ESLint is the linting tool of choice for JavaScript in Drupal. In this tutorial we’ll show how to install the ESLint application and then use it to verify that your JavaScript files are meeting the Drupal coding standards.

Drupal (as of version 8.4) has adopted the Airbnb JavaScript coding standards. In this tutorial, we'll walk through how to install the necessary package dependencies to run eslint on JavaScript files within your Drupal site.

An asset library is a bundle of CSS and/or JavaScript files that work together to provide a style and functionality for a specific component. They are frequently used to isolate the functionality and styling of a specific component, like the tabs displayed at the top of each node, into a reusable library. If you want to include CSS and/or JavaScript in your Drupal theme or module you'll need to declare an asset library that tells Drupal about the existence, and location, of those files. And then attach that library to a page, or specific element, so that it gets loaded when needed.

In this tutorial we’ll:

• Define what an asset library is.
• Explain why asset libraries are used to include JavaScript and CSS files.
• Look at some example asset library definitions.

By the end of this tutorial you should be able to define what asset libraries are, and when you'll need to create one.

Preprocess functions allow Drupal themes to manipulate the variables that are used in Twig template files by using PHP functions to preprocess data before it is exposed to each template. All of the dynamic content available to theme developers within a Twig template file is exposed through a preprocess function. Understanding how preprocess functions work, and the role they play, is important for both module developers and theme developers.

In this tutorial we'll learn:

• What preprocess functions are and how they work
• The use case for preprocess functions
• The order of execution for preprocess functions

By the end of this tutorial you should be able to explain what preprocess functions are and the role they play in a Drupal theme.

Template files are responsible for the HTML markup of every page generated by Drupal. Any file ending with the .html.twig extension is a template file. These files are composed of standard HTML markup as well as tokens used by the Twig template engine to represent dynamic content that will be substituted into the HTML markup when the template is used. As a theme developer, you'll work with this a lot.

In this tutorial we’re going to learn about:

• What template files are, and how they fit into the big picture of creating a theme
• How template files are used in order to allow theme developers to modify the HTML markup output by Drupal
• Naming conventions for, and specificity of, template files

Themes are the part of Drupal that you, and anyone else visiting your Drupal powered application, see when they view any page in their browser. You can think of a theme as a layer, kind of like a screen, that exists between your Drupal content and the users of your site. Whenever a page is requested Drupal does the work of assembling the content to display into structured data which is then handed off to the presentation layer to determine how to visually represent the data provided.

Drupal themes are created by front-end developer. Frequently referred to as themers, or theme developers. Themes consist of standard web assets like CSS, JavaScript, and images, combined with Drupal-specific templates for generating HTML markup, and YAML files for telling Drupal about the file and features that make up each individual theme.

In this tutorial we'll:

• Explain what a Drupal theme is.
• Explain the role of a Drupal themer in the process of building a Drupal site.
• Get a high level overview of the types of files/code that themes are made of.

By the end of this tutorial you should be able to explain what a Drupal theme is, and the kind of work a Drupal theme developer will be expected to do.

Maybe you've heard of anonymous closures but you're not quite sure how they apply in Drupal, or why using them is considered a best-practice. Anonymous closures allow you to avoid accidentally clashing with anything in the global scope, as well as to alias the jQuery object to the more commonly used $. This is necessary because Drupal runs jQuery in no-conflict mode. This tutorial will look at the syntax used for placing your custom JavaScript code inside an anonymous closure, and why it's a good idea to do so.

In this tutorial we'll:

• Explain what a closure is (briefly), and what immediately invoked function expressions are
• Show how typically Drupal JavaScript gets wrapped in a closure
• Provide a copy/paste example you can use in your own code

By the end of this tutorial you should be able to explain what an anonymous closure is, and how to use one in your custom JavaScript for Drupal.

Like most output in Drupal, Views relies on Twig templates for a significant amount of its rendering. In this tutorial we'll identify where you can find the default Views templates within your file system, what the common templates are for, and how to name your templates so that they are applied to specific views.

By the end of this tutorial, you should be able to:

• Identify where to find default views templates
• Understand which templates apply to what part of a view
• Get a sense of the template suggestions and how to use them to limit where your custom templates are applied
• Identify a view's machine name
• Identify a display's machine name
• Identify a field's machine name

Now that we understand what templates are and how we can use them, let's override some templates! In this tutorial we'll copy the views wrapper template to our theme and override it so that we can customize the markup for the Baseball Players view. Then we'll modify the template so that our view's pager appears both above and below our table of players.

As a theme developer you can extend an existing asset library to include custom CSS and/or JavaScript from your theme. This is useful when you want to add styles or behaviors to components provided by Drupal core or another module.

Sometimes there are CSS or JavaScript asset libraries attached to the page by Drupal core, a contributed module, or another theme, that do something you don't like, and you want to change it or even exclude it all together. There are a couple of different ways that themes can override, alter, or extend, an existing asset library in order to modify the CSS and JavaScript that get attached the page by other code belonging to another theme or module.

In this tutorial we'll learn how to:

• Extend an existing asset library using libraries-extend, so that our custom CSS and JavaScript is included whenever that library is used.
• Override an existing asset library using libraries-override, to alter the definition of the library, and replace or exclude individual assets (or the entire library).

By the end of this tutorial you should be able to use your custom theme to override, extend, or alter any of the asset libraries added to the page by another theme or module.

Twig is the theme template engine in Drupal as of version 8. Like any code, there are guidelines and standards that dictate both the style and the structure of the code. In this tutorial we will explain how to adhere to the Drupal code standards while implementing Twig templating.

By the end of this tutorial you will be able to adhere to Drupal's coding standards when writing Twig, and know where to find more information about the guidelines when necessary.