diff --git a/implementations/javascript/src/fold.ts b/implementations/javascript/src/fold.ts
index de98672..a5b455e 100644
--- a/implementations/javascript/src/fold.ts
+++ b/implementations/javascript/src/fold.ts
@@ -68,9 +68,9 @@ export class IdentityFold<T extends object> extends ValueFold<T, T> {
 }
 
 export class MapFold<T extends object, R extends object> extends ValueFold<T, R> {
-    readonly f: (t: T) => R;
+    readonly f: (t: T) => Value<R>;
 
-    constructor(f: (t: T) => R) {
+    constructor(f: (t: T) => Value<R>) {
         super();
         this.f = f;
     }
@@ -127,7 +127,7 @@ export function fold<T extends object, R>(v: Value<T>, o: FoldMethods<T, R>): R
 
 export function mapPointers<T extends object, R extends object>(
     v: Value<T>,
-    f: (t: T) => R,
+    f: (t: T) => Value<R>,
 ): Value<R>
 {
     return fold(v, new MapFold(f));