Acknowledgements

The beginning of your journey with Web Components.

Foreword

A brief introduction to the set of specifications called Web Components. We'll introduce you to concepts such as Shadow DOM, HTML templates, Custom Elements v1 and uncover typical use cases for developing Web Components.

Introduction to Web Components

We'll introduce you to the mockups in Figma, the development environment used throughout the book, and other conventions used in this book.

Conventions Used In The Book

A guide for installing various technologies needed prior to coding the exercises in this book.

Setting Up The Development Environment

We'll look at what you'll learn throughout Fullstack Web Components.

Module 1 Summary

Introduction

In Part 1, you'll code a form using several Web Components specifications.

Specification

Part One

By developing the CardComponent, you'll get hands on experience with autonomous custom elements.

Autonomous custom elements

Before you can develop the CardComponent, you'll need to scaffold the component in Storybook.

Template Slots

Shadow DOM provides encapsulation for styling and template. You'll style CardComponent with CSS in the context of the autonomous custom element's Shadow DOM.

Styling CardComponent

Write integration and snapshot tests for CardComponent using Cypress and Storybook Storyshots.

Testing the card

You built an autonomous custom element with Shadow DOM, HTML templates, and ES Modules. Let's reflect on that.

Chapter One Summary

Getting to Know Web Components

Form-associated custom elements allow custom elements to participate in HTML forms. From validation to submission, you'll code a text input end-to-end.

Form-associated custom elements

Scaffold a form-associated custom element named TextInputComponent in Storybook.

TextInputComponent

You'll learn what distinguises a form-associated custom element from an autonomous custom element, the formAssociated property and ElementInternals interface.

Form Associated and ElementInternals

Form-associated custom elements don't come with a pattern that validates an entire form, so you'll implement a pattern that allows form validation.

Handling Validation

Using observedAttributes and attributeChangedCallback to listen and react to changes in custom element attributes.

Listening for attribute changes

You'll style the various interaction states of the TextInputComponent form-associated custom element.

User feedback

We've created a problem by encapsulating the HTMLInputElement by Shadow DOM. The HTMLInputElement API cann't be accessed by the outside world. In this section, you'll fix that by implementing getters and setters that emulate the HTMLInputElement on TextInputComponent.

Emulating HTMLInputElement

You'll update the Story and provide the missing APIs to TextInputComponent that allow the component to fully interact with a HTML form.

Making a form

Reviewing the work completed on the form-associated custom element TextInputComponent.

Chapter Two Summary

You'll code ButtonComponent, a customized built-in element that inherits accessibility characteristics from HTMLButtonElement.

Customized built-in elements

Scaffold the customized built-in element ButtonComponent with Storybook.

Scaffolding ButtonComponent

Customized built-ins can't benefit from the encapsulation provided by Shadow DOM, so you'll have to find another way to style ButtonComponent.

Styling ButtonComponent

Revist the HTML form by making the user experience more accessible.

Making the form accessible

How customized built-in elements can make custom elements more accessible.

Chapter Three Summary

You'll learn best practices for developing a UI Library with Web Components by coding a micro-library, developing two custom elements with the micro-library, and figuring out how to package the UI library for distribution.

UI Library

Part Two

You'll abstract common patterns intro reusable code in a micro-library powered by TypeScript Decorators.

Micro-library

How to code an interface for Web Components with TypeScript decorators that enables declaration of the selector, styles, and template.

Class Decorator

You'll code a method decorator named Listen that binds event listeners to class methods.

Method Decorator

In this chapter you learned how to code a micro-library with TypeScript decorators that streamlines Web Component development.

Chapter Four Summary

You'll investigate multiple methods for components to communicate by coding an editable table.

Component Communication

How to use mock data in Storybook to code Web Components.

Mocking Table Data in Storybook

You'll learn about three methods for custom elements to communicate, attribute drilling, CustomEvent, and BroadcastChannel.

Coding Compound Components

You'll learn about how to create a separation of concerns between the many custom elements that comprise the editable table.

Making the Table Editable

You learned about how to create a separation of concerns between various custom elements in the same user experience flow.

Summary

Compound Custom Elements

You'll learn how to code a common UX pattern (dialogs) using HTML templates and custom elements.

HTML Templates

You'll mock the DialogComponent in Storybook.

Mocking the Dialog in Storybook

You'll make a configurable dialog with custom elements.

DialogComponent

You'll scaffold two custom elements that leverage DialogComponent to display modals and tooltips.

Modal and Tooltip

You'll fix an issue faced in the last section by learning how to query for elements across Shadow boundaries.

Querying DOM Across Shadow Boundaries

You'll control the ordering and event cycle of dialogs with a custom class named DialogStack.

