fix: repeat does not use fallback on initial length 0

Author: webstrand

packages/range/src/repeat.ts

@@ -24,7 +24,7 @@ export function repeat<T>(
   options: { fallback?: Accessor<T> } = {},
 ): Accessor<T[]> {
   let prev: readonly T[] = [];
-  let prevLen = 0;
+  let prevLen: number | undefined;
   const disposers: (() => void)[] = [];
   onCleanup(() => {
     for (let index = 0; index < disposers.length; index++) {
@@ -75,7 +75,7 @@ export function repeat<T>(
     const next = prev.slice(0, len);
 
     // Create new elements as needed
-    for (let index = prevLen; index < len; index++) {
+    for (let index = prevLen ?? 0; index < len; index++) {
       next[index] = createRoot(dispose => {
         disposers[index] = dispose;
         return mapFn(index);

packages/range/test/repeat.test.ts

@@ -82,6 +82,17 @@ describe("repeat", () => {
       setLength(3);
       expect(mapped(), "mapped after dispose").toEqual(["fb"]);
     }));
+
+  it("uses fallback when length is initially 0", () =>
+    createRoot(disposer => {
+      const map = repeat(
+        () => 0,
+        i => i,
+        { fallback: () => NaN },
+      );
+      expect(map()).toEqual([NaN]);
+      disposer();
+    }));
 });
 
 describe("<Repeat/>", () => {