This app is meant to be used as a workshop for teaching react hooks. Workshop content is below, excercises can be done by running the app.
This project was bootstrapped with Create React App.
Some experience with React. You should know about the component lifecycle, lifecycle methods, etc.
To get the app up and running, run:
npm install
npm start
This should start a server on localhost:3000.
The components we will be writing are in the toDo directory.
In the past, functional components in React could take props and manipulate them, but were otherwise fairly limited. React hooks allow function components to have state, to use side effects, and become first-class citizens.
This is a syntax you'll see a lot in hooks documentation and usage
const answersArr = ['Yeah', 'Nah']
const [yes, no] = answersArr
// equivalent to
const yes = answersArr[0]
const no = answersArr[1]In object destructuring, the name of the variable being assigned to must be the same as the key of the object.
const fruitCounts = { apples: 2, bananas: 10 }
const { apples, bananas } = fruitCounts
// equivalent to
const apples = fruitCounts.apples
const bananas = fruitCounts.bananasYou can also do fun things like rename variables that were based on array keys or set defaults for variables that are undefined. MDN docs
This hook does what it says on the tin, it adds state to a previously stateless component. Suppose I have a component that contains a button, now I want to keep track of how many times that button has been clicked.
Before hooks, we would have had to convert the button component that needed state into a class component. Now, we can use the useState hook.
Any functional component that needs to preserve some state through rerenders.
useState can be called multiple times in one function, and is generally used multiple times rather than using an object as you might in a
class component. When this becomes unwieldy, I often turn to useReducer.
useState takes one argument, an initial state. That initial state can be of any type; a string, a number, an object, and array, etc.
useState returns an array of two values: first, the current state, and second, a setter that can set the next state.
Usually, destructuring syntax is used to give these values useful names, i.e. [dogName, setDogName] = useState('Fluffy') would give us a
variable dogName which would initially be set to 'Fluffy', and a function, setDogName that can be used to change the value of dogName.
setDogName can take either a value, which will set the state to that value when the component rerenders, or a function, which receives the
previous state, so that that state can be used to set the new value.
So if I want to change the dog's name directly, I would call setState('Floofster'), but if I need to keep track of the previous value, I
would use setState((prevDogName) => prevDogName + 'Jr.').
Open up your app (by running npm run start if you haven't already), and let's complete the useState example.
The file to work on is src/toDo/KittenVotes.jsx.
At the end of the day, hooks are just functions. Any hook that returns a function is a closure over a function that you write. The closure tells React when to run that function. Different types of hooks get called at different points in the React lifecycle.
- function components cannot access
componentDidMount,componentDidUpdateor other lifecycle methods directly - function components have no need for a constructor or state object
- functional components are generally shorter/faster to write, expecially for small components
- functional components use hooks to "hook in" to the react lifecycle. Hooks can't be used in class components.
- Hooks can only be called at the top level of a component
- do not call hooks from loops, if statements, or nested functions
- hooks must be called in exactly the same order each time the component renders
- Only call hooks from React functions
- cannot be called from class components
- can be called from custom hook functions
- You can only have one version of React at a time
- hooks can break if your app has multiple versions of React
In order to preserve state between re-renders of the component, React needs to create an object that persists outside of the component.
That persistent object is an array of "state cells" that store the values from the hook functions in the component. Because it's an array, the value that's preserved must be in the same location each time the function is called. If you had three useState hooks in a function, but one was conditionally rendered, then sometimes your third useState would get the wrong value! (OK, not really. Instead react or your linter will yell at you.)
It can't just be an array, can it?? Well, no. Here are some resources that will give you a deeper dive into hooks. You can still use hooks without knowing what's under there.
Suppose we need our component to make an API call in order to set it's state? That's where useEffect comes in handy.
When your component needs side effects to happen on mount or on update.
useEffect takes two arguments, a function that will be run when the component mounts, and an array of values, which tells React when (or if) the function should be run when the component updates.
