Initial render

// Example of changing data and re-rendering
function addPizza() {
  allPizzas.push({
    name: 'Hawaiian',
    toppings: ['tomato sauce', 'mozzarella', 'ham', 'pineapple'],
    description: 'A tropical twist with ham and pineapple.'
  });

  renderPizzas(); // Re-render the updated list
}

// Call this function to add a new pizza and re-render the list
addPizza();

addPizza demonstrates how to change the data by adding a new pizza recipe to the list and then re-rendering the list to reflect the changes.

The main drawback of this approach is you blow away the entire DOM on every render. You can more intelligently update only the bits of DOM that change using a library like lit-html (lit-html usage guide). We do this with several highly dynamic components on Frontend Masters, like our data grid component.

See examples of other approaches in the Vanilla TodoMVC repo and associated Vanilla TodoMVC article.

Reactive DOM Attributes: MutationObserver

One way to make DOM reactive is to add and remove attributes. We can listen to changes in attributes using the MutationObserver API.

const mutationCallback = (mutationsList) => {
  for (const mutation of mutationsList) {
    if (
      mutation.type !== "attributes" ||
      mutation.attributeName !== "pizza-type"
    ) return;

    console.log('old:', mutation.oldValue)
    console.log('new:', mutation.target.getAttribute("pizza-type"))
  }
}
const observer = new MutationObserver(mutationCallback);
observer.observe(document.getElementById('pizza-store'), { attributes: true });

Now we can update the pizza-type attribute from anywhere in our program, and the element itself can have behavior attached to updating that attribute!

Reactive Attributes in Web Components

With Web Components, there is a native way to listen and react to attribute updates.

class PizzaStoreComponent extends HTMLElement {
  static get observedAttributes() {
    return ['pizza-type'];
  }

  constructor() {
    super();
    const shadowRoot = this.attachShadow({ mode: 'open' });
    shadowRoot.innerHTML = `<p>${this.getAttribute('pizza-type') || 'Default Content'}</p>`;
  }

  attributeChangedCallback(name, oldValue, newValue) {
    if (name === 'my-attribute') {
      this.shadowRoot.querySelector('div').textContent = newValue;
      console.log(`Attribute ${name} changed from ${oldValue} to ${newValue}`);
    }
  }
}

customElements.define('pizza-store', PizzaStoreComponent);

<pizza-store pizza-type="Supreme"></pizza-store>

document.querySelector('pizza-store').setAttribute('pizza-type', 'BBQ Chicken!');

This is a bit simpler, but we have to use Web Components to use this API.

Reactive Scrolling: IntersectionObserver

We can wire reactivity to DOM elements scrolling into view. I’ve used this for slick animations on our marketing pages.

var pizzaStoreElement = document.getElementById('pizza-store');

var observer = new IntersectionObserver(function(entries, observer) {
  entries.forEach(function(entry) {
    if (entry.isIntersecting) {
      entry.target.classList.add('animate-in');
    } else {
      entry.target.classList.remove('animate-in');
    }
  });
});

observer.observe(pizzaStoreElement);

Here’s an example scrolling animation CodePen in very few lines of code using IntersectionObserver.

Animation & Game Loop: requestAnimationFrame

When working with game development, Canvas, WebGL, or those wild marketing sites, animations often require writing to a buffer and then writing the results on a given loop when the rendering thread becomes available. We do this with requestAnimationFrame.

function drawStuff() {
  // This is where you'd do game or animation rendering logic
}

// function to handle the animation
function animate() {
  drawStuff();
  requestAnimationFrame(animate); // Continually calls animate when the next render frame is available
}

// Start the animation
animate();

This is the method games and anything that involves real-time rendering use to render the scene when frames become available.

Reactive Animations: Web Animations API

You can also create reactive animations with the Web Animations API. Here we will animate an element’s scale, position, and color using the animation API.

const el = document.getElementById('animatedElement');

// Define the animation properties
const animation = el.animate([
  // Keyframes
  { transform: 'scale(1)', backgroundColor: 'blue', left: '50px', top: '50px' },
  { transform: 'scale(1.5)', backgroundColor: 'red', left: '200px', top: '200px' }
], {
  // Timing options
  duration: 1000,
  fill: 'forwards'
});

// Set the animation's playback rate to 0 to pause it
animation.playbackRate = 0;

// Add a click event listener to the element
el.addEventListener('click', () => {
  // If the animation is paused, play it
  if (animation.playbackRate === 0) {
    animation.playbackRate = 1;
  } else {
    // If the animation is playing, reverse it
    animation.reverse();
  }
});

What’s reactive about this is that the animation can play relative to where it is located when an interaction occurs (in this case, reversing its direction). Standard CSS animations and transitions aren’t relative to their current position.

Reactive CSS: Custom Properties and calc

Lastly, we can write CSS that’s reactive by combining custom properties and calc.

barElement.style.setProperty('--percentage', newPercentage);

In JavaScript, you can set a custom property value.

.bar {
  width: calc(100% / 4 - 10px);
  height: calc(var(--percentage) * 1%);
  background-color: blue;
  margin-right: 10px;
  position: relative;
}

And in the CSS, we can now do calculations based on that percentage. It’s pretty cool that we can add calculations right into the CSS and let CSS do its job of styling without having to keep all that rendering logic in JavaScript.

FYI: You can also read these properties if you want to create changes relative to the current value.

getComputedStyle(barElement).getPropertyValue('--percentage');

The Many Ways to Achieve Reactivity

It’s incredible how many ways we can achieve reactivity using very little code in modern vanilla JavaScript. We can combine these patterns in any way we see fit for our apps to reactively render, log, animate, handle user events, and all the things that can happen in the browser.

The code has received over 🤩 600 stars on GitHub:

In general, the responses were very positive. But as with all popular things, eventually, they spark debate.

React / Frameworks vs. Vanilla JS: Top Four Arguments for Frameworks

#1: “Frameworks Enable Declarative UI”

Modern frameworks like React and Vue don’t exist to fill in the gap left by native JS, they exist so that you write your application in a declarative way where the view is rendered as a function of state.

IMO this is simply a design pattern. Patterns apply in any language.

You can accomplish roughly the same thing in vanilla JavaScript. In my code, when the model changes, it fires a save event, and then I wire App.render() to it, which renders the App using the Todos model.

Todos.addEventListener('save', App.render);

Template strings end up pretty easy to work with when you want to re-render parts of the App from scratch as a framework would:

`
  <div class="view">
    <input class="toggle" type="checkbox" ${todo.completed ? 'checked' : ''}>
    <label></label>
    <button class="destroy"></button>
  </div>
  <input class="edit">
`

The entire App render method is only eleven lines, and it re-renders everything the App needs to based on the state of the App:

render() {
  const count = Todos.all().length;
  App.$.setActiveFilter(App.filter);
  App.$.list.replaceChildren(
    ...this.Todos.all(this.filter).map((todo) => this.renderTodo(todo))
  );
  App.$.showMain(count);
  App.$.showFooter(count);
  App.$.showClear(Todos.hasCompleted());
  App.$.toggleAll.checked = Todos.isAllCompleted();
  App.$.displayCount(Todos.all('active').length);
}

Here I could have chosen to rebuild the entire UI as a template string as a function of state, but instead, it is ultimately more performant to create these DOM helper methods and modify what I want.

#2: “Frameworks Provide Input Sanitization”

The best way to sanitize user input is to use node.textContent.

insertHTML(li, `
  <div class="view">
    <input class="toggle" type="checkbox" ${todo.completed ? 'checked' : ''}>
    <label></label>
    <button class="destroy"></button>
  </div>
  <input class="edit">
`);
li.querySelector('label').textContent = todo.title;

Any user input must be set to the DOM using textContent. If you do that, then you’re fine.

Beyond this, there is a new Trusted Types API for sanitizing generated HTML. I would use this new API if I were generating nested markup with dynamic, user-input data. (Note that this new API isn’t available yet in Safari, but hopefully, it will be soon)

Trusted Types not being everywhere is fine. You can use them where they’re supported and get early warning of issues. Security improves as browsers improve, and usage turns into an incentive for lagging engines (source)

Suppose you want a library to build your app template strings without using textContent manually. In that case, you can use a library like DOMPurify, which uses Trusted Types API under the hood.

#3: “Frameworks Provide DOM Diffing and DOM Diffing is Necessary”

The most common criticism was the lack of DOM Diffing in vanilla JavaScript.

