In addition to the material we saw in class, the ReactJS Basics by Mindspace is a great introductory video series on the subject of React. At any point during this workshop, feel free to pause and take a look at it.

Let's create our initial React environment using create-react-app. This will allow us to concentrate on learning React rather than wasting time setting up Webpack, Babel and friends. In class we learned how these tools work with each other to allow us to develop modern apps that work across a majority of browsers. Know that under the hood, create-react-app uses these exact tools, but automates the process for us.

If you're going to do this in Cloud9, ideally you'll want your /home/ubuntu/workspace directory to be directly the place where create-react-app creates its files. Unfortunately if you try to run it, it will tell you that the directory is not empty. Here's how we'll do it. First, install create-react-app globally with:

npm install --global create-react-app

Then, move to the /home/ubuntu directory, remove the workspace directory and recreate it with create-react-app:

cd /home/ubuntu
rm -r workspace && create-react-app workspace

The second command says to recursively remove the workspace directory, and the && says to run create-react-app only if that is successful. After a few minutes, you should have your structure created for you.

Pure components are the simplest ones to work with. They pretty much only have a render method, and render uses the component instance's props -- this.props -- exclusively to output a tree of components.

Given the same props, a stateless, pure component will always return the same output. It won't rely on a global variable, Math.random() or anything other than the props that can change between calls.

Unfortunately, all our app can't be stateless otherwise there would be no dynamism nor interactivity. For example, if a component represents a countdown timer, then it needs to keep track of the current remaining time somewhere. But as we'll see later, even if we need to store some state, we can still make use of pure components to display a representation of this state.

For now, let's take a look at creating some pure components...

In the src/ directory, create a file called ImageCaption.js with the following code:

import React from 'react';

class ImageCaption extends React.Component {
  render() {
    return (
      <figure>
      </figure>
    );
  }
}

export default ImageCaption;

This is a standard layout for a pure component. Pretty much every component you create will have this base layout.

Now, complete the code of your component's render method. Your component will be used like this:

<ImageCaption source="http://placekitten.com/g/210/210" text="This is a kittenz!"/>

This will render an instance of the ImageCaption component. This instance of the component will receive two props. The first prop is source and the second one is text. The set of props will be available to the component instance's render method under this.props.

Your component should output a figure like this:

<figure>
  <img src="http://placekitten.com/g/210/210"/>
  <figcaption>
    This is a kittenz!
  </figcaption>
</figure>

Once you have completed your render method, go to your App.js. Use import to import your ImageCaption component. Then, in the render method of App, after the <hr/>, add an <h2> that says "testing ImageCaption", and use <ImageCaption> to output the following image http://i.imgur.com/D8JWn.jpg with an appropriate caption. End it all with another <hr/> to create a spacing for the next exercise.

For this exercise, we will use the same ImageCaption component to render a list of images with captions. The data will come from an array of objects, something that you should be quite familiar with by now. Data often comes in as arrays of objects, and applications often display lists of things.

Add the following array as a global variable at the top of your App.js:

var imageList = [
  {id: 42, source: "http://placekitten.com/g/210/210", text: "Hello kittenz!"},
  {id: 43, source: "https://facebook.github.io/react/img/logo.svg", text: "React Logo"},
  {id: 44, source: "https://media.giphy.com/media/EldfH1VJdbrwY/giphy.gif", text: "Mind Blown!"}
];

In your App component, add a renderImage method that can be used with map. It should take an object like the one in the array above, and map it to an <ImageCaption> element.

To use your mapping function, you can take advantage of the fact that JSX lets you use an array of elements as a value, like this:

<div>
  {imageList.map(this.renderImage)}
</div>

This will call renderImage once for each object in the array, and end up with a new array of three <ImageCaption> elements. They'll be placed inside the <div>.

As you test this, open your console and notice that React gives you the following warning:

Warning: Each child in an array or iterator should have a unique "key" prop. Check the render method of Constructor. See https://fb.me/react-warning-keys for more information.