The function that is passed to useEffect can do any side effect. The most common usage that I've seen for calling an API and setting state as a
result of that API call.
Optionally, the function you pass to useEffect can return a cleanup function. This is very similar to something that might get run on componentWillUnmount. So, if you're calling an API, it might prevent the API from setting state after the component is unmounted. If you
have subscribed to a service, it might unsubscribe you.
This is an array of values, and if any one of the values in the array changes, React will call the function you passed to useEffect. If you
only want the effect to run on mount, you can pass it an empty array. If there are props or state which affect your useEffect function, you can add them to the array, and the function you passed to useEffect will be called if the component updates and one of those values changed, similar to logic you might put in componentDidUpdate.
useEffect has no return value.
The code for the example is in src/examples/MovieFacts.jsx.
The code for you to finish is in src/toDo/GhibliMovies.jsx.
What if I had some state in one component, but I now want to share it accross multiple components or multiple parts of my app?
This is where useContext becomes useful. Instead of moving state up the component tree and drilling down props, we can move state into a
context and access it via the useContext hook.
So, if I want to create an app where on one page I report the amount of cookies I've eaten, and on another page I can click on cookies to add to
the cookie count, I could wrap both components in a context provider and use the useContext hook to access the shared state.
A context is created using React.createContext().
If we're creating a context that keeps track of the number of cookies I've eaten, I could create it like so:
const CookieContext = React.createContext().
The context provider is the component that holds whatever values you need it to and makes them available to it's child components.
My cookie context is going to provide a getter and setter for my cookie count to various components. It's a special type of React component that returns a Provider that is part of the CookieContext. It takes a value prop. This value can be any value that you want the children to have
access to. In this case, we're just going to pass the values we got from useState.
const CookieProvider = ({ children }) => {
const [cookieCount, setCookieCount] = useState(0)
return (
<CookieContext.Provider value={[cookieCount, setCookieCount]}>
{children}
</CookieContext.Provider>
)
}To use the context, children must use the useContext hook. So, a component that reports the cookie count might look like:
const CookieNumber = () => {
const [cookieCount] = useContext(CookieContext)
return <div>I ate {cookieCount} cookies!</div>
}We'll do a context example together. It's in src/toDo/TwoButtons.jsx.
In the project directory, you can run:
Runs the app in the development mode.
Open http://localhost:3000 to view it in the browser.
The page will reload if you make edits.
You will also see any lint errors in the console.
Launches the test runner in the interactive watch mode.
See the section about running tests for more information.
Builds the app for production to the build folder.
It correctly bundles React in production mode and optimizes the build for the best performance.
The build is minified and the filenames include the hashes.
Your app is ready to be deployed!
See the section about deployment for more information.
Note: this is a one-way operation. Once you eject, you can’t go back!
If you aren’t satisfied with the build tool and configuration choices, you can eject at any time. This command will remove the single build dependency from your project.
Instead, it will copy all the configuration files and the transitive dependencies (webpack, Babel, ESLint, etc) right into your project so you have full control over them. All of the commands except eject will still work, but they will point to the copied scripts so you can tweak them. At this point you’re on your own.
You don’t have to ever use eject. The curated feature set is suitable for small and middle deployments, and you shouldn’t feel obligated to use this feature. However we understand that this tool wouldn’t be useful if you couldn’t customize it when you are ready for it.
You can learn more in the Create React App documentation.
To learn React, check out the React documentation.
This section has moved here: https://facebook.github.io/create-react-app/docs/code-splitting
This section has moved here: https://facebook.github.io/create-react-app/docs/analyzing-the-bundle-size
This section has moved here: https://facebook.github.io/create-react-app/docs/making-a-progressive-web-app
This section has moved here: https://facebook.github.io/create-react-app/docs/advanced-configuration
This section has moved here: https://facebook.github.io/create-react-app/docs/deployment
This section has moved here: https://facebook.github.io/create-react-app/docs/troubleshooting#npm-run-build-fails-to-minify