fix: include undefined values if formatting with zero option set to true

src/formatDuration/index.ts

@@ -105,7 +105,7 @@ export function formatDuration(
       const token = `x${unit.replace(/(^.)/, (m) =>
         m.toUpperCase(),
       )}` as FormatDistanceToken;
-      const value = duration[unit];
+      const value = duration[unit] ?? (zero ? 0 : undefined);
       if (value !== undefined && (zero || duration[unit])) {
         return acc.concat(locale.formatDistance(token, value));
       }

src/formatDuration/test.ts

@@ -77,4 +77,38 @@ describe("formatDuration", () => {
         "9 months, 2 days",
     );
   });
+
+  it("allows to provide a custom locale formatter", () => {
+    assert(
+      formatDuration(
+        {
+          hours: 1,
+          minutes: 10,
+          seconds: 30,
+        },
+        {
+          delimiter: ":",
+          format: ["hours", "minutes", "seconds"],
+          locale: {
+            formatDistance: (_, count) => String(count).padStart(2, "0"),
+          },
+          zero: true,
+        },
+      ) === "01:10:30",
+    );
+  });
+
+  it("includes undefined duration values when formatting if zero is true", () => {
+    assert(
+      formatDuration(
+        {
+          seconds: 10,
+        },
+        {
+          format: ["hours", "minutes", "seconds"],
+          zero: true,
+        },
+      ) === "0 hours 0 minutes 10 seconds",
+    );
+  });
 });