diff --git a/javascript/index.d.ts b/javascript/index.d.ts
index 566b649..be679a4 100644
--- a/javascript/index.d.ts
+++ b/javascript/index.d.ts
@@ -18,28 +18,28 @@ export as namespace Olm;
 
 declare class Account {
     constructor();
-    free();
-    create();
+    free(): void;
+    create(): void;
     identity_keys(): string;
     sign(message: string | Uint8Array): string;
     one_time_keys(): string;
-    mark_keys_as_published();
+    mark_keys_as_published(): void;
     max_number_of_one_time_keys(): number;
-    generate_one_time_keys(number_of_keys: number);
-    remove_one_time_keys(session: Session);
-    generate_fallback_key();
+    generate_one_time_keys(number_of_keys: number): void;
+    remove_one_time_keys(session: Session): void;
+    generate_fallback_key(): void;
     fallback_key(): string;
     unpublished_fallback_key(): string;
     forget_old_fallback_key(): void;
     pickle(key: string | Uint8Array): string;
-    unpickle(key: string | Uint8Array, pickle: string);
+    unpickle(key: string | Uint8Array, pickle: string): void;
 }
 
 declare class Session {
     constructor();
     free(): void;
     pickle(key: string | Uint8Array): string;
-    unpickle(key: string | Uint8Array, pickle: string);
+    unpickle(key: string | Uint8Array, pickle: string): void;
     create_outbound(
         account: Account, their_identity_key: string, their_one_time_key: string,
     ): void;
@@ -67,7 +67,7 @@ declare class InboundGroupSession {
     constructor();
     free(): void;
     pickle(key: string | Uint8Array): string;
-    unpickle(key: string | Uint8Array, pickle: string);
+    unpickle(key: string | Uint8Array, pickle: string): void;
     create(session_key: string): string;
     import_session(session_key: string): string;
     decrypt(message: string): object;
@@ -80,7 +80,7 @@ declare class OutboundGroupSession {
     constructor();
     free(): void;
     pickle(key: string | Uint8Array): string;
-    unpickle(key: string | Uint8Array, pickle: string);
+    unpickle(key: string | Uint8Array, pickle: string): void;
     create(): void;
     encrypt(plaintext: string): string;
     session_id(): string;