React.js is a front-end JavaScript library for building user interfaces. Many React frameworks integrate React in a way that’s better suited for a production context.
Components
At the core of React apps are components, which are pieces of the UI with its own logic and appearance. Components are defined in JavaScript functions that return markup — then, in a function that defines a document, we can use an “HTML tag” with the same name as the function. Functions must begin with a capital, which distinguishes from regular HTML tags (which must be lowercase).
Development environment
Million Lint is a linter intended for improving the performance of React code.