remove lastIndex from return, update tests

lastIndex is actually _exclusive_, which would require math, and I don't
feel like it's likely to be as useful as firstIndex

Author: lygaret

packages/virtual/src/index.tsx

@@ -16,7 +16,6 @@ type VirtualListReturn<T extends readonly any[]> = [
     viewerTop: number;
     visibleItems: T;
     firstIndex: number;
-    lastIndex: number;
   }>,
   onScroll: (e: Event) => void,
 ];
@@ -51,13 +50,12 @@ export function createVirtualList<T extends readonly any[]>({
         Math.floor(offset() / rowHeight) + Math.ceil(rootHeight / rowHeight) + overscanCount,
       );
 
-      return ({
+      return {
         containerHeight: items.length * rowHeight,
         viewerTop: firstIndex * rowHeight,
         visibleItems: items.slice(firstIndex, lastIndex) as unknown as T,
         firstIndex,
-        lastIndex
-      })
+      };
     },
     e => {
       // @ts-expect-error

packages/virtual/test/index.test.tsx

@@ -61,17 +61,6 @@ describe("createVirtualList", () => {
     expect(virtual().firstIndex).toEqual(0);
   });
 
-  test("returns lastIndex representing the index of the visibleList", () => {
-    const [virtual] = createVirtualList({
-      items: TEST_LIST,
-      rootHeight: 20,
-      rowHeight: 10,
-    });
-
-    expect(virtual().lastIndex).toEqual(2);
-  });
-
-
   test("returns onScroll which sets viewerTop and visibleItems based on rootElement's scrolltop", () => {
     const el = document.createElement("div");
 
@@ -84,46 +73,47 @@ describe("createVirtualList", () => {
     expect(virtual().visibleItems).toEqual([0, 1, 2]);
     expect(virtual().viewerTop).toEqual(0);
     expect(virtual().firstIndex).toEqual(0);
-    expect(virtual().lastIndex).toEqual(2);
 
     el.scrollTop += 10;
 
     // no change until onScroll is called
     expect(virtual().visibleItems).toEqual([0, 1, 2]);
     expect(virtual().viewerTop).toEqual(0);
     expect(virtual().firstIndex).toEqual(0);
-    expect(virtual().lastIndex).toEqual(2);
 
     onScroll(TARGETED_SCROLL_EVENT(el));
 
     expect(virtual().visibleItems).toEqual([0, 1, 2, 3]);
     expect(virtual().viewerTop).toEqual(0);
     expect(virtual().firstIndex).toEqual(0);
-    expect(virtual().lastIndex).toEqual(3);
 
     el.scrollTop += 10;
     onScroll(TARGETED_SCROLL_EVENT(el));
 
     expect(virtual().visibleItems).toEqual([1, 2, 3, 4]);
     expect(virtual().viewerTop).toEqual(10);
     expect(virtual().firstIndex).toEqual(1);
-    expect(virtual().lastIndex).toEqual(4);
 
     el.scrollTop -= 10;
     onScroll(TARGETED_SCROLL_EVENT(el));
 
     expect(virtual().visibleItems).toEqual([0, 1, 2, 3]);
     expect(virtual().viewerTop).toEqual(0);
-    expect(virtual().firstIndex).toEqual(1);
-    expect(virtual().lastIndex).toEqual(3);
+    expect(virtual().firstIndex).toEqual(0);
 
     el.scrollTop -= 10;
     onScroll(TARGETED_SCROLL_EVENT(el));
 
     expect(virtual().visibleItems).toEqual([0, 1, 2]);
     expect(virtual().viewerTop).toEqual(0);
-    expect(virtual().firstIndex).toEqual(1);
-    expect(virtual().lastIndex).toEqual(2);
+    expect(virtual().firstIndex).toEqual(0);
+
+    el.scrollTop += 7_000;
+    onScroll(TARGETED_SCROLL_EVENT(el));
+
+    expect(virtual().visibleItems).toEqual([699, 700, 701, 702]);
+    expect(virtual().viewerTop).toEqual(6990);
+    expect(virtual().firstIndex).toEqual(699);
   });
 
   test("onScroll handles reaching the bottom of the list", () => {
@@ -160,7 +150,6 @@ describe("createVirtualList", () => {
 
     expect(virtual().visibleItems).toEqual([8, 9, 10, 11, 12, 13]);
     expect(virtual().firstIndex).toEqual(8);
-    expect(virtual().lastIndex).toEqual(13);
   });
 
   test("overscanCount defaults to 1 if undefined or zero", () => {