diff --git a/types/relay-runtime/index.d.ts b/types/relay-runtime/index.d.ts
index 42cd633b57..dbafde4fe1 100644
--- a/types/relay-runtime/index.d.ts
+++ b/types/relay-runtime/index.d.ts
@@ -60,6 +60,11 @@ export interface OperationDefaults {
     response: Variables;
 }
 
+// ~~~~~~~~~~~~~~~~~~~~~
+// Constants
+// ~~~~~~~~~~~~~~~~~~~~~
+export const ROOT_ID: string;
+
 // ~~~~~~~~~~~~~~~~~~~~~
 // RelayQL
 // ~~~~~~~~~~~~~~~~~~~~~
@@ -189,7 +194,7 @@ export interface RecordProxy {
 export interface RecordSourceProxy {
     create(dataID: DataID, typeName: string): RecordProxy;
     delete(dataID: DataID): void;
-    get(dataID: DataID): Array<RecordProxy | null> | null;
+    get(dataID: DataID): RecordProxy | null;
     getRoot(): RecordProxy;
 }
 
@@ -1010,7 +1015,7 @@ export type Observable<T> = RelayObservable<T>;
 // commitLocalUpdate
 // ~~~~~~~~~~~~~~~~~~~~~
 // exposed through RelayModern, not Runtime directly
-export type commitLocalUpdate = (environment: Environment, updater: StoreUpdater) => void;
+export function commitLocalUpdate(environment: Environment, updater: StoreUpdater): void;
 
 // ~~~~~~~~~~~~~~~~~~~~~
 // commitRelayModernMutation
diff --git a/types/relay-runtime/relay-runtime-tests.tsx b/types/relay-runtime/relay-runtime-tests.tsx
index 4cbbacf680..72f262cbfa 100644
--- a/types/relay-runtime/relay-runtime-tests.tsx
+++ b/types/relay-runtime/relay-runtime-tests.tsx
@@ -6,6 +6,8 @@ import {
     ConnectionHandler,
     ViewerHandler,
     RecordSourceInspector,
+    commitLocalUpdate,
+    ROOT_ID,
 } from "relay-runtime";
 
 const source = new RecordSource();
@@ -60,3 +62,12 @@ function handlerProvider(handle: any) {
 // ~~~~~~~~~~~~~~~~~~~~~
 
 const inspector = new RecordSourceInspector(source);
+
+// ~~~~~~~~~~~~~~~~~~~~~
+// commitLocalUpdate
+// ~~~~~~~~~~~~~~~~~~~~~
+
+commitLocalUpdate(environment, store => {
+  const root = store.get(ROOT_ID)!;
+  root.setValue("foo", "localKey");
+});