What React is telling you here is that to do its work well -- its work being to efficiently update the real DOM of the page -- it needs you to give a unique identifier to each item you output in a list. This way, React can quickly tell which item is which without having to look inside.

To do this, add a key prop to each <ImageCaption> in your renderImage method. In this case, because each object your receive has a unique id, you can use that as the key. You'll know you did it successfully when you refresh your page and the warning is gone.

NOTE: This pattern of having reusable components used to display lists/arrays of things pops up everywhere when dealing with UIs. It's one of the few things you should memorize and know how to do by heart. The steps are always the same:

1. Create a component for a single item in the list
2. map from an array of data to an array of instances of the component
3. Find a unique piece of data to use as the key of each component in the list

If you don't want to create a named mapping function, the following pattern is also commonplace for one-off lists:

<div>
{
  imageList.map(function(item) {
    return <ImageCaption ...>;
  });
}
</div>

Written in ES2015 syntax with arrow functions, it becomes even nicer:

{imageList.map(item => <ImageCaption .../>)}

Earlier on, you saw how to pass props to component instances. It looks a lot like writing the attributes of an HTML element like <a href="...". Sometimes, we want to create components that are wrappers.

Let's say we want to create a component that renders as a box with a 5px red border all around it, and puts any elements we pass to it inside the box. Passing data to components is done through props. But how would you pass elements -- children -- to a component? It's easy, you simply nest them inside the tag for the component, and they'll be available as a special prop called "children". You can access it as this.props.children in the receiving component.

Here's how the RedBox component would look:

import React from 'react';

class RedBox extends React.Component
  render() {
    return (
      <div className="red-box">
        {this.props.children}
      </div>
    );
  }
};

And here's how it would be used:

<RedBox>
  <p>This will be in the red box</p>
  <img alt="This too!" src="http://placekitten.com/g/200/200"/>
</RedBox>

Which would give the following output:

<div class="red-box">
  <p>This will be in the red box</p>
  <img alt="This too!" src="http://placekitten.com/g/200/200">
</div>

All that would be left to do is add some CSS for the red-box class name.

For this exercise, we want to create a Layout wrapper component. This component will output a common navigation bar, some varying content that will be passed as the children prop, and a common footer at the end.

Here's what the output of Layout should look like:

<div class="layout">
  <nav class="main-nav">
    <ul>
      <li><a href="/">Home</a></li>
    </ul>
  </nav>
  <main>
    <!-- the varying content should go here -->
  </main>
  <footer>
    Copywhat 2016 Kittens
  </footer>
</div>

Create this component in a file called src/Layout.js. Then, test it inside App.js. Below the last <hr/>, write an <h2> saying "Testing Layout". Then, create an instance of the <Layout> component. Its children should be a level 2 heading with the text "About us" and a paragraph with the text "We are React developers!". The word "React" should link to the ReactJS home page. If you don't know what the URL is, find it and bookmark it. Check it often if you use React to build applications.

This concludes the section on pure components. As you can see, pure components is easy. Everything happens in the render function, and the output is always based on the props. Because the same input (props) always gives the same output, pure components can be made super performant by using the Pure Render Mixin. This is a more advanced topic so feel free to skip it completely and come back to it later ;)

In this section, we are going to create some React components that receive user input. We'll start with a stateless component, then we'll look at our first stateful component.

In src/, create a file called GuessTheNumber.js. In it, create a component called GuessTheNumber. Your component will accept a prop called numberToGuess. The prop is required and should be a number.

Your component should output a text input box and a button. When the button is clicked, your component should retrieve the current value of the text input. If the input is equal to the numberToGuess prop, then you should alert('You are correct'). If not, you should alert whether the user guessed a lower or higher number.

Since this component is stateless, there will not be a "number of tries" or anything like that. Start by creating a render method that outputs a text input and a button. Add an onClick prop to the button and give it the value this._handleButtonClick. Because you're inside the render method's code, this represents an instance of the GuessTheNumber component.

