Properties are like state, but they are passed down from a component higher in the component hierarchy. Thus we can define a property in one place, but use it in multiple components without having to redeclare it, or having to perform other gymnastics.

Understanding Props

Let’s see it in action. First create a default React project in the root of your repository where you can muck about with properties:

create-react-app week04-prop-basics

Modify App.js to use a property called testProp in the JSX

import React, {Component} from 'react';
import logo from './logo.svg';
import './App.css';

class App extends Component {

    render() {
        return (
            <div className="App">
                <header className="App-header">
                    <img src={logo} className="App-logo" alt="logo"/>
                    <h1 className="App-title">Welcome to React</h1>
                <p className="App-intro">
                    To get started, edit <code>src/App.js</code> and save to reload.
                <p>My prop: {this.props.testProp}</p>  <=== THIS LINE ===<

export default App;

The only change it App.js is to add this one line which uses a property:

<p>My prop: {this.props.testProp}</p>

Now let’s define the property in index.js and pass it down to App.js:

    <App testProp={2} />,

As you can see, we pass a “parameter” or attribute to App. It defines the value of the testProp property we use in App.js.

We could also do this in index.js:

const myProps = {
    testProp: 3

    <App myProps={myProps} />,

And add this in App.js:

<p>My prop: {this.props.myProps.testProp}</p>

Add PropTypes

We can get some runtime type checking for our properties with PropTypes.

PropType warnings at run time

It is very important that we always keep the Developer Tools (F12 or Ctrl-Shift-I) open when we are running our programs. If we do so, we will see runtime errors in the Console pane, and see a little red icon indicating that there are warnings or errors when we are on that page or some other page. You can see both the warning and the small circular icon with the x in the above screenshot. I’m running Chrome in this example, but the warning is also clearly visible on the console page in Firefox.

Install the prop-types package:

npm install --save prop-types

Do this at the top of App.js:

import PropTypes from 'prop-types';

And then at the bottom of App.js:

App.propTypes = {
    appInit: PropTypes.shape({
      testProp: PropTypes.number

You should install PropTypes:

npm install --save prop-types

You can read more about PropTypes here and here and here.

Do Something on Your Own

The only work you need to is modify index.js and App.js so that App takes two props, and can display both the values 2 and 3 on separate lines in the final product.

React Props Basics

For instance, pass in both of these variables to App.js as props:

const myProps = {
    testProp: 3

const testProp=2;

Now change ReactDom.render so that it instantiates App and takes both variables as parameters, as attributes.

Turn it in

Push your work specify the follwoing when you turn it in:

  • Branch (If relevant)
  • Folder

Be sure that you are passing in to separate props. Here I pass in one prop, even if myProps is an object literal with two properties.

const myProps = {
    testProp: 3,
    anotherProp: 5

<App myProps={myProps} />,

Here I pass in two props called myProps and b:

<App myProps={myProps} b={something} />,

One prop:

<App a={...} />

Two props:

<App a={...} b={...}

Please pass in two props.