React Components Basics

Overview

We'll introduce the heart of React: components. This will include explaining why they're important and examining a few examples. If the idea and application of components don't click immediately, do not worry! The different moving parts required to understand how to use them will fall into place as we move forward.

Objectives

1. Understand React components
2. Create React components and show the HTML they create

Introduction

Let's examine a high level overview of what a React component is before we implement one. The official React documentation on components says it best:

Components let you split the UI into independent, reusable pieces, and think about each piece in isolation.

Components modularize both functionality and presentation in our code. In order to understand how powerful this is, consider just how intricate web applications can become. The difficulty in logically arranging, architecting, and programming these web applications increases with their size. Components are like little packages: they help us keep everything organized and predictable while abstracting the 'boiler plate' code. Each component contains a snippet of code that describes what it should render to the DOM.

React Application Idea

Enough of a description — let's see some examples! While the possibilities of what we can do with components are endless, the first thing we need to understand about them is the ways in which they act as code templates. Let's start simply and build up from there using the following as an example:

Let's imagine we want a blog article describing the fact (note: not opinion) of why Bjarne Stroustrup has the perfect lecture oration. We also want our blog article to display comments made by readers.

Step 1: write the components

First, let's make a component to showcase an opinion:

function Article() {
  return (
    <div>Dear Reader: Bjarne Stroustrup has the perfect lecture oration.</div>
  );
}

Take a moment to read that code line by line:

• a function, Article, is declared
• what it should return is explicitly provided (we tell React "Hey, when you want to put this component on the DOM, here is what it should become!")

When React creates this element and adds it to the DOM, the resulting HTML will look just as you would expect:

<div>Dear Reader: Bjarne Stroustrup has the perfect lecture oration.</div>

Let's see what it would look like, were we to only render this one component, in the DOM:

Ok, that takes care of our Article part of our application. Now let's make a component to display a single user's comment:

function Comment() {
  return <div>Naturally, I agree with this article.</div>;
}

Take the time to read that component line by line. Here is the HTML that this would create when added to the DOM:

<div>Naturally, I agree with this article.</div>

In both of our examples, React is taking JavaScript code, interpreting that special HTML/JavaScript syntax within the return() statement, and spitting out plain old HTML that browsers will know how to represent to the user.

Once we have our components in hand, it's time to actually use them.

Step 2: use the components

Now that we have these components written, all we need to do is make sure some other component is making use of them in its return statement. Every React application has some top level component(s). Very often, this top level component is simply called App. For our example, here's what it might look like:

function App() {
  return (
    <div>
      <Article />
      <Comment />
    </div>
  );
}

Here we can see JSX coming into play a bit more. The code inside the return() still looks a lot like regular HTML, but in addition to rendering a regular old HTML div element we're also rendering our two components. We've created code that is not only well structured and modular, but also a straightforward description of what we want the App component to do: render the article first, followed by the comment. Here is what the resulting HTML will look like:

<div>
  <div>Dear Reader: Bjarne Stroustrup has the perfect lecture oration.</div>
  <div>Naturally, I agree with this article.</div>
</div>

This unpacks logically. The App component (being our top level component) wraps around both Article and Comment, and we already know what they look like when they are turned into HTML.

As you may expect, we refer to the App component as both the Comment and Article component's parent component. Inversely, we refer to Comment and Article as children components of App.

A Note on Classes

As you're exploring the React documentation, and finding other resources on React on the internet, you'll probably notice there are multiple syntaxes you can use for creating components: function components and class components.

A function component looks like this:

function Comment() {
  return <div>Naturally, I agree with this article.</div>;
}

Or this, using the arrow function syntax:

const Comment = () => <div>Naturally, I agree with this article.</div>;

A class component looks like this:

class Comment extends React.Component {
  render() {
    return <div>Naturally, I agree with this article.</div>;
  }
}

For many years, the only way to work with certain key features of React — state and lifecycle — was to use class components. Since the introduction of hooks in React 16.8, this is no longer true, and function components can be used for (almost) everything that class components can.

React's recommendation is that new components are written as function components; but class components will continue to be supported as well. React is also planning to rewrite their documentation to focus on function components and hooks in the near future.

In the meantime, you can also view a community-made version of the React docs that shows all examples with function components here.

You'll learn more about class components later on, so that when you encounter them in legacy code, you'll still be able to work with them. However, for the time being, we'll just be focusing on function components.

Summary

We just introduced simplified, bare bones, React components. They are used to house modularized front end code. In our example, which is often the case, they contain information on how a portion of our application should be turned into HTML.

Going forward, we will continue with this example and show how components can be re-used and how they can be written as dynamic templates, where their content is interchangeable.

Resources

• React Top-Level API
• Introducing JSX
• React Docs with Hooks