Whether you are just starting out with frontend engineering or you are a seasoned veteran, this course will provide a strong understanding of the DOM Event system.

Alex Reardon

Your Ultimate Guide to Understanding DOM Events

 Courses from Alex Reardon

Listening to and responding to events is a foundational skill for building software for the web. The Document Object Model, or DOM for short, has a powerful built-in event system. And yet, many of us have a pretty basic understanding of the DOM event system and only scratch the surface of the available power. 

This lack of understanding leads to bugs in our applications and writing lots of code to work around the DOM event system rather than leveraging it. The goal of the course is that you will walk away with the confidence to successfully and meaningfully engage with the DOM event system.

The DOM event visualizer [domevents.dev](https://domevents.dev) will be used to explain many behaviors throughout the course. You are welcome to play with the visualizer yourself to continue your learning

JavaScript® (often shortened to JS) is a lightweight, interpreted, object-oriented language with first-class functions, most known as the scripting language for Web pages, but used in many non-browser environments as well such as node.js or Apache CouchDB. It is a prototype-based, multi-paradigm scripting language that is dynamic, and supports object-oriented, imperative, and functional programming styles.

javascript

In this lesson, we will use the Chrome performance panel to debug and fix an interaction that feels sluggish.

Debug Event Listener Performance with Chrome Developer Tools

In this lesson we we will look at how you can inspect and debug event listeners in your browser by using the browser developer tools.

Debug and Inspect Event Listeners with Chrome Developer Tools

Some event handlers attached to the `<body>` act as if they were attached to the `window`.

Window Reflecting Body Element Event Handlers

Events are classified as either synchronous or asynchronous (ordered or unordered). This lesson will explore what these categorisations really mean and what it means to you.

Synchronous and Asynchronous Events (Ordered and Unordered Events)

In this lesson, we'll look at how some browsers treat some events added to the `body` as passive by default. However, HTML attribute event handlers on the `body` do not share these default passive values.

Default Passive Values on the Body Element

This lesson will explore what problem passive event listeners are trying to solve, and how you can mark your event listeners as passive to improve scrolling performance. It also covers how some browsers automatically set `touchstart`, `touchmove`, `wheel` and `mousewheel` events added to the `window`, `document` or `body` as passive by default.

Improve Scroll Performance with Passive Event Listeners

Building on our knowledge of the event loop we will see how the execution timings of microtasks queued in event listeners can be different depending on how an event is dispatched.

DOM Events and Microtasks

This lesson presses into the deep inner workings of the event loop in order to understand how events fit into the overall javascript execution lifecycle.

DOM Events and the Event Loop

What happens when you add an event listener to an event target while an event is in the process of being dispatched? Let's find out together!

Add and Remove Event Listeners while an Event is Dispatching

Does the dispatching of an event queue a future task, or is the event dispatched straight away? And when are event listeners executed? Eventually or straight away? This lesson will show you how dispatching an event occurs synchronously and all event listeners on the event path are also executed synchronously.

Events are Dispatched Synchronously

There are some deprecated mechanisms for creating events that should be avoided.

Deprecated Event Creation Mechanisms

In addition to the browser creating events, it is also possible for you to create and dispatch your own events.

Create and Dispatch Synthetic Events

Event delegation is a simple, but powerful leveraging of the DOM event system which allows for easier adding of functionality to dynamically created content; as well as being an interesting performance optimization.

The Event Delegation Pattern

Two separate `event` object methods exist for stopping an event: `event.stopPropagation()` and `event.stopImmediatePropagation()`. This lesson will explain the differences between these two functions as well as cautioning you about some dangers in stopping events.

Stop Events

The `error` and `beforeunload` events have their own bespoke approaches to canceling the event because nothing is easy.

Cancel Bespoke Events

In this lesson, we will look at what it means to cancel an event, why you might want to cancel an event, and how you can cancel an event.

Cancel Events

It is common to log `event` objects to the `console`. However, it is important to keep in mind that object properties are evaluated in objects logged to the `console` when the object is first expanded.

Log Events to the Console

Event listeners are provided with an `event` object that represents an actively dispatched event. An `event` object contains lots of information and functionality related to an event. We will look at the various event properties, constants and methods you have at your disposal on an `event` object and the `Event` constructor.

The Event Object

This lesson explores some of the strangeness of the target phase, including the execution of capture and bubble phase event listeners added to the event target.

The Execution Order of Event Listeners in the Target Phase

It is possible to add many event listeners using different binding approaches to an `EventTarget`. In this lesson, I explain in what order event listeners will be executed in.