DialogStack

Summarizing the work completed in DialogComponent, ModalComponent, TooltipComponent, and DialogStack.

Chapter Six Summary

Powering Dialogs with HTML Templates

You'll learn best practices for organizing and distributing Web Component UI libraries.

UI Libraries

You'll learn brest practices for distributing Web Components by formatting the code with named exports, configuring the build with Rollup, providing a standard package format, and writing a custom Node.js script that enables you to publish packages.

Distribution

Custom Elements Manifest provides a wealth of information about each component. After generating a manifest, it will provide documentation for each component in Storybook.

Documentation

You'll learn how to setup continuous integration for the UI library with Github Actions.

Continuous Integration

Let's reflect on the best practices you learned for distributing UI libraries filled with Web Components.

Chapter Seven Summary

Distributing Custom Element Libraries

You'll integrate the UI library with a single-page application and code the end-to-end user experience with custom elements.

Application

Part Three

You'll provide a separation of concerns with custom elements in a single page application.

Application Development with Custom Elements

You'll scaffold the landing page with an autonomous custom element named MainView.

Landing Page

You'll code a global header with autonomous custom elements named AppHeader.

Header

You'll code another autonomous custom element that handles cookie authorization named CookieFooter.

CookieFooter

You'll lazyload an image with customized built-ins.

Background

You developed the landing page of the application, learning how to architect a separation of concerns between the UI components.

Chapter Eight Summary

To handle multiple views in the single page application, you'll need to implement a router that can navigate between each view.

Routing Custom Elements in a SPA

You'll code the Router class that handles the creation and removal of views in the application.

Routing

You'll code the necessary components and services required in the login view.

Login View

Integrate TableCardComponent into a third view of the single page application, the dashboard.

Dashboard View

You coded a single page web application with custom elements and a custom router.

Chapter Nine Summary

You'll server-side render the views of the application with Declarative Shadow DOM and @lit-labs/ssr

Server-Side Rendering Custom Elements

You'll declare a server-side renderable Declarative Shadow DOM HTML template for each component.

Declarative Shadow DOM Templates

To get custom elements to work in Node.js, you'll need to polyfill DOM APIs. Luckily, @lit-labs/ssr provides a tool that enables just that.

Shim

You'll develop Express middleware that server-side renders Declarative Shadow DOM HTML Templates with @lit-labs/ssr.

Express Middleware

You rendered custom elements server-side using Declarative Shadow DOM and @lit-labs/ssr.

Chapter Ten Summary

Server-side Rendering with Declarative Shadow DOM

The conclusion of the book Fullstack Custom Elements.

Conclusion

Fullstack Web Components

Enter your name and email address below, then click the **"Send my free chapter"** button to get started

There are several books about Web Components, but none go into how to integrate Web Components in a typical enterprise web development workflow. This book is designed to help you learn Web Components and integrate it into your work.

In this book, we cover the basics of Web Components, how to code complex user interfaces with custom elements in Storybook, how to test and maintain UI libraries built with custom elements, and how to build web applications entirely with Web Components and browser specifications. It comes with complete code examples. We walk through the entire process step-by-step.

It's taught by Stephen Belovarich, a software engineer and web developer. Stephen has provided what he's learned about UI libraries and web applications while working at Ubiquiti Networks, Norton, Nike, NBC Universal, and Workday, that can benefit other developers, students, and anyone making a website.

The Complete Guide to Building UI Libraries with Web Components

In this book, we will learn Web Components through the lens of a JavaScript developer.

The basics of working with and manipulating Web Components

The best practices for maintaining a UI library made of custom elements

How to create a micro-library for Web Components with TypeScript Decorators

What problems Web Components can solve in everyday development

How to build web applications entirely from Web Components and JavaScript

Luckily, custom elements provide an API for observing changes to attributes and reacting to those changes. Every custom element has access to a 

, you can establish which attributes the component listens to for changes. For performance sake, you have to whitelist each attribute that should be observed.

 corresponding to the name of the attribute that should be observed. Add 

 because those are the properties the condition in the 

 is referencing to validate whether or not the field has value. You'll add more attributes that should be observed later, but for now, 

  static get observedAttributes() {
    return ["required", "value"];
  }

After declaring which attributes should be observed, a second method named 

 is another method available to all custom elements that must be used in conjunction with 

 that pass the name of the changed attribute, the previous value of the attribute, and the current value of the attribute, respectively. For the sake of brevity, we'll call these arguments 

. You could strictly type define the arguments and what the method returns, but for now we'll ignore the types for this method. 

