05-props.md

Props

Learning Objectives

After this lesson, you will be able to:

• Describe props
• Create a component that renders props

Component Data with Props

The React framework was built to handle data that changes over time.

So far, we have defined a Hello component in App.js. The component's render method returns a div with a few headings, written in JSX.

In index.js, we are importing this component, appending what the Hello component render method returns to the virtual DOM, and rendering that.

This is great, but it doesn't involve any data yet, let alone data that changes over time! Let's make it more interesting.

Rather than simply display "Hello world", let's display a greeting to the user. We'll make the greeting change dynamically based on the user's name.

The question is, how do we add a name to our Hello component without hardcoding it into the component's render method?

Find out! Try it yourself alongside this video in this codepen (note: right click both for new tab!)

Hello World exercise - You do!

Code along: Adding props to our component

Let's use props to make our "Hello World" app more flexible.

First, a single prop

We want to make a greeting that's reusable for many different users, so we'll have a name prop for the name of the user.

In your src/index.js, we'll change the line that renders the Hello component to include this name prop. The new line will be:

<Hello name={"Carl Sagan"} />

We pass in data wherever we are rendering our component. In rendering the Hello component above, we pass in a prop called "name" with a value of "Carl Sagan".

Your index.js should now look like this:

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

ReactDOM.render(
  <Hello name={"Carl Sagan"} />,
  document.getElementById('root')
);

Now, every time we render our component, we will pass in data.

• When the Hello component renders, we pass the Hello component a prop called name with a value of Carl Sagan.

If you check your application now, nothing has changed. We're passing the name prop into the component, but the component isn't using it yet.

In our component definition, we will change the <h1>Hello World!</h1> to <h1>Hello {this.props.name}!</h1>. The portion {this.props.name} deserves a closer look:

• this refers to the specific component instance
• this.props will collect all the props for this component instance
• this.props.name pulls out the name property from this.props.

The {} syntax in JSX renders the result of any expression inside it. It works even without props. If you wrote {2+2} in your JSX, 4 would be rendered.

In App.js, your Hello class should now look like this:

class Hello extends Component {
  render () {
    return (
      <div>
        <h1>Hello {this.props.name}!</h1>
        <h3>It is time for tea.</h3>
      </div>
    );
  }
}

In the above example, we replaced "world" with {this.props.name}.

Check it out! You should be able to browse to http://localhost:3000 to view this change!