Support for working with markdown front matter

packages/core/src/frontmatter.test.ts

@@ -1,6 +1,6 @@
 import { describe, test } from "node:test"
 import assert from "node:assert/strict"
-import { frontmatterTryParse } from "./frontmatter"
+import { frontmatterTryParse, splitMarkdown, updateFrontmatter } from "./frontmatter"
 
 describe("replace frontmatter", () => {
     test("only", () => {
@@ -21,3 +21,55 @@ foo bar
         assert.deepEqual(res, { foo: "bar" })
     })
 })
+
+describe("splitMarkdown", () => {
+    test("split markdown with yaml frontmatter", () => {
+        const markdown = `---
+title: Test
+---
+This is a test.`
+        const { frontmatter, content } = splitMarkdown(markdown)
+        assert.deepEqual(frontmatter, { title: "Test" })
+        assert.equal(content, "This is a test.")
+    })
+
+    test("split markdown with json frontmatter", () => {
+        const markdown = `---
+{
+  "title": "Test"
+}
+---
+This is a test.`
+        const { frontmatter, content } = splitMarkdown(markdown, { format: "json" })
+        assert.deepEqual(frontmatter, { title: "Test" })
+        assert.equal(content, "This is a test.")
+    })
+})
+
+describe("updateFrontmatter", () => {
+    test("update yaml frontmatter", () => {
+        const markdown = `---
+title: Old Title
+---
+This is a test.`
+        const newFrontmatter = { title: "New Title" }
+        const updatedMarkdown = updateFrontmatter(markdown, newFrontmatter)
+        const { frontmatter, content } = splitMarkdown(updatedMarkdown)
+        assert.deepEqual(frontmatter, { title: "New Title" })
+        assert.equal(content, "This is a test.")
+    })
+
+    test("update json frontmatter", () => {
+        const markdown = `---
+{
+  "title": "Old Title"
+}
+---
+This is a test.`
+        const newFrontmatter = { title: "New Title" }
+        const updatedMarkdown = updateFrontmatter(markdown, newFrontmatter, { format: "json" })
+        const { frontmatter, content } = splitMarkdown(updatedMarkdown, { format: "json" })
+        assert.deepEqual(frontmatter, { title: "New Title" })
+        assert.equal(content, "This is a test.")
+    })
+})

packages/core/src/frontmatter.ts

@@ -1,6 +1,6 @@
 import { JSON5TryParse } from "./json5"
 import { TOMLTryParse } from "./toml"
-import { YAMLTryParse } from "./yaml"
+import { YAMLTryParse, YAMLStringify } from "./yaml"
 
 export function frontmatterTryParse(
     text: string,
@@ -34,3 +34,33 @@
     }
     return res !== undefined ? { end: end + 1, value: res } : undefined
 }
+
+export function splitMarkdown(
+    text: string,
+    options?: { format: "yaml" | "json" | "toml" }
+): { frontmatter: any; content: string } {
+    const { value: frontmatter, end } = frontmatterTryParse(text, options) || {}
+    const content = end ? text.split(/\r?\n/g).slice(end).join("\n") : text
+    return { frontmatter, content }
+}
+
+export function updateFrontmatter(
+    text: string,
+    newFrontmatter: any,
+    options?: { format: "yaml" | "json" }
+): string {
+    const { format = "yaml" } = options || {}
+    const { content } = splitMarkdown(text, options)
+    let fm: string
+    switch (format) {
+        case "json":
+            fm = JSON.stringify(newFrontmatter, null, 2)
+            break
+        case "yaml":
+            fm = YAMLStringify(newFrontmatter)
+            break
+        default:
+            throw new Error(`Unsupported format: ${format}`)
+    }
+    return `---\n${fm}\n---\n${content}`
+}

packages/core/src/markdown.ts

@@ -2,6 +2,7 @@
 import { randomHex } from "./crypto"
 import { extractFenced } from "./fence"
 import { trimNewlines } from "./util"
+import { splitMarkdown, updateFrontmatter } from "./frontmatter"
 
 export function prettifyMarkdown(md: string) {
     let res = md
@@ -138,3 +139,13 @@
         return details(node.label, node.content.map(renderTraceTree).join("\n"))
     else return ""
 }
+
+export function mergeFrontmatter(
+    text: string,
+    newFrontmatter: any,
+    options?: { format: "yaml" | "json" }
+): string {
+    const { frontmatter, content } = splitMarkdown(text, options)
+    const updatedFrontmatter = { ...frontmatter, ...newFrontmatter }
+    return updateFrontmatter(text, updatedFrontmatter, options)
+}

packages/core/src/toml.ts

@@ -8,3 +8,4 @@ export function TOMLTryParse(text: string, options?: { defaultValue?: any }) {
         return options?.defaultValue
     }
 }
+