Intro to React Workshop

Many thanks to ReactTraining's hooks-workshop repo. Much of this tutorial is borrowed directly from their work. I've also quoted liberally from the ReactJS website.

What you will get

A stripped-down, quick start intro to the React.js front-end framework that requires little to no previous knowledge of web development or JavaScript.

What you won't get

Unfortunately, we won't discuss styling React components. We also won't get into best accessibility practices.

Installation requirements

Node version 10 or higher is required for this tutorial. To check your current version of node, run:

node -v

Intro to React Workshop

Part 1: Intro

What is React?

React, also known as React.js, is a JavaScript library for building user interfaces.

It is the front-end framework that powers Auspice and Nextstrain.org.

React, created by Facebook, rose to popularity for its pleasant developer experience. React views, also called User Interfaces or UIs, are declarative and composable.

What is the DOM?

The Document Object Model, or DOM, is a

language-independent interface that treats an HTML document as a tree structure wherein each node is an object representing a part of the document. (Wikipedia)

Let's see the DOM in action. Go to https://github.com and open up the developer tools by right-clicking and selecting "Inspect Element". Find this node near the top of the tree:

<div class="application-main " data-commit-hovercards-enabled="">

Now find the node nested under this one that begins with:

<div class="d-flex flex-wrap bg-gray" style="min-height: 100vh;">

Expand the node by clicking on the arrow on the left-hand side of the <div tag opening. Now the nested nodes are visible. Mouseover them to see their corresponding UI elements on the web page.

DOM elements, or nodes, like these are what we will be manipulating with React.

Getting started with React

Let's see React in action. Make sure you've cloned this git repo:

git clone [email protected]:kairstenfay/intro-react.git

Now, change into your newly cloned repo and install the required dependencies:

cd intro-react
npm install

Once your dependencies are done installing, fire up the React app wih:

npm run start

In your browser, at localhost:3000, you should now see a mostly empty view with a tiny message saying "Hello, React!"

If we open the file at src/index.js, we can see that our app consists of only a few lines of code:

import React from "react"
import ReactDOM from "react-dom"

// All React needs to render is a dom element, and a react element
const reactElement = <div>Hello, React!</div>
const domElement = document.getElementById("root")

// and away we go!
ReactDOM.render(reactElement, domElement)

The HTML-looking syntax, similar to what you saw in the DOM, in JavaScript is called JSX. It’s special to React, and we'll discuss it again in a bit.

Inside of the <div> tag, edit "Hello, React!" to greet you by name. The React server detects the change in src/index.js and reloads the browser accordingly. Neat!

Now, if you open up your developer tools (right-click, "Inspect Element"), you can see under the <body> node, a DOM element that starts with <div id="root">. This is the DOM element we told React to modify in our code. We targeted it by referring to its ID, "root".

Let's modify our UI again, this time replacing the declaration of reactElement with the following code:

const reactElement = (
  <button>
    + Add
  </button>
)

In your browser, you can now see a button that says "+ Add."

We mentioned that React is composable, so let's make the code a little more reusable. Declare a JavaScript function that returns the text, "Add".

const getTitle = () => "Add"

Now, invoke your new function, getTitle(), within the reactElement declaration. Altogether, it looks like:

const getTitle = () => "Add"

const reactElement = (
  <button>
    + {getTitle()}
  </button>
)

You should see no change in the view.

What is JSX?

JSX is a syntax extension to JavaScript. We use it when defining React components to tell React what the UI should look like. JSX is similar to a template language, but with the full power of JavaScript.

You can escape JSX and write JavaScript with curly brackets {}. In the most recent example, we "escaped" the HTML-looking JSX to invoke pure JavaScript after the "+" sign.

const reactElement = (
  <button>
    + {getTitle()}
  </button>
)

When we close the curly bracket, we return to JSX syntax. To further illustrate this point, let us temporarily replace the code inside the curly brackets with an equation like this one:

const reactElement = (
  <button>
    + {333 + 9 / 4}
  </button>
)  // 335.25!

