What is React?

react logo

What are components?

Components are individual pieces of a web page interface like:

Components Example

react twitter components

Virtual DOM

DOM Components

Imagine an application with several parts.

DOM Example

react component tree

Automatic Updates

For every change in the components React:

Reacting to Changes

This lets you as the programmer:

The React Lifecycle

React builds and handles changes to the elements on the page through several methods native to React components the most common of which are:

We can access these methods using React hooks to make our component do whatever we want when it loads, changes, or is removed from the page. More on hooks later!

Lifecycle Diagram

react component lifecycle

Declarative UI

React lets you to declare what you want the page to be.

Declarative means that you do not instruct the computer about what steps to take in order to achieve your desired result.

"make it so"

Declarative vs Imperative

Declarative is different than Imperative code which:

An imperative example would be manipulating the DOM like this:

// When the Winow 'load' event fires, run the following JavaScript
(window.onLoad = function() {

  // Create the H1 Element
  const heading = document.createElement('h1');

  // Create the Text node and content
  const text = document.createTextNode('Hello DOM!');

  // Add the Text node to the parent H1

  // Add the H1 Element to the Body Element

Simple Example

    <h1>Reasons to love React!</h1>


Hold up! What's that stuff that looks like HTML doing in my JavaScript!

It's not really HTML, it's JSX which is:

ReactDOM function API

Accepts a description of the components that make up the page, and what DOM node to render the results to.

// Render function on ReactDOM

// API signature
  [callback] /* Optional*/

React.createElement function API

Under the hood React is compiling the JSX into HTML using the createElement method which accepts an element type, props of the element, and child elements, and creates corresponding HTML elements.

// CreateElement function on ReactDOM

// API signature
  [props], /* Optional*/
  [...children] /* Optional*/


React allows you to: