@speajus/mermaid-to-svg

Mermaid diagrams to SVG strings without a browser. No Puppeteer, no Playwright, no headless Chrome — just Node.js (also works in browser).

Install

npm install @speajus/mermaid-to-svg

Quick Start

import { renderMermaid } from '@speajus/mermaid-to-svg';

const { svg, bounds } = await renderMermaid(`
  flowchart LR
    A[Start] --> B{Decision}
    B -->|Yes| C[OK]
    B -->|No| D[Cancel]
`);

console.log(svg); // <svg xmlns="...">...</svg>

Themes

Four built-in themes matching mermaid's defaults: default, dark, forest, neutral. Try it out in the Theme Builder.

// Use by name
const { svg } = await renderMermaid(diagram, { theme: 'dark' });

// Or import the theme object
import { darkTheme } from '@speajus/mermaid-to-svg';
const { svg } = await renderMermaid(diagram, { theme: darkTheme });

// Or create a custom theme
import { createTheme } from '@speajus/mermaid-to-svg';
const myTheme = createTheme({
  background: '#1a1a2e',
  primaryColor: '#e94560',
  primaryTextColor: '#ffffff',
});
const { svg } = await renderMermaid(diagram, { theme: myTheme });

API

renderMermaid(input, options?)

Full pipeline: parse → layout → render. Returns { svg, bounds, diagramType }.

const result = await renderMermaid(mermaidText, {
  theme: 'dark', // 'default' | 'dark' | 'forest' | 'neutral' | Theme object
  padding: 20, // pixels around the diagram
  idPrefix: 'mermaid', // unique prefix for SVG element IDs
  fontMetrics: provider, // custom FontMetricsProvider for text measurement
  layoutOptions: {}, // ELK layout options override
});

Individual Pipeline Steps

For advanced usage, each step is available separately:

import { parse, layout, renderSvg } from '@speajus/mermaid-to-svg';

const ir = await parse(mermaidText); // Mermaid text → IR
const positioned = await layout(ir); // IR → positioned graph
const svg = renderSvg(positioned); // positioned graph → SVG string

Theme Utilities

import { createTheme, mergeThemes, defaultTheme } from '@speajus/mermaid-to-svg';

// Create from defaults with overrides
const theme = createTheme({ background: '#111' });

// Merge two themes
const merged = mergeThemes(baseTheme, overrides);

SVG to PNG

Use @resvg/resvg-js to convert SVG output to PNG (no browser needed):

import { renderMermaid } from '@speajus/mermaid-to-svg';
import { Resvg } from '@resvg/resvg-js';
import { writeFileSync } from 'node:fs';

const { svg, bounds } = await renderMermaid(diagram);
const resvg = new Resvg(svg, {
  fitTo: { mode: 'width', value: bounds.width * 2 },
});
writeFileSync('diagram.png', resvg.render().asPng());

How It Works

Mermaid Text → Parser (mermaid) → IR → Layout (ELK.js) → SVG (React renderToStaticMarkup)

1. Parse — Uses mermaid's internal parser to extract nodes, edges, subgraphs, and direction
2. Layout — ELK.js computes positions and edge routing (layered algorithm, orthogonal edges)
3. Render — React components produce clean SVG via renderToStaticMarkup() — no browser DOM needed

Diagram Support

Flowchart

flowchart TD
  A[Christmas] -->|Get money| B(Go shopping)
  B --> C{Let me think}
  C -->|One| D[Laptop]
  C -->|Two| E[iPhone]
  C -->|Three| F[fa:fa-car Car]

Sequence

sequenceDiagram
  participant Alice
  participant Bob
  Alice->>Bob: Hello Bob, how are you?
  Bob-->>Alice: Great!
  Alice->>Bob: See you later!

Class

classDiagram
  class Animal {
    +int age
    +String gender
    +isMammal() bool
  }
  class Duck {
    +String beakColor
    +swim()
  }
  Animal <|-- Duck

State

stateDiagram-v2
  [*] --> Idle
  Idle --> Processing : submit
  Processing --> Done : complete
  Processing --> Error : fail
  Error --> Idle : retry
  Done --> [*]

ER (Entity Relationship)

erDiagram
  CUSTOMER {
    string name
    int id PK
  }
  ORDER {
    int orderId PK
    date created
  }
  CUSTOMER ||--o{ ORDER : places

Gantt

gantt
  title Project Plan
  dateFormat YYYY-MM-DD
  section Design
    Wireframes :a1, 2024-01-01, 10d
    Mockups    :after a1, 5d
  section Development
    Backend    :2024-01-10, 20d

Pie

pie title Pets adopted
  "Dogs" : 386
  "Cats" : 85
  "Rats" : 15

Mindmap

mindmap
  root((Central))
    Origins
      Long history
      Popularisation
    Research
      On effectiveness
      On adoption

Themes

Four built-in themes — here's the flowchart in each:

| Default | Dark | Forest | Neutral | | --------------------------------------------------------------- | ------------------------------------------------------------ | -------------------------------------------------------------- | --------------------------------------------------------------- | | | | | |

Development

npm test          # Run tests (node:test)
npm run build     # Build with tsup (ESM + CJS + types)
npm run typecheck  # Type check with tsc

# Generate theme sample PNGs
node --import tsx examples/generate-theme-pngs.ts

License

MIT