Notice that you only see the result of the calculation. If you remove the curly brackets, however, React renders the numbers as normal HTML text contained within JSX.

const reactElement = (
  <button>
    + 333 + 9 / 4
  </button>
)  // No math, only sadness

React elements

Let's reset our reactElement variable in src/index.js to the following code:

const reactElement = (
  <button>
    + {getTitle()}
  </button>
)

With reactElement, we created what's called a React element. React elements are the smallest building blocks of React apps. They describe what you wnat to see on the screen.

React elements make up React components. We'll get more into those in a bit. For now, let's import an existing component called FaPlus. Add the following line to the top of your src/index.js file:

import { FaPlus } from "react-icons/fa"

Now, replace the "+" in your reactElement with the FaPlus component, like so:

const reactElement = (
  <button>
    <FaPlus /> {getTitle()}
  </button>
)

Make sure to close the FaPlus component with />, or the app will fail to load!

The change in the view may seem subtle, but what we've just done is no small thing! We

imported an existing React component from a React library, and
rendered it within our reactElement.

The ability to mix and match elements and components is what makes React composable! 🎉

React components

Let's turn our button into a React component and remove some stuff.

Delete getTitle and reactElement, replacing both of them with the following component:

const Button = () => (
  <button>
    <FaPlus />
    Add
  </button>
)

In the final line that starts with ReactDOM.render, replace reactElement with <Button />, once again remembering to close the component with />. You should see no difference in the UI.

One of the great benefits of using components is that they're resuable. Right now, this button always renders the same stuff inside. But, if we use "props", or arbitrary inputs that get passed in, we can use it in multiple contexts.

Edit your declaration of Button to look like this:

const Button = (props) => {
    return (
      <button>
        {props.children}
      </button>
    )
  }

Import FaMinus in the same way you imported FaPlus, with:

import { FaPlus, FaMinus } from "react-icons/fa"

Add a second React component, App, defined as:

const App = () => (
  <div>
    <Button>
      <FaMinus /> Subtract
    </Button>
    <Button>
      <FaPlus /> Add
    </Button>
  </div>
)

Finally, update the ReactDOM.render method to call <App /> instead of <Button />. Compare your final version of src/index.js with final-examples/part1-final.index.js.

Part 2: State

Let's revisit a modified version of the final code from Part 1. Replace the contents of src/index.js with the code from initial-examples/part2-initial.index.js.

So far, we have only learned one way to update the UI. We called ReactDOM.render() to change the rendered output. Next, we're going to modify our React app so that the view uses state instead of a hard-coded value for minutes.

React state

In your newly refreshed src/index.js file, replace the declaration of minutes with:

const [minutes, setMinutes] = useState(5)

useState() is a React hook. Hooks are a new addition in React 16.8. They let you use state and other React features without writing a class. From the ReactJS website:

We call it inside a function component to add some local state to it. React will preserve this state between re-renders. useState returns a pair: the current state value and a function that lets you update it.

To illustrate this concept, let's put our add and subtract buttons to work! Modify the subtract and add <button> elements to pass them an onClick handler equal to the following, respectively:

<button onClick={() => setMinutes(minutes - 1)}>
    <FaMinus />
</button>

<button onClick={() => setMinutes(minutes + 1)}>
    <FaPlus />
</button>

Now, when we click the add or subtract buttons, React re-renders our App component while preserving state! We can see the updating in our browser dev tools. If you expand the node that begins with <div id="root">, you can see the text within the DOM element update as we click our plus or minus buttons.

Let's refactor our code to pass a named function to the button's onClick attribute.

Rewrite the App component to look like:

const App = () => {
    const [minutes, setMinutes] = useState(5)

    const handleSubtract = () => setMinutes(minutes - 1)
    const handleAdd = () => setMinutes(minutes + 1)

    return (
        <div>
            <div id="Minutes">
                <button onClick={handleSubtract}>
                    <FaMinus />
                </button>
                {minutes} Minutes
                <button onClick={handleAdd}>
                    <FaPlus />
                </button>
            </div>
        </div>
    )
}

You can use the state hook multiple times within one component. Add the following lines to App, before the return() statement.

const [counter, setCounter] = useState(0)

Within return(, add another <div> tag after after the one for id="Minutes".

<div id="Counter">
    <button onClick={() => setCounter(counter + 1)}>
        You've clicked this button {counter} times!
    </button>
</div>

With a bit of refactoring, we can utilize React's composability to create clean, readable, and reusable code. Try your hand at breaking down the Minutes and Counter functionality into two, separate React components. When you're done, compare with final-examples/part2-final.index.js.

