fix(markdown-core): use Object.hasOwn instead of in operator in parseFrontmatterBlock (#99129)

zenglingbiao · vincentkoc · web-flow · commit acdaf8ae31e6 · 2026-07-03T22:39:25.000-07:00
* fix(markdown-core): use Object.hasOwn instead of in operator in parseFrontmatterBlock

* test(markdown-core): add regression test for prototype-named null frontmatter keys

* style(markdown-core): fix unbound-method lint in regression test

---------

Co-authored-by: Vincent Koc &lt;vincentkoc@ieee.org&gt;
diff --git a/packages/markdown-core/src/frontmatter.test.ts b/packages/markdown-core/src/frontmatter.test.ts
@@ -103,6 +103,28 @@ metadata:
     expect(parseFrontmatterBlock(content)).toStrictEqual({});
   });
 
+  it("preserves prototype-named keys when YAML value is null", () => {
+    const content = `---
+title: Hello
+toString: null
+constructor: null
+valueOf: null
+hasOwnProperty: null
+---
+Body text`;
+    const result = parseFrontmatterBlock(content);
+    expect(Object.hasOwn(result, "toString")).toBe(true);
+    expect(result["toString"]).toBe("null");
+    expect(Object.hasOwn(result, "constructor")).toBe(true);
+    expect(result["constructor"]).toBe("null");
+    expect(Object.hasOwn(result, "valueOf")).toBe(true);
+    expect(result["valueOf"]).toBe("null");
+    expect(Object.hasOwn(result, "hasOwnProperty")).toBe(true);
+    expect(result["hasOwnProperty"]).toBe("null");
+    // normal key unaffected
+    expect(result.title).toBe("Hello");
+  });
+
   it("parses frontmatter after a leading UTF-8 BOM", () => {
     const content = "\uFEFF---\nname: windows-skill\ndescription: Written by PowerShell\n---\n";
     const result = parseFrontmatterBlock(content);
diff --git a/packages/markdown-core/src/frontmatter.ts b/packages/markdown-core/src/frontmatter.ts
@@ -222,7 +222,7 @@ export function parseFrontmatterBlock(content: string): ParsedFrontmatter {
   }
 
   for (const [key, lineEntry] of Object.entries(lineParsed)) {
-    if (!(key in merged)) {
+    if (!Object.hasOwn(merged, key)) {
       merged[key] = lineEntry.value;
     }
   }

Original file line number	Diff line number	Diff line change
`@@ -222,7 +222,7 @@ export function parseFrontmatterBlock(content: string): ParsedFrontmatter {`
`222`	`222`	`}`
`223`	`223`
`224`	`224`	`for (const [key, lineEntry] of Object.entries(lineParsed)) {`
`225`		`- if (!(key in merged)) {`
	`225`	`+ if (!Object.hasOwn(merged, key)) {`
`226`	`226`	`merged[key] = lineEntry.value;`
`227`	`227`	`}`
`228`	`228`	`}`