 could have many different types, so type defining these attributes anything other than 

  attributeChangedCallback(name, prev, next) {

, it appears we need a way to do something based on the value of 

    switch (name) {
      case "value":
        this.value = next;
        break;
      case "required":
        this.required = next;
        break;
    }

 properties do not exist, so let's add them. When you are finished 

 attributeChangedCallback(name, prev, next) {
    switch (name) {
      case "value":
        this.value = next;
        break;
      case "required":
        this.required = next;
        break;
    }
 }

Using getters and setters to communicate state

 set their own state through attributes, and in some cases, properties. 

 can reliably be set through properties on 

; required state must be set through an attribute. In this section, you'll wrangle these odd behaviors by adding getters and setters to 

, but it failed because there currently isn't a 

 setter, so let's add one. A setter method in JavaScript starts with the 

 keyboard, followed by the name of the method: 

. The first argument of this method is the value that should be set. The setter passes the value argument to the 

  set value(value: string) {
    this.$input.value = value;
  }

. There's no arguments here, but you can set the return value as 

, then in the curly brackets of the method, 

  get value(): string {
    return this.$input.value;
  }

Repeat the same process for getters and setters for the required state, although this time, you'll have to set state of the 

 attribute. When passing the required state from the 

, the type of the required state will be 

, because that's usually the type of anything passed through attributes. This is a problem because when coding the getter for required, you can reliably access the required state of the 

 property, which returns the required value with the type 

  get required(): boolean {
    return this.$input.required;
  }

In TypeScript, getter and setter types must match, so the setter has to accept both a 

 setter, adding conditions that check the 

  set required(value: boolean | string) {
    if (value === "true" || value === true) {
      this.$input.setAttribute("required", "true");
    }
    if (value === "false" || value === false) {
      this.$input.removeAttribute("required");
    }
  }

 getters and setters, return to TextInput.stories.js and add the 

. However, if you use Dev Tools to observe the changes in DOM, you'll notice the 

 because it doesn't exist yet. Remember, the 

 fires almost immediately, but the browser hasn't yet fully parsed all the nodes in the 

To handle situations like this, custom elements include another lifecycle method named 

 that runs when the custom element is added to DOM. 

 offers a space where you can reliably query DOM nodes in the custom element 

So, what does this mean for setting attributes reliably when you need to pass the value of the attribute from parent to child node? This means you need to store the values of the attribute somehow and defer setting the child node's attributes to 

. Eventually, you're going to handle many more different attributes in 

 with whatever passes through the callback.

---
title: Listening for attribute changes
description: Using observedAttributes and attributeChangedCallback to listen and react to changes in custom element attributes.
isPublicLesson: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/fullstack-web-components-book-app/-/tree/main/code/module_04
---

## Listening for attribute changes

Luckily, custom elements provide an API for observing changes to attributes and reacting to those changes. Every custom element has access to a `static` getter called `observedAttributes`. Using `observedAttributes`, you can establish which attributes the component listens to for changes. For performance sake, you have to whitelist each attribute that should be observed.

Just below the `constructor` in the `TextInputComponent class`, declare the `observedAttributes` getter. `observedAttributes` returns an `Array` of `String`, each `String` corresponding to the name of the attribute that should be observed. Add `required` and `value` to the `Array`. You're declaring `observedAttributes` so that developers can set the `value` and `required` attributes on `TextInputComponent` because those are the properties the condition in the `Validator` is referencing to validate whether or not the field has value. You'll add more attributes that should be observed later, but for now, `required` and `value` should suffice.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=28 startLine=26 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.05/protected/src/TextInput.validation.ts#L26 }
  static get observedAttributes() {
    return ["required", "value"];
  }
```

After declaring which attributes should be observed, a second method named `attributeChangedCallback` handles attribute changes. `attributeChangedCallback` is another method available to all custom elements that must be used in conjunction with `observedAttributes`.

`attributeChangedCallback` has three arguments, `name`, `previousValue`, and `currentValue` that pass the name of the changed attribute, the previous value of the attribute, and the current value of the attribute, respectively. For the sake of brevity, we'll call these arguments `name`, `prev`, and `next`. You could strictly type define the arguments and what the method returns, but for now we'll ignore the types for this method. `prev` and `next` could have many different types, so type defining these attributes anything other than `any` can become tricky. `name` will most certainly always be of type `string`.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=33 startLine=33 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.05/protected/src/TextInput.validation.ts#L33 }
  attributeChangedCallback(name, prev, next) {
```

Looking at the `attributeChangedCallback`, it appears we need a way to do something based on the value of `name` passed through the first argument of `attributeChangedCallback`. A typical convention is to use a `switch` statement.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=42 startLine=35 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.05/protected/src/TextInput.validation.ts#L35 }
    switch (name) {
      case "value":
        this.value = next;
        break;
      case "required":
        this.required = next;
        break;
    }
```

Add the `switch` to `attributeChangedCallback`, passing in the `name`. Set up `case` for `value` and `required`. If you try to set `this.value` and `this.required` based on the value of `next`, you'll be prompted with errors the `value` and `required` properties do not exist, so let's add them. When you are finished `attributeChangedCallback` should appear like the example below.

