GitHub - lukilabs/beautiful-mermaid · GitHub

beautiful-mermaid

Render Mermaid diagrams as beautiful SVGs or ASCII art

Ultra-fast, fully themeable, zero DOM dependencies. Built for the AI era.

Why We Built This

Diagrams are essential for AI-assisted programming. When you're working with an AI coding assistant, being able to visualize data flows, state machines, and system architecture—directly in your terminal or chat interface—makes complex concepts instantly graspable.

Mermaid is the de facto standard for text-based diagrams. It's brilliant. But the default renderer has problems:

Aesthetics — Might be personal preference, but wished they looked more professional
Complex theming — Customizing colors requires wrestling with CSS classes
No terminal output — Can't render to ASCII for CLI tools
Heavy dependencies — Pulls in a lot of code for simple diagrams

We built beautiful-mermaid at Craft to power diagrams in Craft Agents. It's fast, beautiful, and works everywhere—from rich UIs to plain terminals.

The ASCII rendering engine is based on mermaid-ascii by Alexander Grooff. We ported it from Go to TypeScript and extended it. Thank you Alexander for the excellent foundation! (And inspiration that this was possible.)

Features

6 diagram types — Flowcharts, State, Sequence, Class, ER, and XY Charts (bar, line, combined)
Dual output — SVG for rich UIs, ASCII/Unicode for terminals
Synchronous rendering — No async, no flash. Works with React useMemo()
15 built-in themes — And dead simple to add your own
Full Shiki compatibility — Use any VS Code theme directly
Live theme switching — CSS custom properties, no re-render needed
Mono mode — Beautiful diagrams from just 2 colors
Zero DOM dependencies — Pure TypeScript, works everywhere
Ultra-fast — Renders 100+ diagrams in under 500ms

Installation

npm install beautiful-mermaid

or

bun add beautiful-mermaid

or

pnpm add beautiful-mermaid

Quick Start

SVG Output

import { renderMermaidSVG } from 'beautiful-mermaid'

const svg = renderMermaidSVG(graph TD A[Start] --> B{Decision} B -->|Yes| C[Action] B -->|No| D[End])

Rendering is fully synchronous — no await, no promises. The ELK.js layout engine runs synchronously via a FakeWorker bypass, so you get your SVG string instantly.

Need async? Use renderMermaidSVGAsync() — same output, returns a Promise<string>.

ASCII Output

import { renderMermaidASCII } from 'beautiful-mermaid'

const ascii = renderMermaidASCII(graph LR; A --> B --> C)

┌───┐ ┌───┐ ┌───┐ │ │ │ │ │ │ │ A │────►│ B │────►│ C │ │ │ │ │ │ │ └───┘ └───┘ └───┘

React Integration

Because rendering is synchronous, you can use useMemo() for zero-flash diagram rendering:

import { renderMermaidSVG } from 'beautiful-mermaid'

function MermaidDiagram({ code }: { code: string }) { const { svg, error } = React.useMemo(() => { try { return { svg: renderMermaidSVG(code, { bg: 'var(--background)', fg: 'var(--foreground)', transparent: true, }), error: null, } } catch (err) { return { svg: null, error: err instanceof Error ? err : new Error(String(err)) } } }, [code])

if (error) return

{error.message}

return

}

Why this works well:

No flash — SVG is computed synchronously during render, not in a useEffect
CSS variables — Pass var(--background) etc. instead of hex colors. The SVG inherits from your app's CSS, so theme switches apply instantly without re-rendering
Memoized — Only re-renders when code changes

Theming

The theming system is the heart of beautiful-mermaid. It's designed to be both powerful and dead simple.

The Two-Color Foundation

Every diagram needs just two colors: background (bg) and foreground (fg). That's it. From these two colors, the entire diagram is derived using color-mix():

const svg = renderMermaidSVG(diagram, { bg: '#1a1b26', // Background fg: '#a9b1d6', // Foreground })

This is Mono Mode —a coherent, beautiful diagram from just two colors. The system automatically derives:

Element | Derivation

Enriched Mode

For richer themes, you can provide optional "enrichment" colors that override specific derivations:

const svg = renderMermaidSVG(diagram, { bg: '#1a1b26', fg: '#a9b1d6', // Optional enrichment: line: '#3d59a1', // Edge/connector color accent: '#7aa2f7', // Arrow heads, highlights muted: '#565f89', // Secondary text, labels surface: '#292e42', // Node fill tint border: '#3d59a1', // Node stroke })

If an enrichment color isn't provided, it falls back to the color-mix() derivation. This means you can provide just the colors you care about.

