grok-mermaid

Render Mermaid diagrams as Unicode box-drawing art, for terminals.

A TypeScript port of the terminal Mermaid renderer in xai-org/grok-build (crates/codegen/xai-grok-markdown/src/mermaid.rs). No browser, no headless Chrome, no SVG — a self-contained layout engine that emits text.

      ┌──────────────┐
      │ Parse source │
      └───────┬──────┘
              │
              ▼
       ╭────────────╮
       │ Supported? │
       ╰──────┬─────╯
      ┌───────┴────────┐
      ▼yes             ▼no
 ┌─────────┐   ┌───────────────┐
 │ Lay out │   │ Framed source │
 └────┬────┘   └───────┬───────┘
      └───────┬────────┘
              ▼
       ┌─────────────┐
       │ Unicode art │
       └─────────────┘

Install

npm install grok-mermaid

Usage

import { render } from 'grok-mermaid'

const art = render('flowchart LR\n  A[Start] --> B[Done]')
if (art) console.log(art.plain.join('\n'))

render draws the diagram at whatever size it needs and reports that as art.width. It returns null when there is no art to show: blank input, a syntax error, a diagram type it does not draw, or one large enough that laying it out is refused.

Syntax errors

Rendering is best-effort: a source that does not fully parse still draws what it can, and reports the rest in art.warnings.

// Flowcharts are lenient, as mermaid.js is: the parseable prefix survives.
render('graph TD\n A[Start --> B')
// plain     one box labelled `Start --> B` — the edge you wrote is gone
// warnings  ['node "A": label is missing its closing `]`']

// The rest fail on any unreadable statement, but retry once without the last
// line — the one a half-finished source ends on.
render('stateDiagram-v2\n A --> B\n some garbage line')
// plain     the A --> B transition, drawn
// warnings  ['dropped, unreadable final line: "some garbage line"']

An empty warnings means the whole source made it into the art.

Warnings are advisory — never gate rendering on them. The art is the best available drawing either way, and a diagram mid-edit warns at nearly every intermediate state: a label bracket is unterminated right up until it is typed.

diagramKind(src) reads the header alone, separating the two nulls worth different messages:

if (render(src) === null) {
  const kind = diagramKind(src) // 'flowchart' | 'state' | 'class' | 'er' | 'sequence' | null
  console.log(kind ? `${kind} diagram: syntax error` : 'diagram type not supported here')
}

Streaming

Call render on each prefix as it arrives — no special handling, no waiting for a complete diagram. Best-effort parsing is what keeps it drawn instead of alternating with the source box.

Fitting a viewport

The renderer takes no width limit. Nothing about a terminal tells it whether a wide diagram should be shrunk, scrolled, linked to an image or just printed, so the decision stays with you — compare art.width against the space you have:

import { render, sourceBox } from 'grok-mermaid'

const cols = process.stdout.columns
const art = render(src)
if (art && art.width <= cols) console.log(art.plain.join('\n'))
else {
  console.log(sourceBox(src, cols).plain.join('\n'))
  console.log(`(diagram needs ${art?.width ?? '?'} columns)`)
}

sourceBox(src, maxWidth?) frames the source in a titled box, hard-wrapping to maxWidth. It is the usual thing to show when the art does not fit or does not exist, but it is yours to choose and yours to caption.

Colour

The core is colour-blind. styled carries the same rows as plain, split into runs tagged with a semantic class, so you map classes to your own theme:

That image is real render() output painted through one such theme.

import { type Cls, render } from 'grok-mermaid'

const art = render(src)!

const theme: Partial<Record<Cls, (s: string) => string>> = {
  border: dim, text: white, edge: cyan, edgeLabel: gray,
}
const out = art.styled.map((row) =>
  row.map((span) => (theme[span.cls] ?? identity)(span.text)).join(''),
)

| Class | What it covers | | --- | --- | | border | box outlines, subgraph frames, compartment rules | | text | node, participant and compartment labels | | edge | connector lines and arrowheads | | edgeLabel | text sitting on an edge | | title | the mermaid: <kind> header of a source box | | none | blank filler |

styled[i] joined is always exactly plain[i], so you can swap between them freely. A render is plain JSON: cacheable across theme changes, transferable to a worker.

For the common case there is a helper:

import { render, toAnsi } from 'grok-mermaid'

console.log(toAnsi(render(src)!).join('\n'))

toAnsi(art, theme) takes Partial<Record<Cls, string>> of SGR parameters ('2' dim, '36' cyan, '38;5;244' for 256-colour), defaulting to a dim frame with cyan connectors.

Supported diagrams

| Type | Notes | | --- | --- | | graph / flowchart | TD/TB, BT, LR, RL; subgraph nesting; node shapes; solid/dotted/thick links; arrow, circle, cross heads; edge labels | | stateDiagram / stateDiagram-v2 | states, transitions, [*] start/end, <<choice>>, descriptions, composite states flattened | | classDiagram | compartments, annotations, generics, cardinalities, inheritance/realization/composition/aggregation/dependency | | erDiagram | entities, attributes, crow's-foot cardinalities | | sequenceDiagram | participants, messages, self-messages, notes, loop/alt/opt dividers, autonumber |

Credits

Inspired by Simon Willison's grok-mermaid.html (source), which compiles the original Rust renderer to WebAssembly so it runs in a browser. That demo is what made the renderer worth having outside the Grok CLI; this port takes the other route and reimplements it in TypeScript, so it needs no WASM and runs anywhere JS does.

100% of the code written by Opus 5, with a healthy dose of feedback and direction from my side.

License

Apache-2.0. See LICENSE.

The Rust original in xai-org/grok-build (crates/codegen/xai-grok-markdown/src/mermaid.rs) is Apache-2.0, Copyright 2023-2026 SpaceXAI. Its layout algorithms, glyph tables, parser behaviour and test corpus are what this port is derived from.