```typescript
 attributeChangedCallback(name, prev, next) {
    switch (name) {
      case "value":
        this.value = next;
        break;
      case "required":
        this.required = next;
        break;
    }
 }
```

### Using getters and setters to communicate state

Existing form controls like `HTMLInputElement` and `HTMLTextAreaElement` set their own state through attributes, and in some cases, properties. `value` and `disabled` can reliably be set through properties on `HTMLInputElement`, but there is no equivalent setter for `required`; required state must be set through an attribute. In this section, you'll wrangle these odd behaviors by adding getters and setters to `TextInputComponent class`.

Earlier, we tried to set `value` in the `attributeChangedCallback`, but it failed because there currently isn't a `value` setter, so let's add one. A setter method in JavaScript starts with the `set` keyboard, followed by the name of the method: `value`. The first argument of this method is the value that should be set. The setter passes the value argument to the `HTMLInputElement` by setting that element's `value` property.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=111 startLine=109 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.05/protected/src/TextInput.validation.ts#L109 }
  set value(value: string) {
    this.$input.value = value;
  }
```

To allow the `Validator` access to the current `value` of the `HTMLInputElement`, make a getter on the `TextInputComponent class`. Getters in JavaScript start with the `get` keyword, followed by the method name: `value`. There's no arguments here, but you can set the return value as `string`, then in the curly brackets of the method, `return` the `HTMLInputElement` current value state: `this.$input.value`.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=104 startLine=102 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.05/protected/src/TextInput.validation.ts#L102 }
  get value(): string {
    return this.$input.value;
  }
```

Repeat the same process for getters and setters for the required state, although this time, you'll have to set state of the `HTMLInputElement` by setting its `required` attribute. When passing the required state from the `TextInputComponent` attribute through `attributesChangedCallback`, the type of the required state will be `string`, because that's usually the type of anything passed through attributes. This is a problem because when coding the getter for required, you can reliably access the required state of the `HTMLInputElement` through the `required` property, which returns the required value with the type `boolean`.

Add the `required` getter to `TextInputComponent`.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=118 startLine=116 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.05/protected/src/TextInput.validation.ts#L116 }
  get required(): boolean {
    return this.$input.required;
  }
```

In TypeScript, getter and setter types must match, so the setter has to accept both a `string` and `boolean` in the first argument. Code the `required` setter, adding conditions that check the `value` and either call `setAttribute` or `removeAttribute` on the `HTMLInputElement`.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=130 startLine=123 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.05/protected/src/TextInput.validation.ts#L123 }
  set required(value: boolean | string) {
    if (value === "true" || value === true) {
      this.$input.setAttribute("required", "true");
    }
    if (value === "false" || value === false) {
      this.$input.removeAttribute("required");
    }
  }
```

Once you've completed the `value` and `required` getters and setters, return to TextInput.stories.js and add the `required` attribute to `in-textinput` in the story template.

You'd expect to see a `required` attribute set on the `HTMLInputElement`. However, if you use Dev Tools to observe the changes in DOM, you'll notice the `required` attribute is still missing.

![](https://d2uusema5elisf.cloudfront.net/courses/fullstack-web-components/module_04/lesson_04.05/public/assets/TextInput.no-required.png)

This is something we need to fix since `TextInputComponent` relies on the state of `HTMLInputElement`, but what is the problem?

### Passing attribute values to children

It's likely the `HTMLInputElement` hasn't received a call to `setAttribute` because it doesn't exist yet. Remember, the `HTMLInputElement` is encapsulated by Shadow DOM. When `TextInputComponent` instantiates, `attributeChangedCallback` fires almost immediately, but the browser hasn't yet fully parsed all the nodes in the `ShadowDOM`.

To handle situations like this, custom elements include another lifecycle method named `connectedCallback` that runs when the custom element is added to DOM. `connectedCallback` offers a space where you can reliably query DOM nodes in the custom element `ShadowRoot`.

So, what does this mean for setting attributes reliably when you need to pass the value of the attribute from parent to child node? This means you need to store the values of the attribute somehow and defer setting the child node's attributes to `connectedCallback`. Eventually, you're going to handle many more different attributes in `attributeChangedCallback`, so create a new property on the `TextInputComponent` class called `$attr` and make it equal to an empty `Object`.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=10 startLine=10 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.05/protected/src/TextInput.validation.ts#L10 }
  private $attr = {};
```

In the first line of `attributeChangedCallback`, set properties on `this.$attr` with whatever passes through the callback.