CSS Custom Properties = Live Switching

All colors are CSS custom properties on the <svg> element. This means you can switch themes instantly without re-rendering:

// Switch theme by updating CSS variables svg.style.setProperty('--bg', '#282a36') svg.style.setProperty('--fg', '#f8f8f2') // The entire diagram updates immediately

For React apps, pass CSS variable references instead of hex values:

const svg = renderMermaidSVG(diagram, { bg: 'var(--background)', fg: 'var(--foreground)', accent: 'var(--accent)', transparent: true, }) // Theme switches apply automatically via CSS cascade — no re-render needed

Built-in Themes

15 carefully curated themes ship out of the box:

Theme | Type | Background | Accent

import { renderMermaidSVG, THEMES } from 'beautiful-mermaid'

const svg = renderMermaidSVG(diagram, THEMES['tokyo-night'])

Adding Your Own Theme

Creating a theme is trivial. At minimum, just provide bg and fg:

const myTheme = { bg: '#0f0f0f', fg: '#e0e0e0', }

const svg = renderMermaidSVG(diagram, myTheme)

Want richer colors? Add any of the optional enrichments:

const myRichTheme = { bg: '#0f0f0f', fg: '#e0e0e0', accent: '#ff6b6b', // Pop of color for arrows muted: '#666666', // Subdued labels }

Full Shiki Compatibility

Use any VS Code theme directly via Shiki integration. This gives you access to hundreds of community themes:

import { getSingletonHighlighter } from 'shiki' import { renderMermaidSVG, fromShikiTheme } from 'beautiful-mermaid'

// Load any theme from Shiki's registry const highlighter = await getSingletonHighlighter({ themes: ['vitesse-dark', 'rose-pine', 'material-theme-darker'] })

// Extract diagram colors from the theme const colors = fromShikiTheme(highlighter.getTheme('vitesse-dark'))

const svg = renderMermaidSVG(diagram, colors)

The fromShikiTheme() function intelligently maps VS Code editor colors to diagram roles:

Editor Color | Diagram Role

Supported Diagrams

Flowcharts

graph TD A[Start] --> B{Decision} B -->|Yes| C[Process] B -->|No| D[End] C --> D

All directions supported: TD (top-down), LR (left-right), BT (bottom-top), RL (right-left).

State Diagrams

stateDiagram-v2 [] --> Idle Idle --> Processing: start Processing --> Complete: done Complete --> []

Sequence Diagrams

sequenceDiagram Alice->>Bob: Hello Bob! Bob-->>Alice: Hi Alice! Alice->>Bob: How are you? Bob-->>Alice: Great, thanks!

Class Diagrams

classDiagram Animal <|-- Duck Animal <|-- Fish Animal: +int age Animal: +String gender Animal: +isMammal() bool Duck: +String beakColor Duck: +swim() Duck: +quack()

ER Diagrams

erDiagram CUSTOMER ||--o{ ORDER : places ORDER ||--|{ LINE_ITEM : contains PRODUCT ||--o{ LINE_ITEM : "is in"

Inline Edge Styling

Use linkStyle to override edge colors and stroke widths — just like Mermaid's linkStyle:

graph TD A --> B --> C linkStyle 0 stroke:#ff0000,stroke-width:2px linkStyle default stroke:#888888

Syntax | Effect

linkStyle 0 stroke:#f00 | Style a single edge by index (0-based) linkStyle 0,2 stroke:#f00 | Style multiple edges at once linkStyle default stroke:#888 | Default style applied to all edges

Index-specific styles override the default. Supported properties: stroke, stroke-width.

Works in both flowcharts and state diagrams.

XY Charts

Bar charts, line charts, and combinations — using Mermaid's xychart-beta syntax.

Bar chart:

xychart-beta title "Monthly Revenue" x-axis [Jan, Feb, Mar, Apr, May, Jun] y-axis "Revenue ($K)" 0 --> 500 bar [180, 250, 310, 280, 350, 420]

Line chart:

xychart-beta title "User Growth" x-axis [Jan, Feb, Mar, Apr, May, Jun] line [1200, 1800, 2500, 3100, 3800, 4500]

Combined bar + line:

xychart-beta title "Sales with Trend" x-axis [Jan, Feb, Mar, Apr, May, Jun] bar [300, 380, 280, 450, 350, 520] line [300, 330, 320, 353, 352, 395]

Horizontal orientation:

xychart-beta horizontal title "Language Popularity" x-axis [Python, JavaScript, Java, Go, Rust] bar [30, 25, 20, 12, 8]

Axis configuration:

Categorical x-axis: x-axis [A, B, C]
Numeric x-axis range: x-axis 0 --> 100
Axis titles: x-axis "Category" [A, B, C]
Y-axis range: y-axis "Score" 0 --> 100

Multi-series: Add multiple bar and/or line declarations. Each series gets a distinct color from a monochromatic palette derived from the theme's accent color.

