React is a JaveScript library for building responsive UIs.


React components are reusable pieces of code that allow us to split the UI into independent pieces.

Components can be written as regular JS functions or as classes.

Functional Components

Functional components are regular JS functions that take a props parameter. props is an object containing properties that are passed to the component.

Example of a functional component:

function Person(props) {
    return <p>Hello {props.lastName}!</p>;

Class Components

Class components are ES6 classes that extend React.Component. They are able to hold state and require a render() function to display.

Similar to functional components, class components make use of a props object. In this case, props is a member of the class rather than a function parameter.

Example of a class component:

class Person extends React.Component {
    render() {
        return <p>Hello {this.props.lastName}</p>

When to Use Functional vs. Class Components

If a component is just taking in some props and rendering, then it's a good idea to use go functional. If it needs more functionality or has to maintain state, a class is the better options.

NOTE: As of React 16.8, this is no longer accurate. Much of the functionality of class components can now be achieved in functional components using hooks. Because I have yet to learn about them, I will not write an explanation here and will update this section at a later date.


The lifecycle of a component is the sequence of stages it goes through during its time in the DOM tree. This includes things like the time it is created, destroyed, or updated.

Lifecycle Methods

Lifecycle methods are special methods in React that run during different stages of a component's lifecycle. Examples of lifecycle methods are listed below.

Note that lifecycle methods can only be used in class components.

  • Common lifecycle methods

    • Render()

      • Contains logic that the component should display on-screen

      • This is the only required method within a class component

      • Has to be a pure function

    • ComponentDidMount()

      • Runs when a component is mounted (added to the DOM tree)

      • This is often used for things like connecting to APIs, setting timers, and adding event listeners

    • ComponentDidUpdate()

      • Runs each time a component is rendered (does not run on very first render)

    • ComponentWillUnmount()

      • Runs when a component is removed from the DOM tree

      • Often used for doing cleanup

  • Newer and more uncommon lifecycle methods

    • shouldComponentUpdate()

      • Lets React know if a component is affected by state/prop changes

      • Useful for when you don't want to render state/prop changes

      • Cannot update component state in this method

    • getDerivedStateFromProps()

      • Safer alternative to componentWillReceiveProps()

      • Called just before each render

      • Static function, so no access to this

      • Returns object to update state in response to prop changes

    • getSnapshotBeforeUpdate()

      • Safer alternative to componentWillUpdate()

      • Called right before DOM is updated

      • Return value is passed to componentDidUpdate()


Hooks are a feature from React 16.8 that allow the use of state, lifecycle methods, and other features in functional components. Without hooks, class components are required to use these.

Why Use Hooks?

Environment Variables

Environment variables can be added to a react project by adding a .env file to the project root. Each variable must start with REACT_APP. For example:


Configured variables can be accessed using process.env:

const myEndpoint = process.env.REACT_APP_API_ENDPOINT

Last updated