Learn Eslint, Polymer and React-storybook

Polymer is a framework created at Google for building reusable web components.

We use HTML elements every day such as div, a, button, form, and table. All HTML elements provide some sort of API of attributes, properties, methods and events that we use to manipulate them. Generally, web elements are styled in a particular way by default, and also allow us to customize their styling with CSS.

Polymer allows you to build fully custom web elements of your own for more complex components composed of the building blocks you are used to with the standard set of HTML elements.

This course is an introduction to Polymer, and should get you started building your own customizable web elements that you can use on their own, or even with your favorite framework like Angular or React!

The Polymer library provides a set of features for creating custom elements. These features are designed to make it easier and faster to make custom elements that work like standard DOM elements.

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Creating Custom Web Elements with Polymer 2

React is one of the web’s most popular frameworks for building JavaScript applications. 

If you know what you’re doing, React can drastically simplify how you build, use, and maintain code.

Whether you’re a React newbie or you’re ready for advanced techniques, you can level-up with egghead. Explore our in-depth courses, lessons, and community resources to build more powerful applications and crack open your career.

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Storybook is a UI component development environment for React, Vue, and Angular. With that, you can develop UI components without running your app.

Here I show how best to configure a React Storybook to use [Typescript](/q/typescript) (with TSX). This lets you write your reusable components with maximal type safety. Please see the provided code for the full source.

Configure React Storybook for use with Typescript

Our first layer of defense when building accessible React applications is to add some auditing tools to our development workflow. This should include adding static analysis checking for common accessibility standards and best practices. We can get this with the latest eslint plugin: "eslint-plugin-jsx-a11y". If you are already linting your projects adding this plugin should fit very smoothly into your workflow.

## Resources
1. [List of all the rules supported by eslint-plugin-jsx-a11y](https://github.com/evcohen/eslint-plugin-jsx-a11y#supported-rules)
2. [Comparison of the difference in rules between recommended and strict mode](https://github.com/evcohen/eslint-plugin-jsx-a11y#difference-between-recommended-and-strict-mode)
3. [Background on how the Accessibility API implemented by browsers interfaces with Assistive Technologies](https://github.com/evcohen/eslint-plugin-jsx-a11y#some-background-on-wai-aria-the-ax-tree-and-browsers)

Set up ESLint to Audit Accessibility Issues in React

The pluggable linting utility for JavaScript and JSX.

eslint

We have a number of options to use when testing for accessibility issues concerning alternative text for images.  The react-axe library and axe browser extension will both report when an image is missing alternative text.  The tota11y browser extension will annotate any images that are missing alternative text.  Running eslint with the eslint-plugin-jsx-a11y plugin installed will report on any `<img>` elements missing an `alt` attribute.  Finally, we can use a screen reader to actually hear what is read when an image has focus.

Test for Image Alternative Text Accessibility Issues

Images must have text alternatives that describe the information or function 
represented by them so they can be read and understood by those using screen readers.

## Alternative text for images
* All `<img>` elements should have an `alt` attribute provided
    * When not provided many screen readers will fallback to reading the file name instead
* The `alt` value should not include words like _“image”_, _“picture”_ or _“icon”_
    * Screen readers are already announcing that the element is an image
* Images that are decorative only and provide no information or function to the page should be hidden from Assistive Technologies by providing an empty `alt` attribute (`alt=””`)

## References
* https://www.w3.org/WAI/tutorials/images/
* https://www.w3.org/WAI/tutorials/images/decision-tree/
* https://www.w3.org/WAI/tutorials/images/tips/
* https://www.w3.org/WAI/tutorials/images/informative/
* https://www.w3.org/WAI/tutorials/images/functional/
* https://www.w3.org/WAI/tutorials/images/textual/

Define Images with Appropriate Text Alternatives

Our accessibility auditing toolkit makes it very easy for us to test for any form controls that are missing labels or are not correctly associated with their labels.  Running Eslint with the eslint-plugin-jsx-a11y installed will show us any form controls with missing or unassociated labels.  Running react-axe or using axe to audit a web page will similarly list any of these findings.  Finally using tota11y we can annotate any form controls on a page that are missing labels.  This is definitely some of the lower hanging fruit that we can easily find and fix when making our applications accessible!

