Next.jsでSSG時にRemarkでMermaidをSVGとして出力する

Next.jsでSSG時にRemarkでMermaidをSVGにして出力してみたいと思うこと、ありますよね。
僕はあるのですがネット上にあまり情報が無かったので自分用にメモです。

今回はremark-mermaidjsを使って実装しました。

このサイトではMarkdownをunifiedで扱っているのでremark-mermaidjsをuseします。
remark-mermaidjsはGoogle Chromeを使ってMermaidをSVGにしているらしくexecutablePathでGoogle Chromeのパスを指定しなければならないようです。
このサイトはGitHub Actionsでビルドしているので、ubuntuのために/opt/google/chrome/google-chromeを指定しました。なお、ローカル環境でも自由に動かせるように.envにもGoogle Chromeへのパスを書くことにしました。

// @ts-ignore
import rehypePrism from '@mapbox/rehype-prism';
import rehypeKatex from 'rehype-katex';
import rehypeStringify from 'rehype-stringify';
import remarkGfm from 'remark-gfm';
import remarkMath from 'remark-math';
import remarkMermaid from 'remark-mermaidjs';
import remarkParse from 'remark-parse';
import remarkRehype from 'remark-rehype';
import { unified } from 'unified';

export const markdownToHtml = async (markdown: string) =>
  (
    await unified()
      .use(remarkParse)
      .use(remarkMath)
      .use(remarkGfm)
      .use(remarkMermaid, {
        launchOptions: {
          executablePath:
            process.env.GoogleChromeExecutablePath ?? // .env
            '/opt/google/chrome/google-chrome', // for GitHub Actions Ubuntu
        },
        svgo: false,
      })
      .use(remarkRehype, { allowDangerousHtml: true, footnoteLabel: '脚注' })
      .use(rehypePrism)
      .use(rehypeKatex)
      .use(rehypeStringify, { allowDangerousHtml: true })
      .process(markdown)
  )
    .toString()
    .replace(//g, process.env.baseUrl || '');

以下にサンプルを置いておきます。図がSVGになっていることがわかると思います。

Flowchart

```mermaid
flowchart LR

A[Hard] -->|Text| B(Round)
B --> C{Decision}
C -->|One| D[Result 1]
C -->|Two| E[Result 2]
```

Sequence diagram

```mermaid
sequenceDiagram
Alice->>John: Hello John, how are you?
loop Healthcheck
    John->>John: Fight against hypochondria
end
Note right of John: Rational thoughts!
John-->>Alice: Great!
John->>Bob: How about you?
Bob-->>John: Jolly good!
```

Gantt chart

```mermaid
gantt
    section Section
    Completed :done,    des1, 2014-01-06,2014-01-08
    Active        :active,  des2, 2014-01-07, 3d
    Parallel 1   :         des3, after des1, 1d
    Parallel 2   :         des4, after des1, 1d
    Parallel 3   :         des5, after des3, 1d
    Parallel 4   :         des6, after des4, 1d
```

Class diagram

```mermaid
classDiagram
Class01 <|-- AveryLongClass : Cool
<<Interface>> Class01
Class09 --> C2 : Where am I?
Class09 --* C3
Class09 --|> Class07
Class07 : equals()
Class07 : Object[] elementData
Class01 : size()
Class01 : int chimp
Class01 : int gorilla
class Class10 {
  <<service>>
  int id
  size()
}
```

Git graph

```mermaid
gitGraph
    commit
    commit
    branch develop
    commit
    commit
    commit
    checkout main
    commit
    commit
```

State diagram

```mermaid
stateDiagram-v2
[*] --> Still
Still --> [*]
Still --> Moving
Moving --> Still
Moving --> Crash
Crash --> [*]
```

Pie chart

```mermaid
pie
"Dogs" : 386
"Cats" : 85.9
"Rats" : 15
```

Entity Relationship Diagram

```mermaid
erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ LINE-ITEM : contains
    CUSTOMER }|..|{ DELIVERY-ADDRESS : uses
```

User Journey diagram

```mermaid
journey
  title My working day
  section Go to work
    Make tea: 5: Me
    Go upstairs: 3: Me
    Do work: 1: Me, Cat
  section Go home
    Go downstairs: 5: Me
    Sit down: 3: Me
```

C4 diagram

```mermaid
C4Context
title System Context diagram for Internet Banking System

Person(customerA, "Banking Customer A", "A customer of the bank, with personal bank accounts.")
Person(customerB, "Banking Customer B")
Person_Ext(customerC, "Banking Customer C")
System(SystemAA, "Internet Banking System", "Allows customers to view information about their bank accounts, and make payments.")

Person(customerD, "Banking Customer D", "A customer of the bank, <br/> with personal bank accounts.")

Enterprise_Boundary(b1, "BankBoundary") {

  SystemDb_Ext(SystemE, "Mainframe Banking System", "Stores all of the core banking information about customers, accounts, transactions, etc.")

  System_Boundary(b2, "BankBoundary2") {
    System(SystemA, "Banking System A")
    System(SystemB, "Banking System B", "A system of the bank, with personal bank accounts.")
  }

  System_Ext(SystemC, "E-mail system", "The internal Microsoft Exchange e-mail system.")
  SystemDb(SystemD, "Banking System D Database", "A system of the bank, with personal bank accounts.")

  Boundary(b3, "BankBoundary3", "boundary") {
    SystemQueue(SystemF, "Banking System F Queue", "A system of the bank, with personal bank accounts.")
    SystemQueue_Ext(SystemG, "Banking System G Queue", "A system of the bank, with personal bank accounts.")
  }
}

BiRel(customerA, SystemAA, "Uses")
BiRel(SystemAA, SystemE, "Uses")
Rel(SystemAA, SystemC, "Sends e-mails", "SMTP")
Rel(SystemC, customerA, "Sends e-mails to")
```

Mermaid記法なら簡単にグラフを書けて便利なので、どんどん使っていきたいと思います。