validate front matter

src/markdown.ts

@@ -43,6 +43,15 @@ export interface ParseContext {
   path: string;
 }
 
+interface FrontMatter {
+  title?: string;
+  toc?: boolean | {show?: boolean; label?: string};
+  index?: boolean;
+  keywords?: string[];
+  draft?: boolean;
+  sql?: {[key: string]: string};
+}
+
 function uniqueCodeId(context: ParseContext, content: string): string {
   const hash = createHash("sha256").update(content).digest("hex").slice(0, 8);
   let id = hash;
@@ -338,7 +347,7 @@ export function createMarkdownIt({
 
 export function parseMarkdown(input: string, options: ParseOptions): MarkdownPage {
   const {md, path} = options;
-  const {content, data} = matter(input, {});
+  const {content, data} = getContents(input);
   const code: MarkdownCode[] = [];
   const context: ParseContext = {code, startLine: 0, currentLine: 0, path};
   const tokens = md.parse(content, context);
@@ -355,6 +364,56 @@ export function parseMarkdown(input: string, options: ParseOptions): MarkdownPag
   };
 }
 
+function getContents(input: string): {content: string; data: FrontMatter & {[key: string]: any}} {
+  try {
+    const {data, content} = matter(input, {});
+    if ("title" in data) data.title = normalizeString(data.title);
+    if ("toc" in data) data.toc = normalizeToc(data.toc);
+    if ("index" in data) data.index = normalizeBoolean(data.index, "index");
+    if ("keywords" in data) data.keywords = normalizeStringArray(data.keywords);
+    if ("draft" in data) data.draft = normalizeBoolean(data.draft, "draft");
+    if ("sql" in data) data.sql = normalizeSqlData(data.sql);
+    return {data, content};
+  } catch (error) {
+    return {
+      data: {},
+      content: '<div class="observablehq--inspect observablehq--error">Invalid front matter</div>'
+    };
+  }
+}
+
+function normalizeString(value: any): string {
+  return value == null ? "" : String(value);
+}
+
+function normalizeToc(toc: any): FrontMatter["toc"] {
+  if (toc == null || typeof toc === "boolean") return toc;
+  if (typeof toc !== "object" || Array.isArray(toc)) console.warn(`Invalid toc format: ${toc}`);
+  if ("show" in toc) toc.show = normalizeBoolean(toc.show, "toc.show");
+  if ("label" in toc) toc.label = normalizeString(toc.label);
+  return toc;
+}
+
+function normalizeBoolean(value: any, name: string): boolean {
+  if (typeof value !== "boolean")
+    console.warn(`the ${name} option should be boolean, ${value} (${typeof value} found instead)`);
+  return Boolean(value);
+}
+
+function normalizeStringArray(value: any): string[] {
+  return value == null ? [] : typeof value === "string" ? [value] : Array.from(value, String);
+}
+
+function normalizeSqlData(sql: any): {[key: string]: string} {
+  if (!sql || typeof sql !== "object" || Array.isArray(sql)) return console.warn("Unsupported sql definition", sql), {};
+  const tables = new Map<string, string>();
+  for (const [name, source] of Object.entries(sql)) {
+    if (typeof source !== "string") return console.warn("Unsupported sql table source definition", source), {};
+    tables.set(name, source);
+  }
+  return Object.fromEntries(tables);
+}
+
 function getHtml(
   key: "head" | "header" | "footer",
   data: Record<string, any>,