To add a _handleButtonClick method to your component, all you have to do is add it to the class, on top of the render method. In this method, you'll need to retrieve the value of the rendered input box. How can you do that? If this was jQuery, you'd probably select the input box using a class name or ID or something, then call .val() to retrieve the value.

In React, when we need to refer to the DOM -- we need to do it in this case because the DOM is holding the value of the input box -- we can add a ref property to the element we want to reference, like this:

<input type="text" ref="userGuess"/>

Then, in the _handleButtonClick method, we can refer to the DOM element related to this ref by using this.refs.userGuess. Since this DOM element is an input, it will have a DOM property called value that will have its current value. So this.refs.userGuess.value will give you the current guess of the user. Use it to alert the appropriate message :)

While the component itself seems stateless on the surface, it's actually referring to a piece of data from the DOM, namely the input box. In the next few exercises, we'll see that sometimes this is not enough and we'll need to create our own state to hold on to extra information that is not -- and should not -- be present in the DOM, as it often is with jQuery.

Once you have implemented your component, test it by importing it in App.js and add it to your ever-growing list of components :) It should look like this (but with the rest of your exercises above it):

For now, we'll put this exercise on hold to explore a few easy stateful components. Then we'll come back to this and create a complete, stateful number guessing game.

Create a component -- you should know where by now -- called YouClicked. This component should output a button and a message below the button. At first the message should say "You have not clicked the button". When the button is clicked once, the message should change to "You have clicked the button once". Then, "You have clicked the button twice". After that, "You have clicked the button X times".

To do this, you'll need to keep track of a value over time. In React components, you can do this by using the state object and this.setState. You should only store the minimal amount of data necessary to display your component properly. In our case, if we have the number of times the button is clicked, we can derive the message to be output in the render method.

Add a method constructor to your YouClicked component. In it, call super() and then assign this.state an object with a single key timesClicked with the value 0. Then, in your render method, add a {this.state.timesClicked} somewhere to see the output. It should say 0 if you preview it in the browser.

Add an onClick event handler to the button of your component. This should call a method of your component called _handleButtonClick. In this method, you should use this.setState to change the state of your component. The reason why you call this.setState instead of simply re-assigning with this.state.timesClicked++ has to do with how React detects that it has to render your component. Here, what you want to do in your event handler is:

this.setState({
  timesClicked: this.state.timesClicked + 1
});

As you know by now, calling setState will trigger a re-render of your component. Since the value of this.state.timesClicked will have changed, render will give a different output and the DOM will be updated to reflect this.

When you get it working, make your component display the appropriate message (you clicked once, twice, ...) based on the value of this.state.timesClicked. All the display logic will be in the render method.

When you get the basics working, add a RESET button to your component. Clicking the reset button should reset the click counter to zero. It should also add 1 to the timesReset state, or create it if it doesn't exist. Use the timesReset state to display an additional message "You have reset X times". You should only display the additional message if the user has reset at least once.

Here's what your component should look like:

Create a component called CharacterCounter. Your component should have a text input, and a character count next to it. Add an onInput event handler to the input field, and point it to a method called _handleInput. In the _handleInput method, you can get the current value of the input field like this:

({
  _handleInput(event) {
    var value = event.target.value;
    // continue here...
  }
})

You'll receive the DOM event, and you'll get one every time the value of the input field changes. target is the DOM element, and value is its current value. Use this value along with this.setState to keep track of the current value in a state called currentInput. To do this the right way, you should also add a constructor method and assign an empty string as the currentInput state with this.state = {currentInput: ""}. Then, in the render method, display the number of characters in this.state.currentInput -- hint: it's just a string.

As always you'll be testing this component by importing it in App.js and adding it with the rest of the exercises.

This exercise expands on the previous one. You can copy your current CharacterCounter to a new component called CharacterLimit, or you can start it from scratch.

