React

Fatma Kahveci

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react

React is a Javascript library for building interactive user interfaces.

The fundamentals of React

1. Component

React composes complex UIs from small and isolated pieces of code called components.
We use components to tell React what we want to see on the screen. When our data changes, React will efficiently update and re-render our components.
A component is written as either a .JSX or .JS file. It is simply a JS class or function that returns some HTML code.
A component file contains both logic and view written in JS and HTML, respectively.
A component name and file name is always written in TitleCase.
Imports are made at the top of a React component.
React components are imported into other React components before using them.
Using a component is as simple as declaring it inside of tags, for example: <HelloWorld />
A component takes in parameters, called props and returns a hierarchy of views to display via the render method.
The render method returns a description of what you want to see on the screen. React takes the description and displays the result. In particular, render returns a React element, which is a lightweight description of what to render.
All React component classes that have a constructor should start with a super (props) call:

class <className> extends React.Component {
  constructor(props) {
    super(props);
    // ...
  }
}

Server and client components

component

2. Props

React components use props to communicate with each other.
- Every parent component can pass some information to its child components by giving them props.
- You can pass any JS value through them, including objects, arrays, and functions.

Passing props to a component

import { getImageUrl } from './utils.js';

function Photo(props) {
  let person = props.person;
  let size = props.size;

  return (
    <img
      className="photo"
      src={getImageUrl(person)}
      alt={person.name}
      width={size}
      height={size}
    />
  );
}

// Step 1: Pass props to the child component
export default function Profile() {
  return (
    <Photo
      person={{
        name: 'Bob Alice',
        imageId: '12ab'
      }}
      size={100}
    />
  );
}

// Step 2: Read props inside the child component
function Photo({ person, size = 50 }) {
  // variables are available here
}

[Prop drilling] (threading) refers to the process you have to go through to get data to parts of the React component tree.
It may cause problems while your project is growing. Below are some cases:
- Refactoring the data: {user: {name: 'Bob'}} => {user: {name: 'Bob', age: 70}}
- Passing redundant props or missing props due to moving a component
- Renaming props makes tracking difficult for your brain

3. State

All the React components can have a state associated with them.
A state can change either due to
- a response to an action performed by the user OR
- en event triggered by the system.
Whenever the state changes, React re-renders the component to the browser.
The following lets you use local state within a function component.

const [<currentState>, <functionToUpdateTheState>] = useState(`<initialState>`);

`setState`

After building an initial state setup, we use setState() method to change the state object.

setState({ stateName : updatedStateValue })
// OR
setState((prevState) => ({ 
  stateName: prevState.stateName + 1 
}))

`package.json`

One of the most important files is package.json which is React app’s configuration file.
It is central to provide any metadata about the project, add any additional libraries to our project, and configuring things like run scripts.

`fontawesome`

fontawesome will provide svgs as react components.

# installation
npm i --save @fortawesome/react-fontawesome @fortawesome/free-solid-svg-icons @fortawesome/fontawesome-svg-core

JSX

You can put any JS expressions within braces inside JSX.

const element = <h1>Hello, world!</h1>;
const element = (
  <div>
    <h1>Hello!</h1>
  </div>
);

Hooks

Hooks let you use state and other React features without writing a class. They don’t work inside classes.
They let you hook into React state and lifecycle features from function components.
Hooks are JS functions imposing two additional rules:
- Only call hooks at the top level. Don’t call hooks inside loops, conditions, or nested functions.
- Only call hooks from React function components. Don’t call hooks from regular JS functions.

1. `useState` hook

const [<currentState>, <functionToUpdateTheState>] = useState(<initialState>); lets you use local state within a function component.

import React, { useState } from 'react';

function Example() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

2. `useReducer` hook

It is an alternative to useState. It is preferable to useState when you have complex state logic that involves multiple subvalues or when the next state depends on the previous one.

const [state, dispatch] = useReducer(reducer, initialArg, init);

// specifying the initial state
// pass the initial state as a second argument
const [state, dispatch] = useReducer(
  reducer,
  {count: initialCount}
);

3. `useEffect` hook

Fetching, subscriptions, or manually changing the DOM called “side effects (effects)” because they can affect other components and can’t be done during rendering.

// Runs on every update
import { useEffect } from 'react';

function <ComponentName> {
  useEffect(() => {});
}

// Runs ONCE after the initial rendering
import { useEffect } from 'react';

function <ComponentName> {
  useEffect(() => {
  },[]);
}

// Runs if any dependency changes
import { useEffect } from 'react';

function <ComponentName> {
  useEffect(() => {
  },[Dependencies]);
}

// Runs on cleanup
import { useEffect } from 'react';

function <ComponentName> {
  useEffect(() => {
    return()=>{}
  },[]);
}

Calling useEffect runs the “effect” function after flushing the changes to the DOM.
- Effects are declared inside the component so they have access to its props and state.
- By default, React runs the effects after every render, including the first.
Just like useState, you can use multiple single effect in a component.

