What is React?

  • React is a JavaScript library.
  • That makes the building complex user interfaces easier.
  • By breaking them up into small, simple, functional components.


  • By providing tools to create components for each part of the interface.
  • And manage the assembly of those components into the final resulting web page.

What are components?

Components are individual pieces of a web page interface like:

  • a user profile
  • a comment
  • a form
  • an up-vote button

Components Example

Virtual DOM 1

  • React controls the rendering of the page by using something called the Virtual DOM.
  • DOM stands for Document-Object-Model, and is managed by the browser.
  • What this means in practice is that React keeps a copy of the web page structure and state in memory.
  • This copy is accessible to JavaScript and React makes changes to the real DOM all at once instead of one at a time.

Virtual DOM 2

Imagine an auction application with several parts.

  • The items on auction right now with current prices.
  • Your current bid, and your maximum bid ceiling.
  • A SOLD label applied to an item that has closed.
  • The items that will come up for auction soon.
  • Recently sold items and their closing prices.

Virtual DOM 3

In this application there are many changes that can happen at once.

  • The items available can increase or decrease.
  • Your current bid can increase in response to another bidder.
  • The SOLD label can be applied or maybe removed.
  • New items can appear with their opening prices.
  • Recently sold items can appear with prices.

Virtual DOM 4

React makes managing a complicated page like this easier by determining what the page needs to look like at any given rendered frame.

Virtual DOM 5

For every given frame React:

  • Records any changes within that time slot.
  • Calculates the interdependencies between components.
  • Generates the next state of the interface.
  • Renders the frame to the browser.

Virtual DOM 6

This lets you as the programmer:

  • Declare the page you want.
  • Decide how to handle change when it occurs.
  • Delegate to React how to build the page for you when changes happen.

Declarative Intent 1

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

  'Hello, React!'
), document.getElementById('root'));

Declarative Intent 2

What is Declarative Intent?

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

  • You only tell the computer what you want.
  • You must describe something that is possible.
  • The computer figures out how to make it so.

"make it so"

Declarative Intent 3

Declarative is different than Imperative code which:

  • Requires a sequence of ordered steps
  • With transitions between the states

An imperative example would be manipulating the DOM like this:

window.onLoad function () {
  var heading = document.createElement('h1');
  var text = document.createTextNode('Hello DOM!');

Declarative Intent 4

See the Pen mLqoGK by Joshua Burke (@Dangeranger) on CodePen.

Declarative Intent 5

Something a little more complicated

        id: "my-form",
        onSubmit: this.handleSubmit
          id: "create",
          type: "text",
          placeholder: "something"
  ), document.getElementById('root'));

Declarative Intent 6

Given a <root> element exists the result is:

  <form id="my-form">
    <input id="create" type="text" placeholder="something"/>

But when the form initiates a onSubmit event React will handle the changes using the handleSubmit handler function.

Declarative Intent 7

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

// API signature
ReactDOM.render(element, container[, callback])
  • element => The DOM element and children to generate.
  • container => What DOM element to generate within.
  • callback => Optional function to call after generation.
  • ReactDOM.render API Docs

Declarative Intent 8

Accepts an element type, props of the element, and child elements.

// API signature
  • type => A DOM element name like div, form or h1.
  • props => The element attributes like id, class, placeholder, onChange, or onSubmit.
  • children => Child elements to nest within the generated element.


React allows you to:

  • Write JavaScript that builds HTML
  • Write functions that update the HTML when state changes
  • Delegate responsibility over the DOM to a library
  • Be confident that the desired application state will be achieved