The CharacterLimit component will accept a prop called limit. It should be a number, and is required.

Then, add a value prop to the input field. Give it the value of "hello" and view your component in the browser. Notice that React gives you a warning about a read-only field? Since the render method gives a constant value to the value property, the value cannot change.

While this seems weird, we'll be able to use this to control the value of the input, but still hook to its onInput method to get the next value. We'll then be able to decide if we want to let it through or not.

Here are the steps to create a controlled input component:

1. In your constructor method, keep track of the current input value. We already have a currentInput from the previous step. This is where we'll keep the "true" value of the input field.
2. In your render method, give the input field a prop value equal to this.state.currentInput.
3. In your render method, attach an onInput event handler to your input field
4. Make your event handler accept one argument -- call it event -- and check under event.target.value for the new value of the input field. If you use setState to change the value of currentInput here, the input field will be updated on the next render.

For this exercise, in your input event handler, you'll use event.target.value's length and check it against this.props.limit to see if you should let the new value go through or not. If the new value is less than the limit, then you can call this.setState to change currentInput to the new value. If not, you don't need to do anything.

Finally, in the render method, instead of displaying the character count, display "X remaining" next to the input field.

Test your input component using <CharacterLimit limit={140}/>. Note that with the current version of this component, you won't be able to retrieve the value of the input field since it's encapsulated in the CharacterLimit component.

For this exercise, you will create a NumberGuessing game component. The component will play a game of guess the number. For every game, it will choose a number between 1 and 100. The user will have an input field and a GUESS button.

They will have 5 tries to guess the number. To guess a number the user must put it in the input field, and click the GUESS button. When they do that, the game will check if the number is the right one. If it is, the user interface is changed to a simple "YOU WIN". If it's not, the user is shown a message that says whether the guess is too low or too high. They will also be shown a list of their guesses, as well as the number of remaining guesses. If the guesses are expired before the user wins, the interface is changed to a simple "YOU LOSE" and we show the winning number.

For both the "YOU WIN" and "YOU LOSE" screens, there will be a button labeled "NEW GAME". Pressing this button will reset the game.

When the component gets mounted in the DOM, we'll want to start a new game. The display of the game will change as a result of calling this.setState at appropriate moments during the lifecycle of the component. Here are the different pieces of state we'll need to keep track of:

({
  gameStatus: 'win|lose|playing',
  numberToGuess: 42, // something between 1 and 100 generated when we start a game
  guesses: [] // initially guesses is an empty array
})

Since you'll be starting a game from multiple places (initial mounting, and clicking NEW GAME), your constructor function should assign an empty object to this.state. You should have a method called _startGame that will use this.setState to reset the game. It will set the gameStatus to playing, the numberToGuess using Math.random, and the guesses to an empty array.

In the render method, if this.state.gameStatus is not defined, simply return null.

The next thing you'll want to do is start a new game when the component gets mounted in the DOM. There happens to be a method that you can implement in your component that will be called exactly as soon as the component gets mounted: componentDidMount. If you add this method to your component, React will call it right after your component is displayed on the screen. In this method, you can call this._startGame() to make this happen. This will in turn call setState, which will re-render your component. At that point the game will start :)

Here's an example of the game being played:

For this exercise, we're going to be doing an AJAX call to the GitHub API to retrieve some information, then build a component that will output this information in a nice way.

The first question is "How do I make AJAX calls with React?" and the answer is, you don't! React is a view library, it deals with user interface components and their interactions.

Loading data with AJAX can be done with any library that supports it, or with Vanilla JS. In our case, the easiest way will be using the modern fetch function. If we want our app to be supported in older browsers, we can polyfill fetch.

Then, let's create a component called GithubProfile in the components directory. You'll mount it in your <App> like the rest of your exercises. The GithubProfile component will take a prop called username, a string that is required.

Next, in the GithubProfile component, implement a constructor method that assigns an empty object to this.state. In this component, we're going to use state to let us know when our AJAX call has completed.

Next, we need to make an AJAX call to the GitHub API to retrieve the user info that is in our this.props.username. Here's an example of a GitHub API output for a user profile:

https://api.github.com/users/gaearon

{
  "login": "gaearon",
  "id": 810438,
  "avatar_url": "https://avatars.githubusercontent.com/u/810438?v=3",
  "gravatar_id": "",
  "url": "https://api.github.com/users/gaearon",
  "html_url": "https://github.com/gaearon",
  "followers_url": "https://api.github.com/users/gaearon/followers",
  "following_url": "https://api.github.com/users/gaearon/following{/other_user}",
  "gists_url": "https://api.github.com/users/gaearon/gists{/gist_id}",
  "starred_url": "https://api.github.com/users/gaearon/starred{/owner}{/repo}",
  "subscriptions_url": "https://api.github.com/users/gaearon/subscriptions",
  "organizations_url": "https://api.github.com/users/gaearon/orgs",
  "repos_url": "https://api.github.com/users/gaearon/repos",
  "events_url": "https://api.github.com/users/gaearon/events{/privacy}",
  "received_events_url": "https://api.github.com/users/gaearon/received_events",
  "type": "User",
  "site_admin": false,
  "name": "Dan Abramov",
  "company": "Facebook",
  "blog": "http://twitter.com/dan_abramov",
  "location": "London, UK",
  "email": "[email protected]",
  "hireable": null,
  "bio": "Created: Redux, React Hot Loader, React DnD. Now helping make @reactjs better at @facebook.",
  "public_repos": 176,
  "public_gists": 48,
  "followers": 10331,
  "following": 171,
  "created_at": "2011-05-25T18:18:31Z",
  "updated_at": "2016-07-28T14:41:02Z"
}

It's a simple, flat JavaScript object. The API URL is always the same prefix, and ends with the username. Easy.

The next thing we need to do is find out what's the best place to make our AJAX call. We need to do it as soon as our component instance gets mounted in the DOM. What's that I hear? componentDidMount! React will call this method as soon as our component has been mounted in the DOM.

Implement componentDidMount for your GithubProfile component. In it, use $.getJSON to load the appropriate GitHub API URL for the username in your props. In the callback, use this.setState to add a user object to your state, and set it to the response of the AJAX call.

⚠️ NOTE: You are going to run into trouble when trying to use this.setState inside the success callback of your fetch AJAX call. That's because the callback is a new function and creates a new this context, so you lose the this that you had access to in the componentDidMount outer function. There are a few different ways to fix it. Use bind if you are comfortable with it, or var that = this otherwise.

Finally, implement the render method for your component. render should check if the state contains a user object. If it does not, then it should return a div with the text "LOADING". If the user object is in the state, then your render method should return something like this:

<div class="github-user">
  <img class="github-user__avatar" src="URL OF THE AVATAR"/>
  <div class="github-user__info">
    <p>USERNAME OF THE USER (REAL NAME OF THE USER)</p>
    <p>BIO OF THE USER IF THERE IS ONE</p>
  </div>
</div>

And the end result should look like this in your browser:

⚠️ NOTE: You'll have to add some CSS if you want your output to look like mine ;) Perhaps a float and width/height on that image, border-radius to make circular, ...

This exercise is going to expand on the previous one. For this exercise, we're going to create two new components, GithubSearch and GithubSearchForm. We'll use these two components along with the GithubProfile component from the previous exercise.

At the end of this exercise, your component should look and act like this:

• Step 1: Create a pure GithubSearchForm component. Here is the code of it:

import React from 'react';

class SearchForm extends React.Component {
  render() {
    return (
      <form>
        <p>Enter a GitHub username:</p>
        <input type="text" />
        <button>Go!</button>
      </form>
    );
  }
};

• Step 2: Create a component called GithubSearch. Start with the following code:

import React from 'react';

class GithubSearch extends React.Component {
  constructor() {
    super();
    this.state = {};
  }
  render() {
    return (
      <div>
        <GithubSearchForm/>
        {this.state.user ? <GithubUser username={this.state.user}/> : null}
      </div>
    );
  }
};

⚠️ Notice that this component is using GithubSearchForm. Make sure to import it.