Test for Form Control Label Accessibility Issues

Labeling inputs, elements, and widgets add context and clarity for assistive technology such as screen readers.  Elements of a `<form>` such as `<input>` or `<textarea>` or `<select>` should be labelled by using a `<label>` element.  Placeholder text can be used in addition to a label but should not be used in place of a label as not all assistive technologies respect placeholder text.

Ensure Form Controls have Accessible Labels

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.

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In this lesson you will learn how to use the eslint yeoman generator to scaffold a new plugin to provide custom rules for your project. 
You will also be able to check the basic skeleton for an eslint ruleand a working example rule to apply a naming convention for async functions.

Scaffold an eslint Plugin with Yeoman

[Storybook](https://storybook.js.org/) is a framework that allows you to develop components in isolation. It is ideal for a variety of use cases, most commonly when you're building some shared component toolkit. As such, Nx comes with a dedicated package `@nrwl/storybook` that adds a series of useful code generators to add storybook to an existing React library, but also to automatically generate stories and more.

Use Storybook to Develop React Components in Nx

[Nx](https://nx.dev) combines a lot of the best tools on the market, integrates them, and provides tooling to provide the best possible developer experience. One such pair is [Cypress](https://www.cypress.io/) and [Storybook](https://storybook.js.org/). Storybook allows developing components in isolation, which is also a great basis for testing components, which is where Cypress shines.
In this lesson, we're exploring some of the `@nrwl/storybook` code generation utilities that allow you to not only configure Storybook for a React application but automatically also set up a Cypress test configuration for it along with some spec files generated from the Storybook stories.

Use Cypress and Storybook to test your React Components in Isolation

Nx already comes with a default TypeScript setup. Although you can opt-out of that and generate plain JS based components, especially in a large setup TypeScript can be beneficial in uncovering potential issues early. To add an extra level, Nx also comes with preconfigured eslint for all the apps and libraries. In this lesson we're going to explore the lint setup and how it can be used.

Linting a React app with Nx

In this lesson we'll look at creating event listeners in our custom element using the listeners object as well as annotated event listeners.

Create Event Listeners in Polymer Elements

In this lesson, we show how to setup React Storybook into an existing React project. We’ll go through common configuration files and how to adjust them.

Add React Storybook to a Project 

Components trigger actions via function props to notify of events that occur within the Component. We can inspect those events using React Storybook.

Inspect Component Events with React Storybook

Custom element property changes can be observed by specifying an observer method which will be executed whenever a change occurs and receive both the new value and old value.

Observe Property Changes on a Custom Polymer Element

In this lesson we'll look at the lifecycle callbacks (Created, Ready, Attached, Detached, and attributeChanged) available to us in Polymer and how to use each one.

Use Lifecycle Callbacks On Custom Polymer Elements

Polymer provides us a variety of data-binding configurations.  In this lesson we'll look at configuring Two-way, One-way-down and One-way-up data binding.

Data Bind Two Polymer Elements

ESLint is a JavaScript linter (static analysis tool) that offers full support for ES6, JSX, and other modern tools via plugins. We walk through setting up ESLint in a project, using the `eslint --init` CLI tool with the JSX and ES6 options, writing a React component in JSX, and adding some extra react linting rules with a plugin. ESLint is built to be "pluggable" with simple, extendable, modular rules and an API for writing and using plugins. ESLint has many rules which are all turned off by default; you can extend the core "recommended" rules which will catch common JavaScript errors, and you can also turn on stylistic rules for code consistency. You can also use plugin rules which we do in this lesson with the `eslint-plugin-react` package.

Linting React JSX with ESLint (in ES6)



[React Storybook](https://github.com/kadirahq/react-storybook) is a UI component development environment for React.
With that, you can develop UI components without running your app.

Here I show how to get started with React Storybook.
For more information, have a look at our [documentation](https://getstorybook.io/docs).

Getting Started with React Storybook

React Component stories can be dynamically loaded into React Storybook instead of doing it manually every time.

Load React Storybook Stories Dynamically

We can utilize the "fire" method from within our custom element to execute a custom event.  In this lesson we'll create a custom event and pass useful data along with the event to the consumer of our element.

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