Created a plugin for frontmatter

src/config/default.docunotion.config.ts

@@ -15,6 +15,7 @@ import { standardNumberedListTransformer } from "../plugins/NumberedListTransfor
 import { standardTableTransformer } from "../plugins/TableTransformer";
 import { standardExternalLinkConversion } from "../plugins/externalLinks";
 import { IDocuNotionConfig } from "./configuration";
+import { standardFrontmatterTransformer } from "../plugins/FronmatterTransformer";
 
 const defaultConfig: IDocuNotionConfig = {
   plugins: [
@@ -35,6 +36,9 @@ const defaultConfig: IDocuNotionConfig = {
     standardInternalLinkConversion,
     standardExternalLinkConversion,
 
+    // Frontmatter transformers, add information to the page frontmatter
+    standardFrontmatterTransformer,
+
     // Regexps plus javascript `import`s that operate on the Markdown output
     imgurGifEmbed,
     gifEmbed,

src/plugins/FronmatterTransformer.spec.ts

@@ -0,0 +1,153 @@
+import { GetPageResponse } from "@notionhq/client/build/src/api-endpoints";
+import { NotionPage } from "../NotionPage";
+import { standardFrontmatterTransformer } from "./FronmatterTransformer";
+
+let getFrontMatter = standardFrontmatterTransformer.frontmatterTransform
+  ?.build as (page: NotionPage) => string;
+
+const sampleMetadata: GetPageResponse = {
+  object: "page",
+  id: "6e6921b9-b1f5-4614-ab3c-bf1a73358a1f",
+  created_time: "2023-04-11T10:17:00.000Z",
+  last_edited_time: "2023-04-13T20:24:00.000Z",
+  created_by: {
+    object: "user",
+    id: "USERID",
+  },
+  last_edited_by: {
+    object: "user",
+    id: "USERID",
+  },
+  cover: null,
+  icon: {
+    type: "file",
+    file: {
+      url: "https:/dummy_URL",
+      expiry_time: "2023-04-15T11:50:20.461Z",
+    },
+  },
+  parent: {
+    type: "workspace",
+    workspace: true,
+  },
+  archived: false,
+  properties: {
+    title: {
+      id: "title",
+      type: "title",
+      title: [
+        {
+          type: "text",
+          text: {
+            content: "Foo",
+            link: null,
+          },
+          annotations: {
+            bold: false,
+            italic: false,
+            strikethrough: false,
+            underline: false,
+            code: false,
+            color: "default",
+          },
+          plain_text: "Foo",
+          href: null,
+        },
+        {
+          type: "text",
+          text: {
+            content: "Bar",
+            link: null,
+          },
+          annotations: {
+            bold: false,
+            italic: false,
+            strikethrough: false,
+            underline: false,
+            code: false,
+            color: "default",
+          },
+          plain_text: "Bar",
+          href: null,
+        },
+      ],
+    },
+    Keywords: {
+      id: "keywords",
+      type: "rich_text",
+      rich_text: [
+        {
+          type: "text",
+          text: {
+            content: "Foo, Bar",
+            link: null,
+          },
+          annotations: {
+            bold: false,
+            italic: false,
+            strikethrough: false,
+            underline: false,
+            code: false,
+            color: "default",
+          },
+          plain_text: "Foo, Bar",
+          href: null,
+        },
+      ],
+    },
+    date_property: {
+      id: "a%3Cql",
+      type: "date",
+      date: {
+        start: "2021-10-24",
+        end: "2021-10-28",
+        time_zone: null,
+      },
+    },
+  },
+  url: "https://www.notion.so/Site-docu-notion-PAGEID",
+};
+
+describe("getFrontMatter", () => {
+  let page: NotionPage;
+
+  beforeEach(() => {
+    page = new NotionPage({
+      layoutContext: "Test Context",
+      pageId: "123",
+      order: 1,
+      metadata: JSON.parse(JSON.stringify(sampleMetadata)),
+      foundDirectlyInOutline: true,
+    });
+  });
+
+  it("should generate frontmatter with all available properties", () => {
+    const expectedFrontmatter = `title: FooBar\nsidebar_position: 1\nslug: /123\nkeywords: [Foo, Bar]\n`;
+    (page.metadata as any).properties.Keywords.rich_text[0].plain_text =
+      "Foo, Bar";
+
+    const result = getFrontMatter(page);
+
+    expect(result).toEqual(expectedFrontmatter);
+  });
+
+  // "title: Foo-Barsidebar_position: 1slug: keywords: [Foo, Bar]"
+  // "title: FooBar\nsidebar_position: 1\nslug: /123\n"
+  it("should generate frontmatter with no keywords", () => {
+    const expectedFrontmatter = `title: FooBar\nsidebar_position: 1\nslug: /123\n`;
+    (page.metadata as any).properties.Keywords = undefined;
+
+    const result = getFrontMatter(page);
+
+    expect(result).toEqual(expectedFrontmatter);
+  });
+
+  it("should replace colons with dashes in the title", () => {
+    const expectedFrontmatter = `title: FooBaz-\nsidebar_position: 1\nslug: /123\nkeywords: [Foo, Bar]\n`;
+    (page.metadata as any).properties.title.title[1].plain_text = "Baz:";
+
+    const result = getFrontMatter(page);
+
+    expect(result).toEqual(expectedFrontmatter);
+  });
+});