Let's see what's going on in GithubSearch: first off, we're initializing our state to an empty object. This signals that this component will need to be stateful. Then, in render, we are outputting a GithubSearchForm. This component will manage the user input, and will be aware of when the user clicked on the Go! button.

Our problem is this: the submit event happens in the GithubSearchForm component, but we need to know about this inside the GithubSearch parent component. The two components need to communicate.

React's documentation provides a recipe to follow for splitting up and communicating between components. In our case, it boils down to adding a method _handleSearch in the GithubSearch component, and passing this method as a prop to GithubSearchForm. When the search form detects an input, it will be able to call the method passed down to it, without needing to know where the method came from. Let's implement this slowly:

• Step 3: detect form submission in GithubSearchForm If we're going to communicate to our parent component that the user want to do a search, we first have to detect the search in the child component.

  1. In the render method of GithubSearchForm, add a ref to the input box and set it to userInput.
  2. Add an onSubmit event handler to your <form> element, and point it to a method called _handleSubmit.
  3. In _handleSubmit, use this.refs.userInput.value to find the value of the input. console.log it for now.
  4. Test your component and note that submitting the form makes the whole page reload! We have to PREVENT the form from submitting. You should know how to do this by now :)

• Step 4: pass a handling function from GithubSearch In the GithubSearch component, add a function called _handleSearch(searchTerm). For now, just console.log the value of searchTerm. Then, pass a prop called onSearch to the GithubSearchForm component and make it the _handleSearch method you just created:

<GithubSearchForm onSearch={this._handleSearch}/>

• Step 5: accept the onSearch prop in your GithubSearchForm component In the _handleSubmit of the GithubSearchForm component you created in a previous step, instead of using console.log, use this.props.onSearch and pass it the value of the search form from your refs.

When all this is wired up, you've basically created an inverse data flow between your components: a child component is communicating to a parent component using a function that was passed down to it as a prop.

• Step 6: closing the loop Now that your GithubSearch component is aware of the new search value, modify the _handleSearch method. Instead of simply console.logging the value, use this.setState to set the value of the user state. This will trigger a re-render of your GithubSearch component. When it gets re-rendered, this.state.user will be set to the value that was in the form. The value will be used by render in the following expression: {this.state.user ? <GithubUser username={this.state.user}/> : null}. Since user is now set, render will output a GithubUser component. When this component gets output, its componentDidMount function will be called. It will do its AJAX call and output the user box :)

Here's an example of it "working":

⚠️ OOPS! What's happening? It seems like our component is working the first time around, but whenever we type a second username, the GithubUser component doesn't refresh. If you look at the video closely, you'll notice that the username prop of the GithubUser component does get updated. So why does this not trigger a new AJAX call??

Let's check our GithubUser component. The AJAX call is currently being done in componentDidMount. Mount. In this case, the component is already mounted in the DOM, the only thing that happened is that its prop has changed. When a component's props change, it doesn't get re-mounted in the DOM. Its current DOM is modified instead, using the result of the latest call to render. When this happens, another React Component lifecycle method gets called: componentDidUpdate. If you implement this function you can trigger a new AJAX call if the component gets updated.

Here's how to fix your GithubUser component:

1. Rename the componentDidMount method to fetchData
2. Create a new componentDidMount method, and call this.fetchData in it
3. ⚠️ Test to make sure things are still working as before!
4. Implement the componentDidUpdate method. In it, check if the prevProps username is different from the current props' username. If and only if they are different, do another call to this.fetchData. Since fetchData is using this.props to do its job, it will fetch the new user info :)

When you are done, your component should behave like this (NOTE: this is the same screencast as in the beginning of this exercise):