Picks up on the final two issues during testing.

types/collectionsjs/collectionsjs-tests.ts

@@ -50,7 +50,7 @@ collection.slice(1, 3); // $ExpectType Collection<{ name: string; age: number; }
 collection.sort(); // $ExpectType Collection<{ name: string; age: number; }>
 collection.sortBy('name'); // $ExpectType Collection<{ name: string; age: number; }>
 collection.stringify(); // $ExpectType string
-collection.sum('age'); // $ExpectType any
+collection.sum('age'); // $ExpectType number
 collection.take(2); // $ExpectType Collection<{ name: string; age: number; }>
 Collection.macro('addToMembers', (collection, n) => collection.map((collectionItem: any) => collectionItem + n));
 

types/collectionsjs/index.d.ts

@@ -24,7 +24,7 @@ export default class Collection<T> {
     join(separator?: string): string;
     keys(): Collection<T>;
     last(callback?: ((item: T) => boolean)|null): T;
-    map<R>(callback: (item: T) => R): Collection<T>;
+    map<R>(callback: (item: T) => R): Collection<R>;
     pluck(property: string): Collection<T>;
     push(item: T): Collection<T>;
     reduce<R>(callback: (previous: R, current: T) => R, initial: R): R;