XY Chart Styling

The chart renderer follows a clean, minimal design philosophy inspired by Apple and Craft:

Dot grid — A subtle dot pattern fills the plot area instead of traditional solid grid lines
Rounded bars — All bar corners are rounded for a modern, polished look
Smooth curves — Line series use natural cubic spline interpolation, producing mathematically smooth curves through all data points (not straight segments or staircase steps)
Floating labels — No visible axis lines or tick marks; labels float freely for a clutter-free aesthetic
Drop-shadow lines — Each line series has a subtle shadow beneath it for depth
Monochromatic palette — Series 0 uses the theme's accent color; additional series get darker/lighter shades of the same hue with subtle hue drift, adapting automatically to light or dark backgrounds
Interactive tooltips — When rendered with interactive: true, hovering over bars or data points shows value tooltips. Multi-line tooltips appear when multiple series share an x-position
Sparse line dots — Lines with 12 or fewer data points show data point dots by default for readability
Full theme support — All 15 built-in themes (and custom themes) apply to charts. The accent color drives the entire series color palette
Live theme switching — Chart series colors are CSS custom properties (--xychart-color-N), so theme changes apply instantly without re-rendering

ASCII Output

For terminal environments, CLI tools, or anywhere you need plain text, render to ASCII or Unicode box-drawing characters:

import { renderMermaidASCII } from 'beautiful-mermaid'

// Unicode mode (default) — prettier box drawing const unicode = renderMermaidASCII(graph LR; A --> B)

// Pure ASCII mode — maximum compatibility const ascii = renderMermaidASCII(graph LR; A --> B, { useAscii: true })

Unicode output:

┌───┐ ┌───┐ │ │ │ │ │ A │────►│ B │ │ │ │ │ └───┘ └───┘

ASCII output:

+---+ +---+

| A |---->| B |

+---+ +---+

ASCII Options

renderMermaidASCII(diagram, { useAscii: false, // true = ASCII, false = Unicode (default) paddingX: 5, // Horizontal spacing between nodes paddingY: 5, // Vertical spacing between nodes boxBorderPadding: 1, // Padding inside node boxes colorMode: 'auto', // 'none' | 'auto' | 'ansi16' | 'ansi256' | 'truecolor' | 'html' theme: { ... }, // Partial — override default colors })

ASCII XY Charts

XY charts render to ASCII with dedicated chart-drawing characters:

Bar charts — █ blocks (Unicode) or # (ASCII mode)
Line charts — Staircase routing with rounded corners: ╭╮╰╯│─ (Unicode) or +|- (ASCII)
Multi-series — Each series gets a distinct ANSI color from the theme's accent palette
Legends — Automatically shown when multiple series are present
Horizontal charts — Fully supported with categories on the y-axis

API Reference

`renderMermaidSVG(text, options?): string`

Render a Mermaid diagram to SVG. Synchronous. Auto-detects diagram type.

Parameters:

text — Mermaid source code
options — Optional RenderOptions object

RenderOptions:

Option | Type | Default | Description

XY Charts: Diagrams starting with xychart-beta are auto-detected — no separate function needed. The accent color option drives the chart series color palette.

`renderMermaidSVGAsync(text, options?): Promise<string>`

Async version of renderMermaidSVG(). Same output, returns a Promise<string>. Useful in async server handlers or data loaders.

`renderMermaidASCII(text, options?): string`

Render a Mermaid diagram to ASCII/Unicode text. Synchronous.

AsciiRenderOptions:

Option | Type | Default | Description

`parseMermaid(text): MermaidGraph`

Parse Mermaid source into a structured graph object (for custom processing).

`fromShikiTheme(theme): DiagramColors`

Extract diagram colors from a Shiki theme object.

`THEMES: Record<string, DiagramColors>`

Object containing all 15 built-in themes.

`DEFAULTS: { bg: string, fg: string }`

Default colors (#FFFFFF / #27272A).

Attribution

The ASCII rendering engine is based on mermaid-ascii by Alexander Grooff. We ported it from Go to TypeScript and extended it with:

Sequence diagram support
Class diagram support
ER diagram support
Unicode box-drawing characters
Configurable spacing and padding

Thank you Alexander for the excellent foundation!

License

MIT — see LICENSE for details. Built with care by the team at Craft