React — A JavaScript library for building user interfaces

Sandini Pitawala
3 min readMay 5, 2021


React.js is an open-source JavaScript library that is used for building user interfaces specifically for single-page applications.

React allows developers to create large web applications that can change data, without reloading the page.

The main purpose of React is to be fast, scalable, and simple.

Based On Component

Build encapsulated components that manage their own state, then compose them to make complex UIs.

Since component logic is written in JavaScript instead of templates, you can easily pass rich data through your app and keep state out of the DOM.

If you’ve learned it, you can write it anywhere.

We don’t make assumptions about the rest of your technology stack, so you can develop new features in React without rewriting existing code.

React can also render on the server using Node and power mobile apps using React Native.

A Simple Component

React components implement a render() method that takes input data and returns what to display.

This example uses an XML-like syntax called JSX. Input data that is passed into the component can be accessed by render() via this.props

JSX is optional and not required to use React.

Try the Babel REPL to see the raw JavaScript code produced by the JSX compilation step.


A Stateful Component

In addition to taking input data (accessed via this.props), a component can maintain internal state data (accessed via this.state).

When a component’s state data changes, the rendered markup will be updated by re-invoking render().


An Application

Using props and state, we can put together a small Todo application.

This example uses state to track the current list of items as well as the text that the user has entered.

Although event handlers appear to be rendered inline, they will be collected and implemented using event delegation.


A Component Using External Plugins

React allows you to interface with other libraries and frameworks.

This example uses remarkable, an external Markdown library, to convert the <textarea>’s value in real time.