A reactive UI/diff engine is non-negotiable for me.

Diffing is exactly what you need to do (barring newer methods like svelte) to figure out what to tell the browser to change. The vdom tree is much faster to manipulate than DOM nodes.

However, I think this is a much more balanced take:

Diffing seems necessary when your UI gets complicated to the level that a small change requires a full page re-render. However, I don’t think this is necessary for at least 95% of the websites on the internet.

I agree most websites and web apps don’t suffer from this issue, even when re-rendering the needed components based on vanilla’s application state like a framework.

Lastly, I’ll note that DOM diffing is inefficient for getting reactive updates because it doubles up data structures. Lit, Svelte, Stencil, Solid, and many others don’t need it and are way more performant as a result. These approaches win on performance and memory use, which matters because garbage collection hurts the UX.

Modern frameworks necessitate that you render the entire App client-side which makes your apps slow by default.

My issue with modern frameworks forcing declarative UI (see #1) and DOM diffing (see #2) approach is that they necessitate unnecessary rendering and slow startup times. Remix is trying to avoid this by rendering server-side then “hydrating,” and new approaches like Quik are trying not to have hydration altogether. It’s an industry-wide problem, and people are trying to address it.

In my vanilla JavaScript projects, I only re-render the most minimal parts of the page necessary. Template strings everywhere, and especially adding DOM diffing, is inefficient. It forces you to render all of your App client-side increasing startup time and the amount the client has to do overall each time data changes.

That said, if you do need DOM diffing in parts of a vanilla app, libraries like morphdom do just that. There is also a fantastic templating library called Lit-html that solves this problem of making your App more declarative in a tiny package (~3KB), and you can continue using template strings with that.

#4: “Frameworks Scale, Vanilla JavaScript Will Never Scale”

I have built many large vanilla JavaScript projects and scaled them across developers, making the companies I worked for tons of money, and these apps still exist today. 🕺✨

Conventions and idioms are always needed, no matter if you build on top of a framework or not.

At the end of the day, your codebase will only be only as good as your team, not the framework.

The way vanilla JS scales is the same way any framework scales. You have to have intelligent people talk about the needs of the codebase and project.

App Architecture Branch:

That said, here’s an example of adding ~20 lines of structure to the code in the app architecture branch. It splits the code into a TodoList and App component. Each component implements a render method that optionally renders a filtered view of the data.

Overall I’d argue these solutions are more performant, less code (~200 lines only), and more straightforward than most, if not all, the TodoMVC implementations on the internet without a framework.

Here are Eight Vanilla JavaScript Tips from the Code

#1. Sanitization

User input must be sanitized before being displayed in the HTML to prevent XSS (Cross-Site Scripting). Therefore new todo titles are added to the template string using textContent:

li.querySelector('label').textContent = todo.title;

#2. Event Delegation

Since we render the todos frequently, it doesn’t make sense to bind event listeners and clean them up every time. Instead, we bind our events to the parent list that always exists in the DOM and infer which todo was clicked or edited by setting the data attribute of the item $li.dataset.id = todo.id;

Event delegation uses the matches selector:

export const delegate = (el, selector, event, handler) => {
  el.addEventListener(event, e => {
    if (e.target.matches(selector)) handler(e, el);
  });
}

When something inside the list is clicked, we read that data attribute id from the inner list item and use it to grab the todo from the model:

delegate(App.$.list, selector, event, e => {
  let $el = e.target.closest('[data-id]');
  handler(Todos.get($el.dataset.id), $el, e);
});

#3. insertAdjacentHTML

insertAdjacentHTML is much faster than innerHTML because it doesn’t have to destroy the DOM first before inserting.

export const insertHTML = (el, html) => {
  el.insertAdjacentHTML("afterbegin", html);
}

Bonus tip: Jonathan Neal taught me through a PR that you can empty elements and replace the contents with el.replaceChildren() — thanks Jonathan!

#4. Grouping DOM Selectors & Methods

DOM selectors and modifications are scoped to the App.$.* namespace. In a way, it makes it self-documenting what our App could potentially modify in the document.

