{
  "url": "https://unifast.dev/docs/introduction/key-concepts/",
  "locale": "en",
  "title": "Key Concepts",
  "description": "Understand unifast's multi-stage compilation pipeline, built-in passes, and how MdAst and HAst transformations work.",
  "section": "introduction",
  "body": "unifast compiles Markdown through a multi-stage pipeline. Understanding these stages helps you configure the compiler and choose the right plugins.\n\n### Compilation Pipeline\n\n```\nInput text\n  → Parse (Markdown or MDX)\n  → IR0: MdAst (Markdown structure)\n  → Normalize + Built-in MdAst passes\n  → Lower to IR1: HAst (HTML structure)\n  → HAst passes (sanitize, highlight, etc.)\n  → Emit (HTML string)\n```\n\nEach stage transforms the document through an intermediate representation (IR).\n\n### Intermediate Representations\n\n| IR | Name | Purpose |\n|----|------|---------|\n| **IR0** | MdAst | Markdown structure - headings, paragraphs, lists, code blocks |\n| **IR1** | HAst | HTML structure - elements, attributes, text nodes |\n| **IR2** | JsAst | MDX only - ESM imports and JSX expressions |\n\nThe parser produces **MdAst**, which is then lowered to **HAst** for HTML emission. MDX inputs additionally produce **JsAst** nodes for JavaScript output.\n\n### Passes\n\nPasses are transformations applied to the AST at specific phases. unifast has built-in passes for common tasks:\n\n**MdAst passes** (before lowering to HTML):\n\n- **Normalize** - Consistent structure for downstream passes\n- **Slug** - Generate heading IDs from text content\n- **TOC** - Extract table of contents from headings\n- **Definition Resolution** - Resolve link/image reference definitions\n\n**HAst passes** (after lowering to HTML):\n\n- **Sanitize** - Remove disallowed HTML elements and attributes\n- **Highlight** - Apply syntax highlighting to code blocks\n- **Cleanup** - Remove unnecessary nodes and whitespace\n\nPasses are ordered by phase - you don't need to worry about execution order.\n\n### Plugins\n\nPlugins are TypeScript packages that configure built-in passes. They don't run arbitrary JavaScript during compilation - instead, they set options that control how the Rust core processes your document.\n\n```ts\nimport { compile, gfm, sanitize } from \"@unifast/node\";\n\nconst result = compile(source, {\n  plugins: [gfm(), sanitize()],\n});\n```\n\nEach plugin returns a configuration object that is merged into the compile options before the Rust core runs. This keeps the hot path entirely in native code.\n\n### Output Formats\n\nThe compiler supports multiple output formats via the `outputKind` option:\n\n| Format | Description |\n|--------|-------------|\n| `\"html\"` | HTML string (default) |\n| `\"hast\"` | HAst JSON - the HTML AST for custom rendering |\n| `\"mdast\"` | MdAst JSON - the Markdown AST for analysis |\n| `\"mdx-js\"` | JavaScript module string (MDX only) |\n\n### Diagnostics\n\nThe compiler reports issues via the `diagnostics` array in the result. Each diagnostic includes a severity level, message, and optional source span for precise error location.\n\n```ts\nconst result = compile(source);\n\nfor (const d of result.diagnostics) {\n  console.warn(`[${d.level}] ${d.message}`);\n}\n```",
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