prettier

Prettier is an opinionated code formatter that parses your source code into an abstract syntax tree and re-prints it from scratch, applying its own consistent rules while honoring a configurable maximum line length. By removing all original styling and producing deterministic output, it ends debates over formatting entirely and lets teams focus code review on substance rather than whitespace.

With first-class support for JavaScript, TypeScript, JSX, CSS, SCSS, Less, HTML, Vue, Angular, GraphQL, Markdown, YAML, and JSON, Prettier runs from a CLI, an editor-on-save integration, a pre-commit hook, or a CI check. A programmatic API is also available for tools that need to format code on the fly.

What You Get

• A zero-configuration CLI that formats entire projects in place with a single command
• Built-in printers for 15+ languages including JavaScript, TypeScript, CSS, HTML, Vue, GraphQL, Markdown, and YAML
• Editor integrations that reformat files automatically on save
• A programmatic API (format, check, formatWithCursor, resolveConfig) for embedding formatting in your own tooling
• A —check mode for CI that fails the build when files are not already formatted

Common Use Cases

• Enforcing a single, non-negotiable code style across a team or monorepo
• Reformatting files automatically on save in the editor
• Gating pull requests in CI so unformatted code cannot be merged
• Formatting a mixed-language codebase (JS, CSS, Markdown, YAML) with one tool

Under The Hood

Architecture Prettier is a parse-and-reprint pipeline rather than a rule-patching linter. src/main/core.js orchestrates the flow: source text is parsed by a language-specific parser (src/main/parse.js dispatches to plugins under src/language-js, src/language-css, src/language-html, etc.), then src/main/ast-to-doc.js converts the resulting AST into an intermediate representation called a Doc using the builder combinators in src/document. A Wadler-style pretty-printer (src/document/printer.js) walks the Doc and decides where to break lines based on the configured printWidth, before the result is serialized back to a string. Comments are re-attached to the new tree via src/main/comments, and options flow through normalize-options.js and core-options.evaluate.js.

Tech Stack Written in JavaScript (ESM, "type": "module", Node >=22) with TypeScript type definitions shipped in src/index.d.ts. It bundles a large set of best-in-class parsers as dependencies — @babel/parser, typescript, flow-parser, meriyah, oxc-parser, and hermes-parser for JS/TS; postcss (with less/scss plugins) for CSS; angular-html-parser and @glimmer/syntax for templates; unified/remark for Markdown; and yaml and graphql for their respective languages. The CLI is delegated to @prettier/cli, and the build is bundled for distribution.

Code Quality The project is exceptionally well-tested, with over 4,300 test files under tests/ split across format, unit, integration, and dts suites, run via Jest with snapshot testing to lock formatting output. Code is organized cleanly by language into self-contained src/language-* directories, each exposing parsers and printers behind a consistent plugin interface (Parser/Printer in the type definitions). Options are centrally normalized, and the Doc IR provides a clear, well-abstracted seam between parsing and printing.

API Design The public API is small and ergonomic: format(source, options) and check(source, options) return promises, with formatWithCursor, resolveConfig, resolveConfigFile, getSupportInfo, and getFileInfo covering configuration and tooling needs. The plugin contract (Plugin, Parser, Printer) is fully typed and lets third parties add languages without touching core. Extensive official documentation, a live playground, and near-zero required configuration make it trivial to adopt.