$: {
  input: document.querySelector('[data-todo="new"]'),
  toggleAll: document.querySelector('[data-todo="toggle-all"]'),
  clear: document.querySelector('[data-todo="clear-completed"]'),
  list: document.querySelector('[data-todo="list"]'),
  count: document.querySelector('[data-todo="count"]'),
  showMain(show) {
    document.querySelector('[data-todo="main"]').style.display = show ? 'block': 'none';
  },
  showFooter(show) {
    document.querySelector('[data-todo="main"]').style.display = show ? 'block': 'none';
  },
  showClear(show) {
    App.$.clear.style.display = show ? 'block': 'none';
  },
  setActiveFilter(filter) {
    document.querySelectorAll('[data-todo="filters"] a').forEach(el => el.classList.remove('selected')),
    document.querySelector(`[data-todo="filters"] [href="#/${filter}"]`).classList.add('selected');
  },
  displayCount(count) {
    replaceHTML(App.$.count, `
      <strong>${count}</strong>
      ${count === 1 ? 'item' : 'items'} left
    `);
  }
},

#5. Send Events on a Class Instance with Subclassing EventTarget

We can subclass EventTarget to send out events on a class instance for our App to bind to:

export const TodoStore = class extends EventTarget {

In this case, when the store updates, it sends an event:

this.dispatchEvent(new CustomEvent('save'));

The App listens to that event and re-renders itself based on the new store data:

Todos.addEventListener('save', App.render);

#6. Group Setting Up Event Listeners

It is essential to know exactly where the global event listeners are set. An excellent place to do that is in the App init method:

init() {
  Todos.addEventListener('save', App.render);
  App.filter = getURLHash();
  window.addEventListener('hashchange', () => {
    App.filter = getURLHash();
    App.render();
  });
  App.$.input.addEventListener('keyup', e => {
    if (e.key === 'Enter' && e.target.value.length) {
      Todos.add({ title: e.target.value, completed: false, id: "id_" + Date.now() })
      App.$.input.value = '';
    }
  });
  App.$.toggleAll.addEventListener('click', e => {
    Todos.toggleAll();
  });
  App.$.clear.addEventListener('click', e => {
    Todos.clearCompleted();
  });
  App.bindTodoEvents();
  App.render();
},

Here we set up all the global event listeners, subscribe to the store mentioned above, and then initially render the App.

Similarly, when you create new DOM elements and insert them into the page, group the event listeners associated with the new elements near where they are made.

#7. Use Data Attributes in Markup & Selectors

One issue with JavaScript is your selectors get tightly coupled to the generated DOM.

To fix this, classes should be used for CSS rules, and data atributes for JavaScript behavior.

<div data-jsmodule="behavior"></div>

document.querySelector('[data-jsmodule="behavior"]')

#8. Render the State of the World Based on Data (Data Flowing Down)

Lastly, to reiterate what I said above, render everything based on the state in the render() method. This is a pattern lifted from modern frameworks.

Make sure you update the DOM based on your App state, not the other way around.

It’s even better if you avoid reading DOM to derive any part of your app state aside from finding your target for event delegation.

Side note: I like to rely on the server to generate the markup for faster boot times, then take control of the bits we show. Have the CSS initially hide things you don’t need, and then have the JavaScript show the elements based on the state. Let the server do most of the work where you can, rather than wait for the entire App to render client-side.

In Conclusion

Vanilla JS is Viable Today for Building Web Apps

JavaScript is better today than it has ever been.

The fact that I could shave off 80% of the code over the previous TodoMVC years ago at the drop of a hat feels terrific. Plus, we now have established design patterns that we can lift from modern frameworks to apply to vanilla JavaScript projects to make our UIs as declarative as we like.

As an industry we should consider pure JavaScript as an option for more projects.

Finally, as Web Components get more ergonomic, we will even have a way to share our code in an interoperable and framework-agnostic way.

I hope you enjoyed the post. Please send your feedback to me @1marc on Twitter. Cheers!

Bonus: Performant Rendering of Large Lists

The code for rendering the entire list contents on model change is clean because data flows down, but it potentially will not be as performant for rendering large lists.

More Performant & Granular DOM Updates with Vanilla

Here’s a branch sending specific events with context from the model so we can make DOM updates more selectively as we need them: (see granular DOM updates diff).

performant-rendering branch

More Performant DOM Updates with lit-html (plus animations!)

We can acheieve the same performant DOM updates with far less code by adopting lit-html using the repeat directive: (see adding lit-html diff).

animation-lithtml branch