The Execution Order of Event Listeners

When comparing two visually similar websites, it can often be challenging to precisely identify the subtle differences between them. To facilitate a rapid analysis of these discrepancies, a practical technique involves leveraging Chrome's integrated developer tools to capture full-page screenshots of each site. Subsequently, these screenshots can be uploaded to a platform like AI Studio. There, the AI can analyze the images and generate a report detailing the variations. This entire process, leveraging a few quick shortcuts, can be completed in under a minute, delivering a comprehensive breakdown of the differences for efficient comparison and analysis.

Quickly Compare Two Sites Using AI Studio with Chrome Full Page Screenshots

It's fairly trivial to create a React project, but there's always a big hurdle between creating it locally and making it shareable so that someone else can run it. This lesson walks you through the process of automating creating a React project locally, creating the repository where it's going to be hosted, and creating shareable links where it can be run so that you can quickly and easily create single one-off demos and shoot them over to your friends and coworkers for sharing your code ideas in a working environment.


```bash
share-react-project() {
  if [[ -z "$1" ]]; then
    echo "Usage: share-react-project <project_name>"
    return 1
  fi

  local project_name="$1"
  local github_username=$(gh api /user --jq '.login')

  echo "Creating Vite project: $project_name"
  pnpm create vite "$project_name" --template react

  cd "$project_name"

  echo "Initializing Git repository"
  git init

  echo "Adding all files to Git"
  git add .

  echo "Creating initial commit"
  git commit -m "Initial commit"

  local codesandbox_link="https://codesandbox.io/p/github/${github_username}/${project_name}"

  echo "Adding CodeSandbox link to README.md"
  echo "" >> README.md
  echo "## CodeSandbox" >> README.md
  echo "[![Open in CodeSandbox](https://assets.codesandbox.io/github/button-edit-blue.svg)](${codesandbox_link})" >> README.md

  echo "Adding README.md to Git"
  git add README.md

  echo "Committing README.md changes"
  git commit -m "Add CodeSandbox link"

  echo "Creating GitHub repository: $github_username/$project_name"
  gh repo create "$github_username/$project_name" --public

  echo "Pushing to remote 'origin'"
  git push -u origin main

  echo "Project '$project_name' created successfully!"
  echo "GitHub repository: https://github.com/$github_username/$project_name"
  echo "CodeSandbox link: $codesandbox_link"
}
```

Automate Creating a Local React Project, GitHub Repository, and Live Hosted Demo in a Single Command

[Verdaccio](https://verdaccio.org/) is a fully featured NPM registry that you can run locally on your machine. It’s a great tool for testing your release process, running end-to-end tests, and simulating real-world scenarios like deploying packages, installing them, and verifying functionality as an end user would.

Nx includes a [generator](https://nx.dev/features/generate-code) that simplifies setting up Verdaccio. To configure it, run:

```shell
npx nx g @nx/js:setup-verdaccio   
```

This command installs the necessary NPM packages and creates a `project.json` file with a `local-registry` target. To start the local registry, open a new terminal window and run:

```shell
npx nx local-registry
```

This will:
- Launch the Verdaccio registry.
- Configure NPM to point to the local Verdaccio server at `http://localhost:4873`.

> **Note:** A `project.json` file provides an advanced way to define scripts, offering more flexibility than a traditional `package.json` for managing complex targets.

Setup local Verdaccio server to release NPM packages

Learn how to spin up a local NPM registry powered by Verdaccio to test deploy your NPM packages.

In TypeScript, developers often begin by defining the structure of their data using type definitions or interfaces, which is a powerful way to ensure type safety. However, as applications grow and become more complex, simply defining the shape of an object might not be enough to guarantee its consistent creation and behavior throughout the codebase. This often leads to situations where objects that are intended to be of a certain "type" might be missing crucial properties or have incorrect data, which can be difficult to track down and debug.

This lesson tackles the question of when and why to move beyond basic type definitions and embrace classes in TypeScript. It demonstrates how classes provide a more robust mechanism for defining object structure, enforcing type constraints, and ensuring consistent object creation. By establishing a clear blueprint for objects, classes not only improve compile-time type safety but also offer runtime benefits and better code organization, ultimately helping developers avoid common pitfalls associated with loosely structured objects in larger TypeScript projects.