src/plugins/FronmatterTransformer.ts

@@ -0,0 +1,20 @@
+import { IPlugin } from "./pluginTypes";
+import { NotionPage } from "../NotionPage";
+
+function getFrontmatter(page: NotionPage): string {
+  let frontmatter = "";
+  frontmatter += `title: ${page.nameOrTitle.replaceAll(":", "-")}\n`; // I have not found a way to escape colons
+  frontmatter += `sidebar_position: ${page.order}\n`;
+  frontmatter += `slug: ${page.slug ?? ""}\n`;
+  if (page.keywords) frontmatter += `keywords: [${page.keywords}]\n`;
+
+  return frontmatter;
+}
+
+export const standardFrontmatterTransformer: IPlugin = {
+  name: "standardFrontmatterTransformer",
+
+  frontmatterTransform: {
+    build: getFrontmatter,
+  },
+};

src/plugins/pluginTypes.ts

@@ -35,6 +35,11 @@ export type IPlugin = {
   // simple regex replacements on the markdown output
   regexMarkdownModifications?: IRegexMarkdownModification[];
 
+  // operations on pages to define the markdown's frontmatter
+  frontmatterTransform?: {
+    build: (page: NotionPage) => string;
+  };
+
   // Allow a plugin to perform an async operation at the start of docu-notion.
   // Notice that the plugin itself is given, so you can add things to it.
   init?(plugin: IPlugin): Promise<void>;

src/transform.ts

@@ -30,7 +30,7 @@ export async function getMarkdownForPage(
   logDebugFn("markdown from page", () => JSON.stringify(blocks, null, 2));
 
   const body = await getMarkdownFromNotionBlocks(context, config, blocks);
-  const frontmatter = getFrontMatter(page); // todo should be a plugin
+  const frontmatter = getMarkdownFrontMatter(config, page);
   return `${frontmatter}\n${body}`;
 }
 
@@ -252,14 +252,17 @@ function registerNotionToMarkdownCustomTransforms(
   });
 }
 
-// enhance:make this built-in plugin so that it can be overridden
-function getFrontMatter(page: NotionPage): string {
+function getMarkdownFrontMatter(
+  config: IDocuNotionConfig,
+  page: NotionPage
+): string {
   let frontmatter = "---\n";
-  frontmatter += `title: ${page.nameOrTitle.replaceAll(":", "-")}\n`; // I have not found a way to escape colons
-  frontmatter += `sidebar_position: ${page.order}\n`;
-  frontmatter += `slug: ${page.slug ?? ""}\n`;
-  if (page.keywords) frontmatter += `keywords: [${page.keywords}]\n`;
-
+  config.plugins.forEach(plugin => {
+    if (plugin.frontmatterTransform) {
+      logDebug("transforming page with plugin", plugin.name);
+      frontmatter += plugin.frontmatterTransform?.build(page);
+    }
+  });
   frontmatter += "---\n";
   return frontmatter;
 }