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
React Cycle

How to update components

  • This is how React updates components, in more details
React Cycle

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
Todos 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(
    { title: 'alarm' },
    'The alarm goes off at ',

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 (
      {' '}
      {/* call other components */}
      <DisplayAlarm />
        {' '}
        {/* multiple times */}
        <DisplayAlarm />
//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 (
      <DisplayAlarm alarmTime={props.alarm} />
        <DisplayAlarm alarmTime={otherAlarm} />

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 (
        Hello {}, the count is: {count}{' '}
      <button onClick={increment}>Add 1</button>
// 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 (
        Hello {}, the count is: {count}{' '}
      <button onClick={increment}>Add 1</button>

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 (
      { => {
          return <li key={}>{}</li>;
        }) //keys ⮭⮭ are used by React for performance
        //in reconciliation phase, not part of the DOM

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)

Code sample of useEffect

import React, { useState, useEffect } from 'react';
const url = '';

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

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

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

Returned effect: Cleanup

  • The returned function is cleanup. it is called either:

    • Just before the next effect is called, but after React rendered
    • Just after the component is destroyed (when react "unmounts" the component)
  • Makes it easy to subscribe and unsubscribe to stuff
useEffect(() => {
  return function cleanup() {
}, [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.

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