React Cheatsheet

Components in React

• This is how components works in React
• Data drives the UI
• To change the UI, we must first update the data
• No two-way binding. Only one-way binding

How to update components

• This is how React updates components, in more details

Think in React

• Decompose the UI into components
  • Each component will render itself, based on external and internal data
  • Build trees of the components to create app

Describe the result

• React tries to be declarative instead of imperative
  • Don't try to modify the display (imperative) (like in jQuery)
  • Instead, just describe the result
    • React will figure out the changes that need to happen in DOM

<!-- just describe the following -->
<h3 title="alarm">The alarm goes off at 10:04 AM</h3>

Create components

• Components are functions (or classes)
  • Always start with an uppercase letter
  • Take a single argument called props
• Components return (or render) a 'React element' (also called 'virtual element')

//Simple Component.
//Starts with uppercase
//takes a single argument
function SimpleComponent(props) {
  return null;
}

JSX to the rescue

• React Elements should be created by JSX
• JSX is an easy syntax to create React elements
  • looks like HTML (gets transpiled to createElement())
  • Inside of JSX, curly braces switches to JS

import React from 'react'; //we still need this
//because JSX transpiles to React.createElement();

// This is the code to display the time ⮯⮯.
function DisplayAlarm(props) {
  return (
    // JSX is often enclosed in round brackets ()
    <h3 title='alarm'>The alarm goes off at {props.alarmTime}</h3> 
    //      curly braces {} switches to JS ⮭⮭
  );
}
//Babel converts JSX to React.createElement() ⮭⮭

JSX gets converted to createElement()

• Browsers don't know how to deal with the JSX.
• JSX will be converted to createElement() before being rendered.

//Need this ⮯⮯ to call React.createElement();
import React from 'react';

//JSX gets transpiled to the following JS
function DisplayAlarm(props) {
  return React.createElement(
    'h3',
    { title: 'alarm' },
    'The alarm goes off at ',
    props.alarmTime
  );
}

How React works behind the curtain

• JSX and createElement are going to generate React elements.
  • React elements will describe a Virtual DOM (vDom)
  • Virtual DOM works quicker than the DOM, because React Elements are simple JS objects.
• React has a 'reconciliation' phase
  • Called after a component (and all its children) are generated
    • The elements are generated to the virtual Dom (or vDom):
    • React compares current elements with the previous elements of the vDom
    • Any modification will generate the smallest DOM change possible

//This is what the JSX converted to the
//raw React elements looks like
function DisplayAlarm(props) {
  return {
    type: 'h3',
    props: {
      title: 'alarm',
      children: ['The alarm goes off at ', props.alarmTime],
    },
    key: null,
    ref: null,
    $$typeof: Symbol.for('react.element'),
  };
}

Components create a tree

• You can use components from other components.
• You build a "component tree" to describe your app

function DoubleAlarmClock(props) {
  //code could be found before the return
  return (
    <div>
      {' '}
      {/* call other components */}
      <DisplayAlarm />
      <div>
        {' '}
        {/* multiple times */}
        <DisplayAlarm />
      </boxed>
    </boxed>
  );
}
//all tags in jsx must be closed, or self-closed ⮭⮭

Use props to push data down

• Data coming from a parent component is called "props" (properties)
• Props are pushed from parent to children through JSX attributes
• Props are passed as the argument to the function.

function DoubleAlarmClock(props) {
  //otherAlarm is 10 minutes later (60000ms) ⮯⮯
  const otherAlarm = props.alarm + 60000;

  return (
    <div>
      <DisplayAlarm alarmTime={props.alarm} />
      <div>
        <DisplayAlarm alarmTime={otherAlarm} />
      </boxed>
    </boxed>
  );
}

Store data in State

• Modifiable data is called "state".
• State is defined by the useState function.
  • It returns an array composed of two items
    • First item is the data (state)
    • Second is a function to change that data (setState).
• NEVER change the state directly.
  • Always use the set function
  • This will schedule a rerender of the component (and child components)

import React, { useState } from 'react';
//named export ⮭⮭

function Counter(props) {
  const [count, setCount] = useState(5);
  //set to 5 the first time it's called ⮭⮭
  //use count to read the data
  //use setCount to write the data

  //a function inside a function
  //called by the click event ⮯⮯
  function increment(evt) {
    setCount(count + 1);
  }

  return (
    <div>
      <h1>
        Hello {props.name}, the count is: {count}{' '}
      </h1>
      <button onClick={increment}>Add 1</button>
    </boxed>
  );
}
// events are wired this way in React: onClick={fn} ⮭⮭
// It is a synthetic event, different than
// a real DOM event: onclick="increment".
// (Notice the difference in casing)
// Use synthetic events whenever possible.

