perf(runtime-vapor): preallocate memory

Author: johnsoncodehk

packages/runtime-vapor/src/apiCreateFor.ts

@@ -143,16 +143,18 @@ export const createFor = (
         }
       } else {
         const commonLength = Math.min(oldLength, newLength)
-        const oldKeyToIndexMap = new Map<any, number>()
+        const oldKeyToIndex: [any, number][] = new Array(oldLength)
         const pendingNews: [
           index: number,
           item: ReturnType<typeof getItem>,
           key: any,
-        ][] = []
+        ][] = new Array(newLength)
 
         let defaultAnchor: Node = parentAnchor
         let right = 0
         let left = 0
+        let l1 = 0
+        let l2 = 0
 
         while (right < commonLength) {
           const index = newLength - right - 1
@@ -179,23 +181,27 @@ export const createFor = (
           if (oldKey === key) {
             update((newBlocks[left] = oldBlock), item[0])
           } else {
-            pendingNews.push([left, item, key])
-            oldKeyToIndexMap.set(oldKey, left)
+            pendingNews[l1++] = [left, item, key]
+            oldKeyToIndex[l2++] = [oldKey, left]
           }
           left++
         }
 
         for (let i = left; i < oldLength - right; i++) {
-          oldKeyToIndexMap.set(oldBlocks[i].key, i)
+          oldKeyToIndex[l2++] = [oldBlocks[i].key, i]
         }
 
         const prepareLength = Math.min(newLength - right, commonLength)
         for (let i = left; i < prepareLength; i++) {
           const item = getItem(source, i)
           const key = getKey.apply(null, item)
-          pendingNews.push([i, item, key])
+          pendingNews[l1++] = [i, item, key]
         }
 
+        pendingNews.length = l1
+        oldKeyToIndex.length = l2
+
+        const oldKeyToIndexMap = new Map(oldKeyToIndex)
         const pendingMounts: [
           index: number,
           item: ReturnType<typeof getItem>,