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Rendering engines

The render package is engine-agnostic. It ships with built-in engines, and you can register custom engines at runtime.

Embedded (default)

Uses the mermaid package in-process with a jsdom DOM shim. This provides deterministic rendering without spawning external processes.

Optional dependencies:

  • mermaid
  • jsdom
  • dompurify (sanitizer)

Current pinned version:

  • mermaid@10.9.5

Mermaid CLI (mmdc)

Uses the Mermaid CLI (@mermaid-js/mermaid-cli) via subprocess. This is useful if you already use mmdc in CI.

Set MERMKIT_MMDC_PATH to point to a custom binary.

Stub

Produces a placeholder SVG with the Mermaid source as text. Useful for early integration.

ASCII

A pure-TypeScript ASCII/Unicode renderer. Supports flowcharts (LR and TD directions) and sequence diagrams. Outputs box-drawing characters by default; pass --ascii on the CLI for plain ASCII (+, -, |) mode.

This engine is skipped in auto mode. Request it explicitly:

  • CLI: --engine ascii or --format ascii
  • API: { engine: "ascii" } or { format: "ascii" }

Custom engines

Register your own engine:

import { registerEngine, render } from "@mermkit/render";

registerEngine({
  id: "beautiful-mermaid",
  priority: 40,
  render: async (diagram, options) => {
    // return { bytes, mime, warnings }
  }
});

await render(diagram, { format: "svg", engine: "beautiful-mermaid" });

engine: "auto" will try engines in priority order (highest first).