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mermaid-formatter

A formatter for Mermaid diagram syntax.

Features

• Format all Mermaid diagram types (sequenceDiagram, flowchart, classDiagram, etc.) • Normalize indentation and whitespace • Format Mermaid code blocks in Markdown • CLI tool and programmatic API • Zero dependencies (lightweight)

Installation

Usage

CLI

Prettier Plugin

Programmatic API

API Reference

formatMermaid(input: string, options?: FormatOptions): string

Format Mermaid diagram source code.

Options:

• indentSize (number, default: 4) - Number of spaces for indentation • useTabs (boolean, default: false) - Use tabs instead of spaces

formatMarkdownMermaidBlocks(markdown: string, options?: FormatOptions): string

Format all Mermaid code blocks in a Markdown document.

parse(input: string): Diagram

Parse Mermaid source into an AST.

detectDiagramType(input: string): DiagramType

Detect the diagram type from source code.

Formatting Rules

• Diagram declaration at column 0 • Block keywords (critical, alt, loop, par, opt, break, rect, subgraph, end) indented based on nesting depth • Block continuations (else, option, and) at same level as their opening block keyword • Content inside blocks indented by configured amount • Consecutive blank lines collapsed to single blank line • Trailing blank lines removed • Blank line inserted before block starts • Arrow messages normalized when line matches from ARROW to: message pattern (A->>B:msg → A ->> B: msg) • Flowchart class syntax is preserved (A --> B:::warning is not treated as arrow message) • Whitespace normalized (multiple spaces → single, bracket padding removed)

Supported Diagram Types

• sequenceDiagram • flowchart / graph • classDiagram • stateDiagram / stateDiagram-v2 • erDiagram • journey • gantt • pie • quadrantChart • requirementDiagram • gitGraph • mindmap (preserved, indent-sensitive) • timeline (preserved, indent-sensitive) • sankey-beta • xychart-beta • block-beta • architecture-beta

Editor Integration

See Integrations Guide for:

• VS Code setup (tasks, run-on-save) • Pre-commit hooks • GitHub Actions • Prettier plugin

Contributing

Issues and PRs welcome! See GitHub Issues.

License

MIT

Description from the npm listing.