locks: Fix JSON serialising lock attributes

Signed-off-by: Lewis Marshall <lewis.marshall@ably.com>

examples/locks.ts

@@ -2,7 +2,7 @@
 // application.
 import Ably from 'ably/promises';
 import Spaces from '../dist/cjs/Spaces.js';
-import { Lock } from '../dist/cjs/Locks.js';
+import { Lock, LockAttributes } from '../dist/cjs/Locks.js';
 
 // SlideElement represents an element on a slide which a member can both be
 // located at and attempt to lock (e.g. an editable text box).
@@ -21,8 +21,8 @@ class SlideElement {
   }
 
   // the attributes to use when locking this SlideElement.
-  lockAttributes(): Map<string, string> {
-    const attributes = new Map();
+  lockAttributes(): LockAttributes {
+    const attributes = new LockAttributes();
     attributes.set('slideId', this.slideId);
     attributes.set('elementId', this.elementId);
     return attributes;

src/Locks.test.ts

@@ -3,7 +3,7 @@ import { Realtime, Types } from 'ably/promises';
 
 import Space from './Space.js';
 import type { SpaceMember } from './types.js';
-import { LockStatus } from './Locks.js';
+import { LockAttributes, LockStatus } from './Locks.js';
 import { createPresenceMessage } from './utilities/test/fakes.js';
 
 interface SpaceTestContext {
@@ -70,7 +70,7 @@ describe('Locks (mockClient)', () => {
       const presenceUpdate = vi.spyOn(presence, 'update');
 
       const lockID = 'test';
-      const attributes = new Map();
+      const attributes = new LockAttributes();
       attributes.set('key1', 'foo');
       attributes.set('key2', 'bar');
       const req = await space.locks.acquire(lockID, { attributes });

src/Locks.ts

@@ -22,12 +22,18 @@ export type LockRequest = {
   id: string;
   status: LockStatus;
   timestamp: number;
-  attributes?: Map<string, string>;
+  attributes?: LockAttributes;
   reason?: Types.ErrorInfo;
 };
 
+export class LockAttributes extends Map<string, string> {
+  toJSON() {
+    return Object.fromEntries(this);
+  }
+}
+
 interface LockOptions {
-  attributes: Map<string, string>;
+  attributes: LockAttributes;
 }
 
 type LockEventMap = {