CSS-in-JS

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CSS-in-JS is a styling technique where JavaScript is used to style components. When this JavaScript is parsed, CSS is generated (usually as a <style> element) and attached into the DOM. It allows to abstract CSS to the component level itself, using JavaScript to describe styles in a declarative and maintainable way. There are multiple implementations of this concept in the form of libraries such as

• Emotion^[1]
• Styled Components^[2]
• JSS^[3]

These libraries allow to create styled components using tagged template literals. For example, using styled components in a React (JavaScript library) project would look like the following:

import styled from 'styled-components';
// Create a component that renders a <p> element with blue text
const BlueText = styled.p`
  color: blue;
`;

<BlueText>My blue text</BlueText>

You can do other things using CSS-in-JS which were not possible using traditional CSS techniques. You can make the styles dynamic inline with just a few conditional statements. It also allows you to write more modular code with your CSS being encapsulated in the same block as your javascript, scoping it to that module only.