unified enables new exciting projects like Gatsby to pull in Markdown, MDX to embed JSX, and Prettier to format it. It’s used in about 300k projects on GitHub and has about 10m downloads each month on npm.
unified uses abstract syntax trees, or ASTs, that plugins can operate on. It can even process between different formats. This means you can parse a markdown document, transform it to HTML, and then transpile back to markdown.
unified leverages a syntax tree specification (called unist or UST) so that utilities can be shared amongst different formats. In practice, you can use unist-util-visit to visit nodes using the same library with the same API on any supported AST.