Andy Desmarais

A code ninja and all around tech geek

StencilJs - Part 1 - Quick Introduction

2019-11-17 Andy DesmaraisWebComponents

Cover photo credit: Marija Zaric

I’ve covered building native web components in a past series. There’s a lot of boilerplate that can creep in as you write web components. This is increasingly true if you’re planning to build an entire design system worth of components. Stencil Js (or just Stencil for short) aims to solve the problem of building and maintaining a large number of web components.

I’m not going to dig into what a design system is here. That’s a post unto itself. For the purposes of this article, I’ll focus primarily on getting started with Stencil.

In this article we’ll cover:

Why does Stencil exist

Stencil is built by the Ionic team. They had an issue trying to build their component sets in a way that allowed the creation of PWAs (progressive web apps). They attempted and failed to build a PWA using Angular as the core of the component set, and found they were just shipping too much code to make it possible. When they took a step back and looked at the problem holistically they found that picking any framework was not going to be tenable.

This challenge and the requirement for cross framework functionality drove them to pick Web Components.

There’s an entire talk digging into more detail if you’re interested.

What is Stencil

Stencil is a compiler that generates Web Components (more specifically, Custom Elements).


Stencil’s main goal is to simplify the process of building web components. They have a unique mix of Angular like decorators and React like JSX. This provides a straight-forward way to solve some of the more detail oriented problems that make the native APIs painful.

They start by abstracting away the Custom Element entirely. This gives them the ability to highly optimize the resulting web component. They are also using a virtual DOM to manage change detection. They worked hard to have an asynchronous rendering engine that was inspired by React Fiber.

That’s enough of the what and why. Let’s get started!

Getting started building components

Stencil has a whole section of their site dedicated to getting started. I’m going to be a bit more high level than their walk through.

First let’s init a new project. Head to your favorite folder of choice and run:

npm init stencil

You’ll be presented with three options, each of which will change the way the project is built.

  • ionic-pwa - Bootstrap an entire application based on Stencil that is PWA ready
  • app - The smallest footprint needed to create a basic application (not a PWA)
  • component - Use this for creating a package of reusable components

For the remainder of this article, let’s focus on the component type of project.

Project Structure

The project is structured to focus on component creation. There’s a src/ folder that contains a components/ folder with a basic “hello world” style of component.

They also spin up a `utils/’ folder. For my taste, a folder named utils shouldn’t live long.

If you look at the package.json you’ll notice that all of the scripts are powered by the Stencil CLI. This abstracts a lot of the “who is doing what, and how” from you as a consumer. I personally believe that stuff is worth knowing, so here’s a quick list:

  • stencil build is powered by Rollup under the hood.

    • The stencil.config.ts exposed a custom configuration that does not allow ALL Rollup functionality
    • This can be pretty frustrating depending on what you need.
  • stencil test is powered by jest. Both the unit and e2e tests are run using jest.
  • stencil generate is a custom tool similar to ng generate allowing the quick creation of new components.

Typescript or die

You probably also noticed that there are no js files in this project. Stencil is very heavily bought into the Typescript eco-system. It’s a part of what makes things like decorators more straight forward.

The project is typescript all the way down to their configuration files.

Your first component

If you open src/components/my-component/my-component.tsx you’ll see a small amount of code designed to display a person’s name.

Here’s the guts of the component:

  tag: 'my-component',
  styleUrl: 'my-component.css',
  shadow: true
export class MyComponent {
   * The first name
  @Prop() first: string;

   * The middle name
  @Prop() middle: string;

   * The last name
  @Prop() last: string;

  private getText(): string {
    return `${this.first} ${this.middle} ${this.last}`;

  render() {
    return <div>Hello, World! I'm {this.getText()}</div>;

Just before the class declaration is the @Component decorator. This decorator is very similar to the Angular @Component decorator. It lets you define the tag that will represent the class, the css file to be used, and the shadow boolean to state whether or not the component should be using the shadow DOM.

There are a few other properties you can set on the component, but we’ll cover those in a later post.


There are three @Prop entries. These represent both attributes and properties on the web component.

Properties: Programmatically accessible on an instance of the DOM node. Attributes: Added to the tag in the DOM. Eg. the href attribute on an anchor tag.

We need to have coverage for both properties and attributes because attributes do not support complex data types. The only type attributes support is string. This means you can pass all of the primitive types into attributes easily as strings, but complex types would require serialization to a string.

You should also take note of the comments above each of the props. These comments will be turned into the Description column of the table in the auto-generated README file in the same directory as the component. This README can have custom content added above the delimiter. It’s a super quick and simple way to document each component as it’s built.


If you’ve done any React development then the render() method should look very familiar. It’s a very straightforward jsx statement that returns out the concatenated name.

You can return a JSX object, or an array of JSX objects.


Writing code should always be accompanied by writing tests that help the next dev (or you 6 months from now) better understand the purpose of the code. In the case of Stencil they suggest end to end testing as a way to handle this.

Alongside the component tsx file is an e2e.ts file which has a basic set of tests in it to cover the logic in our component.

Stencil utilizes puppeteer to drive browser based tests with Jest. This means you’ll be testing your components in an actual browser instead a node environment. This makes it a bit closer to the reality the component will experience when deployed to a live site.

Wrapping up

Stencil makes creating web components a very simple process, and with a little bit of work you can have a very well documented and fully functional component.

My plan for the next few parts of this series is to cover:

  • @Component decorator deep dive
  • @Prop, @Watch and @Method decorators deep dive
  • @Event and @Listen decorators deep dive
  • @State decorator deep dive
  • <Host> element and how/why to use it
  • Using Sass with Stencil