refactor: combine frontmatter fields and block, add tests

src/frontmatter/frontmatterFields.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, it } from "vitest";
 import {
+	extractFrontmatterBlock,
 	findFieldRange,
 	formatTagsYamlValue,
 	replaceFrontmatterField,
@@ -76,3 +77,55 @@ describe("setFrontmatterTags", () => {
 		expect(result.content).not.toContain("tags: [old]");
 	});
 });
+
+describe("extractFrontmatterBlock", () => {
+	it("extracts yaml from a valid block", () => {
+		const block = extractFrontmatterBlock(
+			"---\ntitle: Hello\nstatus: todo\n---\nbody",
+		);
+		expect(block).not.toBeNull();
+		expect(block!.yaml).toBe("title: Hello\nstatus: todo");
+		expect(block!.closeLine).toBe(3);
+	});
+
+	it("returns null when content does not start with ---", () => {
+		expect(extractFrontmatterBlock("no frontmatter")).toBeNull();
+		expect(extractFrontmatterBlock("")).toBeNull();
+	});
+
+	it("handles unclosed frontmatter with trailing newline", () => {
+		const block = extractFrontmatterBlock("---\ntitle: x\n");
+		expect(block).not.toBeNull();
+		expect(block!.closeLine).toBe(-1);
+		expect(block!.yaml).toBe("title: x\n");
+	});
+
+	it("handles unclosed frontmatter without trailing newline", () => {
+		const block = extractFrontmatterBlock("---\ntitle: x");
+		expect(block).not.toBeNull();
+		expect(block!.closeLine).toBe(-1);
+		expect(block!.yaml).toBe("title: x");
+	});
+
+	it("handles CRLF line endings (yaml is normalized to LF)", () => {
+		const block = extractFrontmatterBlock(
+			"---\r\ntitle: Hello\r\nstatus: todo\r\n---\r\nbody",
+		);
+		expect(block).not.toBeNull();
+		expect(block!.yaml).toBe("title: Hello\nstatus: todo");
+	});
+
+	it("handles --- with trailing whitespace", () => {
+		const block = extractFrontmatterBlock(
+			"---  \ntitle: Hello\n---  \nbody",
+		);
+		expect(block).not.toBeNull();
+		expect(block!.yaml).toBe("title: Hello");
+		expect(block!.closeLine).toBe(2);
+	});
+
+	it("only matches --- on the first line", () => {
+		const block = extractFrontmatterBlock("text\n---\ntitle: x\n---\n");
+		expect(block).toBeNull();
+	});
+});