init: add source code from src.zip

src/utils/objectGroupBy.ts

@@ -0,0 +1,18 @@
+/**
+ * https://tc39.es/ecma262/multipage/fundamental-objects.html#sec-object.groupby
+ */
+export function objectGroupBy<T, K extends PropertyKey>(
+  items: Iterable<T>,
+  keySelector: (item: T, index: number) => K,
+): Partial<Record<K, T[]>> {
+  const result = Object.create(null) as Partial<Record<K, T[]>>
+  let index = 0
+  for (const item of items) {
+    const key = keySelector(item, index++)
+    if (result[key] === undefined) {
+      result[key] = []
+    }
+    result[key].push(item)
+  }
+  return result
+}