diff --git a/test/rebuild.test.ts b/test/rebuild.test.ts
new file mode 100644
index 0000000..ce0be71
--- /dev/null
+++ b/test/rebuild.test.ts
@@ -0,0 +1,29 @@
+import { expectType } from 'tsd';
+
+import { rebuild } from '../es';
+
+const oldObj = { foo: '123-456', bar: '678' };
+
+const newObj = rebuild(([k, v]) => {
+  return v.split('-').map((n, i) => [`${k}${i}`, n]);
+}, oldObj);
+
+expectType<Record<string, string>>(newObj);
+
+const newObj2 = rebuild(([k, v]) => {
+  return [[k, v.split('-')]];
+}, oldObj);
+
+expectType<Record<string, string[]>>(newObj2);
+
+const newObj3 = rebuild(([k, v]) => {
+  const innerObj = Object.fromEntries(v.split('-').map((n, i) => [i, n]));
+  return [[k, innerObj]];
+}, oldObj);
+
+expectType<Record<string, Record<string, string>>>(newObj3);
+
+const diffValueTypes = { foo: 123, bar: 'blah' };
+
+const updated = rebuild(([k, v]) =>[[k, v]], diffValueTypes);
+expectType<Record<string, string | number>>(updated);
diff --git a/types/mapKeys.d.ts b/types/mapKeys.d.ts
new file mode 100644
index 0000000..f89dcb7
--- /dev/null
+++ b/types/mapKeys.d.ts
@@ -0,0 +1,8 @@
+import { Placeholder } from './util/tools';
+
+// mapKeys(fn)(obj)
+export function mapKeys(fn: (key: string) => string): <T>(obj: Record<string, T>) => Record<string, T>;
+// mapKeys(__, obj)(fn)
+export function mapKeys<T>(__: Placeholder, obj: Record<string, T>): (fn: (key: string) => string) => Record<string, T>;
+// mapKeys(fn, obj)
+export function mapKeys<T>(fn: (key: string) => string, obj: Record<string, T>): Record<string, T>;
diff --git a/types/rebuild.d.ts b/types/rebuild.d.ts
new file mode 100644
index 0000000..d8c92e3
--- /dev/null
+++ b/types/rebuild.d.ts
@@ -0,0 +1,8 @@
+import { Placeholder } from './util/tools';
+
+// rebuild(fn)(obj)
+export function rebuild<T, V>(fn: (kvp: [keyof T, T[keyof T]]) => [string, V][]): (obj: T) => Record<string, V>;
+// rebuild(__, obj)(fn)
+export function rebuild<T>(__: Placeholder, obj: T): <V>(fn: (kvp: [keyof T, T[keyof T]]) => [string, V][]) => Record<string, V>;
+// rebuild(fn, obj)
+export function rebuild<T, V>(fn: (kvp: [keyof T, T[keyof T]]) => [string, V][], obj: T): Record<string, V>;