Change the state to rerender

• When state (or props) change, or the parent re-renders, your component updates AUTOMATICALLY.
• You never have to modify the DOM yourself
  • If you do, you are not using React properly

function Counter(props) {
  const [count, setCount] = React.useState(5);

  function increment(evt) {
    setCount(count + 1);
  }
  //setCount ⮭⮭ doesn't change the state right away
  //it schedules the change for right after the firing
  //of all events. Once all scheduled changes are done
  //changing the states, those component will now
  //"rerender" (along with the child components).

  return (
    <div>
      <h1>
        Hello {props.name}, the count is: {count}{' '}
      </h1>
      <button onClick={increment}>Add 1</button>
    </boxed>
  );
}

Looping over items

• Loop over an array of data with .map()
• It returns an element for each iteration
• For performance reasons, remember to provide an unique key for each element

const authors = [
  { id: 1, name: 'Jeff' },
  { id: 2, name: 'Bill' },
  { id: 3, name: 'Mary' },
];

function App() {
  return <AuthorList authors={authors} />;
}

function AuthorList(props) {
  return (
    <ul>
      {
        props.authors.map((author) => {
          return <li key={author.id}>{author.name}</li>;
        }) //keys ⮭⮭ are used by React for performance
        //in reconciliation phase, not part of the DOM
      }
    </ul>
  );
}

Different ways to style

• use CSS files and then set attribute with className='danger'
  • use 'className' ('class' is a reserved keyword in js)
• Set inline styles with dangerStyle={{backgroundColor: 'red' }}
  • you could refer to a global style variable
  • dangerStyle={backgroundColor: red}

<p className="danger">Text</p>
<p style="{dangerStyle}">Text</p>
<p style={{backgroundColor: 'red', width: '300px', height: 300}}>
<!-- {/*  numeric values will be converted to 'px' units */} -->

Use a third-party library for theming and styling

Here is a list of libraries to help you generate css from js.

• Aphrodite
• Emotion
• Glamor
• Fela
• Styletron
• Jss
• Radium
• React-Native-Web
• Styled-Component
• CSS-in-JS (Facebook 2020?)

Use effect

• First argument of useEffect() takes a function.
  • Runs right after reconciliation and painting the screen.
  • Inside of this callback, you perform async functions and side effects
    • Manipulating the DOM (read or write) is a side effect
    • Network calls are async functions
• The second argument is a "change array" of dependencies
  • Effect gets called only when an item in the array is modified
    • Code should include in array any var that might change.
  • Put null or nothing to execute after all renders
  • Put an empty array [] for the effect to be called just once, right after the first render (when React "mounts" the component)

import React, { useState, useEffect } from 'react';
const url = 'https://randomuser.me/api/';

function DisplayUser(props) {
  const [data, setData] = useState();

  useEffect(() => {
    fetch(`${url}?seed=${props.id}`)
      .then((response) => response.json())
      .then((data) => setData(data.results[0]));
  }, [props.id]);
  // useEffect ⮭⮭ is called after each render
  // where id was modified (by the parent component)

  //we return the render result ⮯⮯
  return (
    <div>
      {data ? ( // if no data, show a "no data" msg
        <p>
          <img src={data.picture.thumbnail} />
          {data.name.first}
        </p>
      ) : (
        <p>No data (yet)!</p>
      )}
    </boxed>
  );
}

useEffect(() => {
  chatSubscribe(friendId);
  return function cleanup() {
    chatUnsubscribe(friendId);
  };
}, [friendId]);
//Each time the chat is re-rendered, if friendId was
//modified, then we unsubscribe to the previous chat,
//just before subscribing to the new friendId. The magic
//of "closures" will ensure the old friendID gets
//unsubscribed, not the current one.