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

 can communicate with a form, we should expose some of the methods used during form validation on the component 

 and possibly not as intuitive for engineers to access validity state. If validity state were to remain on 

 won't behave like a typical form element which stores this state on the root of the 

. We don't necessarily want developers to reference 

 when looping over a form's inputs to check validity. Accessing validity state from 

Create methods on the component class that expose the methods on the 

 interface used for validation. First, make a method called 

. This method checks for any constraints that have been declared, and when the check fails for any of those constraints, triggers the browser to emit an 

  checkValidity() {
    return this.internals.checkValidity();
  }

  reportValidity() {
    return this.internals.reportValidity();
  }

 that includes properties that reflect common validation errors, such as 

  get validity() {
    return this.internals.validity;
  }

The next validation-related getter named 

 returns messages associated with each invalid condition of the form control. Add the 

 getter to the class, and make sure it returns 

  get validationMessage() {
    return this.internals.validationMessage;
  }

 that does most of the heavy lifting during form validation. This method takes three arguments: 

  setValidity(
    flags: ValidityStateFlags,
    message?: string,
    anchor?: HTMLElement
  ): void {
    this.internals.setValidity(flags, message, anchor);
  }

Whenever validation is triggered on the form control, usually when the user changes the value or blurs the input, you can write logic that validates the current value of the form control against some condition.

Consider the scenario of the required form control. The user focuses the input, then immediately navigates away without entering text into the field, effectively blurring the input. You could write a condition like the following that checks if the value of the text input has any content and then triggers validation on the 

 interface by setting a validation error with the key 

if (input.value && input.value.length > 0) {
  input.setValidity(
    {
      valueMissing: true,
    },
    "Error: This field is required, Please type in a value."
  );
}

There is an important distinction between the first and second arguments of 

 in the first argument communicates to the user agent what is invalid, while the message composed from a 

 in the second argument provides user feedback for the user to acknowledge the error and take action. The message 

Error: This field is required, Please type in a value.

 notifies the user there is a problem, prompting them to type something into the text field as a solution.

Two questions arise from the above example. Where does validation logic like the above belong in source code? How do you get the message to display in UI? For now, let's tackle the answer to this first question. Later in this chapter, we'll dive into the second.

Where does validation logic belong in the source code?

 provides a convenient place to listen for events on the 

 and provide a callback to validate the form control. Suppose somewhere in the component, you listened for the blur event on the input and triggered validation on a method called 

. The content in this section is speculative and should not be implemented in 

this.$input.addEventListener("blur", this.onValidate);

 callback, you could check if the element has a required attribute and the validity of the input. When the condition returns false, call 

 flag because it matches this use-case and viable user feedback in the second argument.

---
title: Handling Validation
description: 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.
isPublicLesson: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/fullstack-web-components-book-app/-/tree/main/code/module_04
---

## Handling Validation

Now the `TextInputComponent` can communicate with a form, we should expose some of the methods used during form validation on the component `class`. Validation state is stored on `this.internals`, which is both `private` and possibly not as intuitive for engineers to access validity state. If validity state were to remain on `internals`, it would mean `TextInputComponent` won't behave like a typical form element which stores this state on the root of the `Object`. We don't necessarily want developers to reference `input.internals.validity` when looping over a form's inputs to check validity. Accessing validity state from `input.validity` is much more preferable.

Create methods on the component class that expose the methods on the `ElementInternals` interface used for validation. First, make a method called `checkValidity` that returns `this.internals.checkValidity()`. This method checks for any constraints that have been declared, and when the check fails for any of those constraints, triggers the browser to emit an `invalid` event, and then returns `false`.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=62 startLine=60 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.04/protected/src/TextInput.validation.ts#L60 }
  checkValidity() {
    return this.internals.checkValidity();
  }
```

`reportValidity` is very similar to `checkValidity`, except that it also sends any `validationMessage` to the user agent for display. Add the `reportValidity` method to the `TextInputComponent` class, and make sure it returns `this.internals.reportValidity()`.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=69 startLine=67 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.04/protected/src/TextInput.validation.ts#L67 }
  reportValidity() {
    return this.internals.reportValidity();
  }
```

`validity` returns a `ValidationState Object` that includes properties that reflect common validation errors, such as `tooShort`, `valueMissing`, and `patternMismatch`. This property is read-only. `ValidationState` is updated on calls to `setValidity` and `reportValidity`. Add the `validity` getter to the `class`, and make sure it returns `this.internals.validity`.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=76 startLine=74 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.04/protected/src/TextInput.validation.ts#L74 }
  get validity() {
    return this.internals.validity;
  }
```

The next validation-related getter named `validationMessages` returns messages associated with each invalid condition of the form control. Add the `validationMessages` getter to the class, and make sure it returns `this.internals.validationMessages`.

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=83 startLine=81 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.04/protected/src/TextInput.validation.ts#L81 }
  get validationMessage() {
    return this.internals.validationMessage;
  }
```

`ElementInternals` includes a method called `setValidity` that does most of the heavy lifting during form validation. This method takes three arguments: `flags`, `message`, and `anchor`:

-   `flags` is an `Object` with properties that correspond to the validation flags the browser can interpret
-   `message` allows the engineer to pass a custom human-readable `string` that can be used to display feedback for the end-user
-   `anchor` is set to the `HTMLElement` the engineer wishes to display the user feedback stored in `message`

```javascript { actualFilename=./protected/src/TextInput.validation.ts endLine=97 startLine=91 statedFilename=TextInput.ts url=https&#x3A;//gitlab.com/fullstackio/books/fullstack-web-components/module_04/lesson_04.04/protected/src/TextInput.validation.ts#L91 }
  setValidity(
    flags: ValidityStateFlags,
    message?: string,
    anchor?: HTMLElement
  ): void {
    this.internals.setValidity(flags, message, anchor);
  }
```

Whenever validation is triggered on the form control, usually when the user changes the value or blurs the input, you can write logic that validates the current value of the form control against some condition.

Consider the scenario of the required form control. The user focuses the input, then immediately navigates away without entering text into the field, effectively blurring the input. You could write a condition like the following that checks if the value of the text input has any content and then triggers validation on the `ElementInternals` interface by setting a validation error with the key `valueMissing` equal to `true` and a human-readable message.

```javascript
if (input.value && input.value.length > 0) {
  input.setValidity(
    {
      valueMissing: true,
    },
    "Error: This field is required, Please type in a value."
  );
}
```

There is an important distinction between the first and second arguments of `setValidity`. The `ValidityStateFlags` on the `Object` in the first argument communicates to the user agent what is invalid, while the message composed from a `String` in the second argument provides user feedback for the user to acknowledge the error and take action. The message `Error: This field is required, Please type in a value.` notifies the user there is a problem, prompting them to type something into the text field as a solution.

Two questions arise from the above example. Where does validation logic like the above belong in source code? How do you get the message to display in UI? For now, let's tackle the answer to this first question. Later in this chapter, we'll dive into the second.

### Where does validation logic belong in the source code?

The context of the `TextInputComponent` provides a convenient place to listen for events on the `HTMLInputElement` and provide a callback to validate the form control. Suppose somewhere in the component, you listened for the blur event on the input and triggered validation on a method called `onValidate`. The content in this section is speculative and should not be implemented in `TextInputComponent`.