chore: support index -1

packages/ssz/src/viewDU/listBasic.ts

@@ -38,12 +38,26 @@ export class ListBasicTreeViewDU<ElementType extends BasicType<unknown>> extends
     this.set(index, value);
   }
 
+  /**
+   * Returns a new ListBasicTreeViewDU instance with the values from 0 to `index`.
+   * To achieve it, rebinds the underlying tree zero-ing all nodes right of `chunkIindex`.
+   * Also set all value right of `index` in the same chunk to 0.
+   *
+   * Note: Using index = -1, returns an empty list of length 0.
+   */
   sliceTo(index: number): this {
+    if (index < -1) {
+      throw new Error("Does not support sliceTo() with index less than -1");
+    }
+
+    if (index === -1) {
+      return this.type.defaultViewDU() as this;
+    }
+
     // Commit before getting rootNode to ensure all pending data is in the rootNode
     this.commit();
 
     // All nodes beyond length are already zero
-    // Array of length 2: [X,X,0,0], for index >= 1 no action needed
     if (index >= this._length - 1) {
       return this;
     }

packages/ssz/test/unit/byType/listBasic/tree.test.ts

@@ -219,13 +219,17 @@ describe("ListBasicType.sliceTo", () => {
     const listRoots: string[] = [];
     const listSerialized: string[] = [];
 
-    for (let i = 0; i < 16; i++) {
-      listView.push(i);
+    for (let i = -1; i < 16; i++) {
+      if (i >= 0) {
+        listView.push(i);
+      }
+      // Javascript arrays can handle negative indexes (ok for tests)
       listSerialized[i] = toHexString(listView.serialize());
       listRoots[i] = toHexString(listView.hashTreeRoot());
     }
 
-    for (let i = 0; i < 16; i++) {
+    // Start at -1 to test the empty case.
+    for (let i = -1; i < 16; i++) {
       const listSlice = listView.sliceTo(i);
       expect(listSlice.length).to.equal(i + 1, `Wrong length at .sliceTo(${i})`);
       expect(toHexString(listSlice.serialize())).equals(listSerialized[i], `Wrong serialize at .sliceTo(${i})`);