Stop Avoiding Classes in TypeScript

Karabiner isn't only for your keyboard. It can also hijack your mouse buttons and enable them to be similar to layers on your keyboard, where holding down a mouse button can allow you to tap keys on your keyboard to trigger commands and macros that can completely revolutionize your workflows. For example, you can hold down the right mouse button and tap on R to go to references in VS Code. Or hold down the right mouse button and tap the left mouse button to chop up videos and Screenflow.

This completely unlocks a whole new world of possibilities and use cases for your mouse as you can hover your mouse over anywhere on the page. And just tapping keys on your keyboard allows you to trigger mouse actions, to trigger keyboard actions, or to trigger macros and applications and terminal commands. and anything that your imagination can come up with. 

Karabiner: Rethinking Right-Click as a Macro Command Center

Make an HTML table more readable on mobile devices by using the ::before pseudo-element. We add labels to each cell, so they appear like cards when the table is viewed on smaller screens. The ::before content is set using a data-label attribute for each cell to enhance mobile viewability without horizontal scrolling.



Create a mobile friendly table with the before pseudo-element

Learn to use CSS ::before pseudo-element for mobile-friendly HTML tables. Add labels to cells for enhanced readability on mobile, avoiding horizontal scroll.

Any app of a decent size has event flows and messages being passed around. At a certain point it becomes complicated to explain it through prose and you want to turn towards diagrams. Now setting aside the time to create diagrams that accurately describe what's happening in your codebase can be quite time consuming. Luckily, with today's AI tools, we can now just do basic walkthroughs with rough recordings over codebase dictating what's going on along the way and then upload that to tools like AI Studio and then ask it to generate diagrams in a text-based format such as Mermaid that will then generate the flowcharts and diagrams for us. So this video walks us through this process of recording a rough video, uploading it to AI Studio and then asking AI Studio to generate the proper Mermaid diagram syntax.

Generate Mermaid Flow Chart Diagrams with Gemini 2.0 from Rough Screen Recordings

I'm a diehard Karabiner user and anytime I start doing something repetitively I always move it into my Karabiner configuration. Sometimes I'll fall into scenarios where I want to trigger a command, but I also want to know if that command succeeded or failed. Since Karabiner can trigger any command, setting up the command is fairly straightforward, but knowing whether it succeeded or failed became much more difficult. Luckily for me there is a package called terminal-notifier that you can install from homebrew and you can pipe the output of your command to terminal-notifier. And have a seamless way of triggering your keyboard shortcut which runs a command and you can be notified of the results.

Use Karabiner to Trigger a Terminal Command and Get Notified of the Results

There are a lot of scenarios when inspecting a site in Chrome where you don't want any click events to fire and you don't want any scripts to run. Or maybe something is animating in and out of the page and you want to capture it at a certain moment. This is a great use case for pausing script execution, which is kind of using the debugger manually by going into the Chrome DevTools and the Sources tab and clicking the pause button to essentially pause the entire page. This will give you a chance to inspect any of the elements and styles you want to inspect. And you don't have to worry about fire and click events or anything that might interrupt your investigation into the elements and styles that you want to find.

Pause Script Execution in Chrome to Easily Inspect Elements

Gemini 2.0 has a context window of a million tokens, and this means that you can upload hundreds of files and allow it to inspect and analyze massive refactors across entire code bases. 

This lesson demonstrates concatenating an entire code base into a single file and then uploading it to AI Studio. Once within AI Studio, we ask it to find the types of tasks that it can evaluate for us. Then we select one of the tasks and ask it to generate instructions for that task. Once those instructions are complete, we then turn them over to Windsurf's Cascade agent, and we allow Windsurf's Cascade to do all of the work for us as we sit back, relax, and let the AI do our jobs.

Alex Reardon

Your Ultimate Guide to Understanding DOM Events

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Your Ultimate Guide to Understanding DOM Events

Debug Event Listener Performance with Chrome Developer Tools

Debug and Inspect Event Listeners with Chrome Developer Tools

Window Reflecting Body Element Event Handlers

Synchronous and Asynchronous Events (Ordered and Unordered Events)

Default Passive Values on the Body Element

Improve Scroll Performance with Passive Event Listeners

DOM Events and Microtasks

DOM Events and the Event Loop

Add and Remove Event Listeners while an Event is Dispatching

Events are Dispatched Synchronously

Deprecated Event Creation Mechanisms

Create and Dispatch Synthetic Events

The Event Delegation Pattern

Stop Events

Cancel Bespoke Events

Cancel Events

Log Events to the Console

The Event Object

The Execution Order of Event Listeners in the Target Phase

The Execution Order of Event Listeners