Reject empty items in frontmatter lists

site/data.schema.json

@@ -81,7 +81,7 @@
     }
   },
   "$defs": {
-    "stringList": { "type": "array", "items": { "type": "string" } },
+    "stringList": { "type": "array", "items": { "type": "string", "minLength": 1 } },
     "entryBase": {
       "type": "object",
       "required": [

site/lib/frontmatter.mjs

@@ -13,7 +13,7 @@ import { renderInline } from "./markdown.mjs";
 import { unquoteScalar } from "./shared.mjs";
 
 /** Splits the inside of a flow list on commas that are not inside quotes. */
-function splitFlowList(inner) {
+function splitFlowList(inner, onEmptyItem) {
   const parts = [];
   let cur = "";
   let quote = null;
@@ -33,7 +33,17 @@ function splitFlowList(inner) {
     } else cur += ch;
   }
   parts.push(cur);
-  return parts.map((p) => unquoteScalar(p.trim()));
+
+  // YAML permits one comma after the final item. Other empty items are invalid.
+  if (parts.length && parts[parts.length - 1].trim() === "") parts.pop();
+  return parts.flatMap((part) => {
+    const value = unquoteScalar(part.trim());
+    if (value === "") {
+      onEmptyItem?.();
+      return [];
+    }
+    return [value];
+  });
 }
 
 /**
@@ -77,7 +87,7 @@ export function unclosedQuote(raw) {
   return q === '"' ? "double" : "single";
 }
 
-export function parseScalar(raw) {
+export function parseScalar(raw, onEmptyListItem) {
   const v = stripComment(raw).trim();
   if (v === "true") return true;
   if (v === "false") return false;
@@ -87,7 +97,7 @@ export function parseScalar(raw) {
   }
   if (v.startsWith("[") && v.endsWith("]")) {
     const inner = v.slice(1, -1).trim();
-    return inner ? splitFlowList(inner) : [];
+    return inner ? splitFlowList(inner, onEmptyListItem) : [];
   }
   return v;
 }
@@ -116,7 +126,7 @@ export function parseFrontmatter(text) {
         const value = parseScalar(item[1]);
         const open = unclosedQuote(item[1]);
         if (open) errors.push(`line ${lineNo}: list item opens a ${open} quote that is never closed`);
-        else if (value === null) errors.push(`line ${lineNo}: list item is empty (a value starting with # is a comment; quote it)`);
+        else if (value === null || value === "") errors.push(`line ${lineNo}: list item is empty (a value starting with # is a comment; quote it)`);
         else if (cutAtHash(item[1], value)) errors.push(`line ${lineNo}: list item is cut at " #" (read as a comment); quote the value if the # is part of it`);
         else meta[listKey].push(value);
       }
@@ -142,7 +152,7 @@ export function parseFrontmatter(text) {
       errors.push(`line ${lineNo}: "${line.trim()}" needs a space after the colon ("${key}: value")`);
       return;
     }
-    const value = parseScalar(rawValue);
+    const value = parseScalar(rawValue, () => errors.push(`line ${lineNo}: "${key}" has an empty item in its flow list`));
     const open = unclosedQuote(rawValue);
     if (open) errors.push(`line ${lineNo}: "${key}" opens a ${open} quote that is never closed`);
     if (cutAtHash(rawValue, value)) {

site/lib/lib.test.mjs

@@ -640,6 +640,14 @@ const f1 = fm([
 ]);
 eq("frontmatter undoes quote escapes", f1.meta.attribution_string, 'Music "{title}" by {artist}');
 eq("frontmatter keeps a quoted comma inside one list item", JSON.stringify(f1.meta.tags), JSON.stringify(["sci-fi, space", "ui", "it's"]));
+const trailingFlow = fm(["tags: [a, b,]"]);
+eq("frontmatter accepts a trailing flow-list comma", JSON.stringify(trailingFlow.meta.tags), JSON.stringify(["a", "b"]));
+eq("frontmatter trailing flow-list comma has no errors", trailingFlow.errors.length, 0);
+const emptyFlow = fm(["tags: [x, , y]"]);
+has("frontmatter rejects an empty flow-list item", emptyFlow.errors.join(), 'line 2: "tags" has an empty item in its flow list');
+eq("frontmatter omits an invalid empty flow-list item", JSON.stringify(emptyFlow.meta.tags), JSON.stringify(["x", "y"]));
+const emptyBlock = fm(["tags:", '  - ""']);
+has("frontmatter rejects an empty quoted block-list item", emptyBlock.errors.join(), "line 3: list item is empty");
 eq("frontmatter reads a block list", JSON.stringify(f1.meta.formats), JSON.stringify(["PNG", "SVG"]));
 eq("frontmatter reads booleans", f1.meta.commercial, true);
 eq("frontmatter has no errors on good input", f1.errors.length, 0);