React Crash Course for Beginners, Part 2

  
Discover Your Movie Mojo!

However, you might have noticed there is an issue. In App.js the  component render method returns what looks like HTML. Yet in Header.js there’s just a single call to React.createElement(). What’s going on?

The answer is JSX. In App.js we use JSX to write HTML-like syntax to define our component output. Compare this with our component definition for Header.js.

React.createElement( 'div', null, 'Hello there, this is a React component!' );

This is how we have to write React components without JSX. In fact, under the hood, this is what JSX is compiled into before it can be rendered to the browser.

You’re not required to use JSX at all for your React components; it is entirely up to you. But almost all components you’ll come across will be written in JSX because it’s just so much easier to write.

It’s also highly readable for others new to your code. Imagine having to study a React project containing dozens of different components written in plain JavaScript!

So it should come as no surprise that we’ll be using JSX for component definitions throughout the remainder of this tutorial series.

Go ahead and replace the React.createElement() call with the JSX equivalent we copied from App.js. Your Header.js file should now look like this:

import React, { Component } from 'react';

class Header extends Component {
  render() {
    return (
      

        
Discover Your Movie Mojo!
      
    );
  }
}

export default Header;

While JSX allows us much more flexibility in writing our components, bear in mind that it isn’t actual HTML we’re writing but an abstraction of it.

You can see this in the code snippet above. Notice in the

 tag we used className rather than class to indicate where we want to declare a CSS class? This is because all JSX is compiled down to pure JavaScript, and class is a reserved word in ES6 JavaScript.

Let’s also tweak the header styles. Open App.css and edit the .App-header CSS class to be:

.App-header {
  background-color: steelblue;
  height: 70px;
  padding: 20px;
  color: white;
}

This updates the background color of the header and reduces the height.

Component Props

So far, our

 component is static. That is, it displays fixed content that never changes. But components can be made to be dynamic and display content passed into them, via component props. This makes components suddenly much more useful as they become generic and reusable.

Think of component props as similar to HTML tags. For example, a

 tag may have attributes for id, class, style and so on that enable us to assign unique values for that specific

 element.

We can do the same for React components. Say we didn’t want our header to output the fixed text ‘Discover Your Movie Mojo!’ all the time. Wouldn’t it be better if our header could display any text?

Unlike HTML attributes, we can name our component props whatever we like. Inside App.js, update the

 tag to be:

Then, update the

 component to use the text prop.

  
{this.props.text}

This results in our header displaying whatever text is added to the text prop in App.js.

Let’s take a closer look at how we referenced the text prop inside Header.js using:

{this.props.text}

The curly braces simply tell JSX that we have some JavaScript we want to evaluate. This distinguishes it from text. If we didn’t use any curly braces, the string literal this.props.text would be outputted, which isn’t what we want.

The this keyword refers to the Header component class, and props is an object that contains all the values passed in from 

. In our case, the props object contains just the one entry, but you can add as many as you like in practice.

Our

 component is now much more generic and doesn’t contain a hard-coded string. This is a good practice when writing React components. The more generic you make them, the more reusable they are.

This is good news when developing future React apps as you can reuse components from previous projects so you don’t have to write everything from scratch.

We used props above to pass a fixed string into the

 component, but props can also pass variables, function references, and state to components.

To send a variable via props, we could do something like this, where headerText is a variable:

Inspecting React Components

There’s a very useful tool available for the Chrome browser that lets you inspect information about your React app.

The default developer tools only allow you to view normal HTML elements, but with the React Developer Tools extension installed, you can navigate through all the React components in your app.

Once it’s installed, open your browser inspector tools, and click on the newly available React tab. Notice that instead of HTML elements, you see the hierarchy of React components in your app. Click on the  component to select it.

Once selected, information about a component is displayed in the window to the right. The  component doesn’t have any props and so the window is empty. But if you select the

 component inside  then you’ll see the ‘text’ prop we passed in.

The React developer tools are very useful for debugging, especially when you get onto developing more complex React apps, so it’s well worth getting used to using them on simpler apps.

You can also use the React developer tools to inspect your application state, which we’ll get into in the next tutorial.

Conclusion

In this tutorial you learned how to split your app into separate components to make it more modular. Component props enable you to pass in values to individual components, similar to how you add attributes to HTML elements.

We also saw how to leverage new browser inspector tools to examine components and props data.

In part 3, we’ll add state to our app to help us manage our data more effectively.

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