Part 3: Multiple states

Let's pick up with where we left off at the end of Part 2. Feel free to refresh your src/index.js with the content from final-examples/part2-final.index.js.

Changing state

We can decide when to change state and when not to. Maybe we only want values 1 through 9 for minutes. We can modify the handleSubtrafct and handleAdd functions with our desired logic.

const handleSubtract = () => {
    if (minutes > 1) {
        setMinutes(minutes - 1)
    }
}

const handleAdd = () => {
    if (minutes < 9) {
        setMinutes(minutes + 1)
    }
}

Now, state only updates when minutes is within our desired range. Therefore, React does not detect a change in state and does not re-render.

We can add multiple states, like an error state when a user tries to enter an invalid value.

const [error, setError] = useState(null)

const handleSubtract = () => {
    if (minutes > 1) {
        setMinutes(minutes - 1)
        setError(null)
    } else {
        setError("Greater than 0 please.")
    }
}

const handleAdd = () => {
    if (minutes < 9) {
        setMinutes(minutes + 1)
        setError(null)
    } else {
        setError("Less than 10 please.")
    }
}

To make our error visible, we must add an additional JSX element to our return statement. Add the following code after the final </button> closing tag, but before the final </div> closing tag:

<div id="Error">
    { error }
</div>

Now, when the user tries to enter an invalid number of minutes, they see our error message.

Instead of always showing the error, even when it's null, we can optionally render the error based in its state value. We can do this using JSX and JavaScript's && operator, which we will use similarly to an if statement:

<div id="Error">
    { error && (
      <p>
        { error }
      </p>
    )}
</div>

How does this work? Remember, the curly brackets allow us to escape JSX to enter JavaScript mode. When we hit the part of the code that says { error && (, JavaScript is evaluating the "truthiness" of error. If error evaluates to true, e.g. it isn't null, then JavaScript will proceed in evaluating the rest of the statement.

We then give JavaScript a JSX element wrapped within parentheses. This is similar to what we did when making our first reactElement.

Multiple elements can change the same state. Here, we can close the error message by clicking a button inside it.

<div id="Error">
    { error && (
        <p>
            { error }<br />
            <button onClick={() => setError(null)}>
                dismiss
            </button>
        </p>
    )}
</div>

Part 4: Controlled components

Replace your code in src/index.js with code from initial-examples/part4-initial.index.js.

We have a counter again, but this time the value of the counter is contained within a HTML input field. The buttons are no longer functional, but the default value of 0 is editable within the input field. Here, the user, not the programmer, is in control of the input.

Now, change the <input> element to take a value={minutes} instead of a defaultValue={minutes}. The user can no longer edit the value contained within the input field. Why? This highlights React's declarative nature. We told React to render a minutes value equal to 30, it listened.

As programmers, we can add onClick handlers like we did before to control the value in the input field.

Add the following functions to the Minutes component of src/index.js:

  const handleSubtract = () => {
      if (minutes > 0) {
          setMinutes(minutes - 1)
      }
  }

  const handleAdd = () => {
      setMinutes(minutes + 1)
  }

Next, provide each <button> with its suitable onClick function. For the subtract button:

<button onClick={handleSubtract}>
    <FaMinus />
</button>

And for the add button:

<button onClick={handleAdd}>
    <FaPlus />
</button>

Our add and subtract buttons are now functional! However, if you open your browser dev tools, you'll see an error in the console:

Warning: Failed prop type: You provided a value prop to a form field without an onChange handler. This will render a read-only field. If the field should be mutable use defaultValue. Otherwise, set either onChange or readOnly.

We programmers are still completely in control of the input field. We need to bring in onChange to allow the user's interactions to control the input. Technically, it is still the programmer controlling the input. Let's take a look at the implementation.

Add the following function to the Minutes component:

const handleInputChange = event => {
  setMinutes(parseInt(event.target.value))
}

Now, incorporate it in the <input> element by rewriting it to mimic this:

<input value={minutes} id="minutes"
    onChange={handleInputChange} />

Our user can now update the minutes counter using the buttons or the input field, both of which we control.

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