Virtual DOM

Learning Objectives

After this lesson, you will be able to:

• Describe the Virtual DOM versus the standard DOM

• Understand how components are called

Review and Refactor

Hello in src/App.js is our component class. It has a render method that returns the JSX for our "Hello World!" and heading tags. Keeping components separate and organized is a best practice, so we created that class in its own file.

To show up on the page, though, that component still needs to actually be called from somewhere. The main "hub" of our React app is src/index.js. We'll investigate how src/index.js is currently loading and rendering the component, and we'll improve the code by making it more explicit and readable.

Look at your src/index.js file, and contrast it with the code below.

import React from 'react';
import ReactDOM from 'react-dom';
import Hello from './App.js';

ReactDOM.render(
  <Hello />,
  document.getElementById('root')
)

The first (and smallest) difference is that we've dropped the CSS file import. We just aren't using it.

The next difference is that import App from './App'; has turned into import Hello from './App.js'.

This line imports the Hello component from the src/App.js file. Remember, the default part of export default Hello in src/App.js means that importing other names - like App - actually already brings in the Hello component! As a best practice, though, we're going to explicitly import the Hello component.

The last difference is that ReactDOM.render( <App />, has turned into ReactDOM.render( <Hello />,.

This changes the ReactDOM.render() call to explicitly say "Render whatever the component Hello returns."

Hello World exercise - You do!

Code along: Calling our Hello component explicitly

Update your index.js file to have the three changes listed above:

• Delete the CSS import.

• Change the component name that's imported to be your Hello component.

• Change the component name that's used inside ReactDOM.render to be your Hello component.

Check it out! You should be able to browse to http://localhost:3000 and see that nothing has changed.

Virtual DOM Intro

You should be familiar with the DOM. You may have noticed that our src/index.js code mentions ReactDOM. ReactDOM doesn't refer to the same DOM we know. Instead, it refers to a Virtual DOM. The Virtual DOM is a key piece of how React works.

So, how is different? Watch this video to find out. (note: right click for new tab!)

In React, the virtual DOM is a staging area for changes that will eventually be implemented.

Virtual DOM Diagram

If you're interested in learning more about the Virtual DOM, check this video out. (note: right click for new tab!)

You know every component has, at a minimum, a render method. The render method generates a Virtual DOM node to be added to the actual DOM.

The contents of this node are what we define in the method's return statement, using JSX.

The ReactDOM.render() function takes two arguments:

ReactDOM.render(
  <Hello />,
  document.getElementById('root')
)

• <Hello /> uses the name of the component to render. In our App.js file, the Hello component returns the content to render: a div with "Hello World!" and heading tags (written in JSX). As a reminder, this is the Hello component:

  class Hello extends Component {
    // what should the component render?
    render () {
      // make sure to return some UI
      return (
        <div>
          <h1>Hello World!</h1>
          <h3>It is time for tea.</h3>
        </div>
      )
    }
  }

• The second argument of the ReactDOM.render() function is document.getElementById('root'); this finds the DOM element to append that content to. This argument can be any element on the page. Here, we're simply appending it to an element with the id root. (Look through the index.html file if you're curious about the HTML structure from create-react-app.)

When our index.js is processed, React compares the virtual DOM to the regular DOM and only updates the root element on the page.

Side note: What is <Hello /> written in? JSX! Whenever you use a self-closing tag in JSX, you MUST end it with a /, like <Hello /> in the above example. If you don't use a self-closing tag, JSX will look for a closing tag and never find it!