Getting started with React. Notes and main concepts to understand how it works and how to build a website.


This is a guide to understand React framework to build web applications. It explores its main concepts and common project structure.

React is a declarative JavaScript library for building user interfaces.

declarative programming is a programming paradigm—a style of building the structure and elements of computer programs—that expresses the logic of a computation without describing its control flow. (Opposite: imperative programming)



React is based around a React Components class. Everything in React is a Component, it is said that its architecture is Component-based.

Each component

  1. takes in parameters, called props to customize it when creating them, and

  2. returns a hierarchy of views to display via the render method, which returns a React element (a lightweight description of what to render)


React Elements is what a React Component returns, describing what should appear on the screen.


Props are arbitrary inputs passed to React Components. They can be seen as object arguments that customize components.



State allows React components to change their output over time in response to:

  • user actions,
  • network responses, and
  • anything else

without preventing components acting like pure functions with respect to their props. 1

A pure function doesn’t depend on and doesn’t modify the states of variables out of its scope. Concretely, that means a pure function always returns the same result given same parameters.

React components can have state by setting this.state in the constructor.


JSX produces React “elements”.

JSX adds XML syntax to JavaScript, so it can be thought that its name is a mix of Javascript+XML.

To make the rendering definition easier, we use JSX that allows us to use react components in an XML-syntax style, e.g.:

The <div /> syntax is transformed at build time to React.createElement('div'), so writing the above tag is the same as writing using React.createElement:

return React.createElement('div', {className: '...'},
  React.createElement('h1', ...),
  React.createElement('ul', ...)

You can put any JavaScript expression within braces inside JSX. Each React element is a real JavaScript object that you can store in a variable or pass around your program.


Passing data through Components

Using props when instantiating a component like <MyComponent value=somevalue /> then we can use it in another component accessing this.props in MyComponent, e.g.:

class Board extends React.Component {
  render() {
    return <Square value={i} />;

class Square extends React.Component {
  render() {
    return (
      <button className="square">

Aggregation of many components

When you want to aggregate data from multiple children or to have two child components communicate with each other, move the state upwards so that it lives in the parent component. The parent can then pass the state back down to the children via props, so that the child components are always in sync with each other and with the parent.

Pulling state upwards like this is common when refactoring React components.


There are two ways of changing data:

  1. mutation
  2. replace with new copy

In React it is better to create new versions of data instead of directly modifying it, like instead of directly modifying an array value, make a copy of it.

This has several advantages:

  • better performance.
  • easier to implement complex features (keeping older versions we can switch to them as needed).
  • tracking changes is difficult, determining an immutable object change is trivial, this helps to determine when a component requires being re-rendered.

More at:

Interactive Components

Functional Components

Functional Components is a simpler syntax for component types that only consist of a render method.

Rather than defining a class extending React.Component, have a function that takes props and returns what should be rendered.

function Mycomponent(props) {
  return (
    <button className="foobar">

this won’t be available, so this.props become props


Useful commands

  • Installation of create-react command line utility:

       npm install -g create-react-app

Create react app

This tool creates a fronted build pipeline without backend logic or databases create-react-app my-app.

It makes available the commands start, build, test and eject:

  • To start the app npm start, which will run react-scripts start.
  • To deploy, first create an optimized build in the build folder with npm run build.



Marcelo Canina
I'm Marcelo Canina, a developer from Uruguay. I build websites and web-based applications from the ground up and share what I learn here.