4. `useContext` hook

React context is an interface for sharing information with other components without explicitly passing the data as props.
It is flexible enough to use as a centralized state management system for the project, or you can scope it to smaller sections of your application.
You can hold inside the context:
- global state
- theme
- application configuration
- authenticated user
- user settings
- prefferred language
- a collection of services

4.1. Create the context

It creates a context instance.

import { createContext from 'react' };

const Context = createContext('Default Value');

4.2. Provide the context

It provides the context to its child components, no matter how deep they are.

function Main() {
  const value = 'My Context Value'; // update it to change the context value

  // <MyComponent /> to add the component
  return (
    <Context.Provider value={value}>
      <MyComponent />
    </Context.Provider>
  );
}

4.3. Consume the context

You can have multiple consumers for a single context. All customers will be notified and re-rendered depending on the context value change.

import { useContext } from 'react';

function MyComponent() {
  const value = useContext(Context);

  return <span>{value}</span>;
}

function MyComponent() {
  return (
    <Context.Consumer>
      {value => <span>{value}</span>}
    </Context.Consumer>
  );
}

5. `useMemo` hook

const <cachedValue> = useMemo(<calculateValue>, [<dependencies>]) caches the result of a calculation between re-renders.
It takes two arguments which are dependencies and a function that computes the value you want to memoize.
It memoizes a value.
It can avoid unnecessary recomputing.
Example code

6. `useCallback` hook

const <cachedFunctionName> = useCallback(<functionName>, [<dependencies>]) caches a function definition between re-renders.
It takes two arguments which are dependencies and a function that you want to memoize.
It memoizes a function.
It can avoid re-rendering the child component every time the parent component is rendered.
Example code

7. `useRef` hook

const <ref> = useRef(<initialValue>) creates a reference value that’s not needed for rendering.
- Parameter: <initialValue> can be a value of any type.
- Return: <current> is set to the <initialValue> initially, you can later change it.
Changing a ref doesn’t trigger a re-render so refs are ideal for storing information that doesn’t affect the visual output of your component.
Example: Focusing a text input

import { useRef } from 'react';

export default function Form() {
  const inputRef = useRef(null);

  function handleClick() {
    inputRef.current.focus();
  }

  return (
    <>
      <input ref={inputRef} />
      <button onClick={handleClick}>
        Focus the input
      </button>
    </>
  );
}

8. `useImperativeHandle` hook

useImperativeHandle(<ref>, <createHandle>, [<dependencies>]) exposes a value, state or function inside a child component to the parent component through ref.
- <ref> := ref passed down from the parent component
- <createHandle>:= value to be exposed to the parent component
You can decide which properties the parent component can access so you can maintain the private scoping of the child component.
It is useful when you need bidirectional data.
Example code

9. `useLayoutEffect` hook

useLayoutEffect(<setup>, [<dependencies>]) performs side effects.
- <setup> := function containing the side effect logic
It is a version of useEffect that fires before the browser repaints the screen. The main difference is the timing of when they are executed.
It is useful when measuring the layout before the browser repaints the screen.

10. `useDebugValue` hook

useDebugValue(<value>, <format>) adds a label to a custom hook.
- <value> := any value that you want to display
- <format> := an optional formatting function
It is also useful when deferring the formatting of a debug value.

11. `useDeferredValue` hook

useDeferredValue(<value>) defers updating a part of the UI.
- <value> := any value that you want to defer
It is also useful when showing stale content while fresh content is loading.

12. `useTransition` hook

const [<isPending>, <startTransition>] = useTransition() updates the state without blocking the UI.
- <isPending> flag that tells you whether there is a pending transition.
- <startTransition> function that lets you mark a state update as a transition.
It is useful when marking a state update as a non-blocking transition.

13. `useId` hook

const <id> = useId() generates unique IDs that can be passed to accessibility attributes.

14. `useSyncExternalStore`

const <snapshot> = useSyncExternalStore(<subscribe>, <getSnapshot>, <getServerSnapshot>) subscribes to an external store.
- <snapshot>:= function that takes a single callback argument and subscribes it to the store
- <getSnapshot>:= function that returns a snapshot of the data in the store that’s needed by the component
It is useful when subscribing to an external store or a browser API, extracting the logic to a custom Hook, or adding support for server rendering.

15. `useInsertionEffect`

useInsertionEffect(<setup>, [<dependencies>]) is a version of useEffect that fires before any DOM mutations.
- <setup> := function with your Effect’s logic.
It is useful when injecting dynamic styles from CSS-in-JS libraries.

axios requests

axios is a popular data fetching package on npm.
It is an open-source and promise-based HTTP client.

npm install axios

With `async/await`

async is a function that pauses until the promise is resolved due to await. To pull out the response from the promise object, we use then and catch methods.
await is an operator that is declared with the promise function. It is always defined inside the async function. It holds the execution of the promise function as long as the promise is completely fulfilled.

import axios from "axios";

// GET
const getData = async () => {
  const response = await axios.get("<url>");
};

// POST
try {
  const url = "<url>";
  const data = {key: 'value'};
  const config = {'content-type': 'application/json'};

  const response = await axios.post(url, data, config);
  // <command>
} catch (error) {
  // <command>
}

// PUT
axios.put()

// DELETE
axios.delete()

Handling events

They are named using camelCase.
With JSX you pass a function as the event handler.
You cannot return false to prevent default behaviour in React. You must call preventDefault explicitly.
Generally, if you refer to a method without (), you should bind that method.

class Toggle extends React.Component {
  constructor(props) {
    super(props);
    this.state = {isToggleOn: true};

    // This binding is necessary to make `this` work in the callback
    this.handleClick = this.handleClick.bind(this);
  }

  handleClick() {
    this.setState(prevState => ({
      isToggleOn: !prevState.isToggleOn
    }));
  }
  
  render() {
    return (
      <button onClick={this.handleClick}>
        {this.state.isToggleOn ? 'ON' : 'OFF'}
      </button>
    );
  }
}

Passing arguments to event handlers

With an arrow function, we have to pass the React event e explicitly.

<button onClick={(e) => this.deleteRow(id, e)}>Delete Row</button>

Conditional rendering

function UserGreeting(props) {
  return <h1>Welcome back!</h1>;
}

function GuestGreeting(props) {
  return <h1>Please sign up.</h1>;
}

function Greeting(props) {
  const isLoggedIn = props.isLoggedIn;

  if (isLoggedIn) {
    return <UserGreeting />;
  }
  return <GuestGreeting />;
}

const root = ReactDOM.createRoot(document.getElementById('root')); 

root.render(<Greeting isLoggedIn={false} />);

match

A match object contains information about how a <Route path> matched the URL.
- params are key/value pairs from the URL corresponding to the dynamic segments of the path.
- isExact is true if the entire URL was matched.
- path is the pattern used to match.
- url is the matched part of the URL.

Lists and keys

function ListItem(props) {
  return <li>{props.value}</li>;
}

function NumberList(props) {
  const numbers = props.numbers;
  const listItems = numbers.map((number) =>
    <ListItem key={number.toString()}
              value={number} />
  );
  return (
    <ul>
      {listItems}
    </ul>
  );
}

const root = ReactDOM.createRoot(document.getElementById('root'));
const numbers = [1, 2];
root.render(<NumberList numbers={numbers} />);

Rendering Elements

Rendering converts the code you write into user interfaces (UIs).
An element is the smallest building block of React apps. It describes what you want to see on the screen.
Each React element is a JS object that you can store in a variable or pass around in your program.
The only way to update the UI is to create a new element, and pass it to root.render().

const root = ReactDOM.createRoot(
  document.getElementById('root')
);

function tick() {
  const element = (
    <div>
      <h1>Hello world</h1>;
      <h2>It is {new Date().toLocaleTimeString()}.</h2>
    </div>
  );
  root.render(element);
}

setInterval(tick, 1000);

`setInterval`

setInterval executes a function at specific intervals. It helps in checking a condition regularly or fetching data every so often.
clearInterval(<intervalName>) stops the interval.

Layouts

A layout is UI that is shared between multiple pages.

The fundamentals of React

1. Component

Server and client components

2. Props

Passing props to a component

3. State

setState

package.json

fontawesome

JSX

Hooks

1. useState hook

2. useReducer hook

3. useEffect hook

4. useContext hook

4.1. Create the context

4.2. Provide the context

4.3. Consume the context

5. useMemo hook

6. useCallback hook

7. useRef hook

8. useImperativeHandle hook

9. useLayoutEffect hook

10. useDebugValue hook

11. useDeferredValue hook

12. useTransition hook

13. useId hook

14. useSyncExternalStore

15. useInsertionEffect

axios requests

With async/await

Handling events

Passing arguments to event handlers

Conditional rendering

match

Lists and keys

Rendering Elements

setInterval

Layouts

`setState`

`package.json`

`fontawesome`

1. `useState` hook

2. `useReducer` hook

3. `useEffect` hook

4. `useContext` hook

5. `useMemo` hook

6. `useCallback` hook

7. `useRef` hook

8. `useImperativeHandle` hook

9. `useLayoutEffect` hook

10. `useDebugValue` hook

11. `useDeferredValue` hook

12. `useTransition` hook

13. `useId` hook

14. `useSyncExternalStore`

15. `useInsertionEffect`

With `async/await`

`setInterval`