diff --git a/mw/Map.d.ts b/mw/Map.d.ts
index adfb0c3..cd1bfd5 100644
--- a/mw/Map.d.ts
+++ b/mw/Map.d.ts
@@ -1,10 +1,68 @@
-type KeyOrArray<T> = keyof T | Array<keyof T>;
-type GetOrDefault<V, K extends keyof V, T> = V extends Required<Pick<V, K>>
-    ? V[K]
-    : Required<V>[K] | T;
-type PickOrDefault<V, S extends keyof V | Array<keyof V>, T> = S extends Array<infer SS>
-    ? { [K in SS & keyof V]-?: GetOrDefault<V, K, T> }
-    : GetOrDefault<V, S & keyof V, null>;
+type TypeOrArray<T> = T | T[];
+
+// Get/PickOrDefault<V, S, TD, TX> extracts values from V using key selection S
+//  - TD is the value type of missing properties
+//  - TX is the value type of unknown properties
+
+type GetOrDefault<V, K extends PropertyKey, TD, TX = unknown> = K extends keyof V
+    ? V extends Required<Pick<V, K>>
+        ? V[K]
+        : Required<V>[K] | TD
+    : TX | TD;
+
+type PickOrDefault<V, S extends TypeOrArray<PropertyKey>, TD, TX = unknown> = S extends Array<
+    infer K
+>
+    ? { [P in K & PropertyKey]-?: GetOrDefault<V, P, TD, TX> }
+    : GetOrDefault<V, S & PropertyKey, TD, TX>;
+
+// `ExtensibleMap<V, TX>` is an alternative to `Map<V & { [k: string]: TX; }>`
+// but unlike the latter, ExtensibleMap provides additional overloads to improve selection
+// autocompletion and type checking.
+
+export interface ExtensibleMap<V extends Record<string, any>, TX = unknown> extends mw.Map<V> {
+    /**
+     * Check if a given key exists in the map.
+     *
+     * @param selection Key to check
+     * @returns True if the key exists
+     * @see https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.Map-method-exists
+     */
+    exists<S extends keyof V>(selection: S): selection is S;
+    exists<S extends string>(selection: S): selection is S;
+
+    /**
+     * Get the value of one or more keys.
+     *
+     * If called with no arguments, all values are returned.
+     *
+     * @param selection Key or array of keys to retrieve values for.
+     * @param fallback Value for keys that don't exist.
+     * @returns If selection was a string, returns the value. If selection was an array, returns
+     * an object of key/values. If no selection is passed, a new object with all key/values is returned.
+     * @see https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.Map-method-get
+     */
+    get<S extends TypeOrArray<keyof V>, TD>(
+        selection: S,
+        fallback: TD
+    ): PickOrDefault<V, S, TD, TX>;
+    get<S extends TypeOrArray<string>, TD>(selection: S, fallback: TD): PickOrDefault<V, S, TD, TX>;
+    get<S extends TypeOrArray<keyof V>>(selection: S): PickOrDefault<V, S, null, TX>;
+    get<S extends TypeOrArray<string>>(selection: S): PickOrDefault<V, S, null, TX>;
+    get<T extends Record<string, any> = V | Record<string, TX>>(): T;
+
+    /**
+     * Set the value of one or more keys.
+     *
+     * @param selection Key to set value for, or object mapping keys to values
+     * @param value Value to set (optional, only in use when key is a string)
+     * @returns True on success, false on failure
+     * @see https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.Map-method-set
+     */
+    set<S extends keyof V>(selection: S, value: V[S]): boolean;
+    set<S extends string>(selection: S, value: TX): boolean;
+    set<S extends Partial<V> & Record<string, TX>>(selection: S): boolean;
+}
 
 declare global {
     namespace mw {
@@ -40,8 +98,11 @@ declare global {
              * an object of key/values. If no selection is passed, a new object with all key/values is returned.
              * @see https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.Map-method-get
              */
-            get<S extends KeyOrArray<V>, T>(selection: S, fallback: T): PickOrDefault<V, S, T>;
-            get<S extends KeyOrArray<V>>(selection: S): PickOrDefault<V, S, null>;
+            get<S extends TypeOrArray<keyof V>, TD>(
+                selection: S,
+                fallback: TD
+            ): PickOrDefault<V, S, TD>;
+            get<S extends TypeOrArray<keyof V>>(selection: S): PickOrDefault<V, S, null>;
             get<T extends V = V>(): T;
 
             /**
diff --git a/mw/config.d.ts b/mw/config.d.ts
index 9cbef8a..ba74e7f 100644
--- a/mw/config.d.ts
+++ b/mw/config.d.ts
@@ -1,3 +1,5 @@
+import { ExtensibleMap } from "./Map";
+
 declare global {
     namespace mw {
         /**
@@ -11,7 +13,7 @@ declare global {
          *
          * @see https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw-property-config
          */
-        const config: Map<{
+        const config: ExtensibleMap<{
             /**
              * Since MediaWiki 1.36+, 0 means debug mode is off, and a positive non-zero number means debug mode is on (e.g. 1 or 2).
              *
@@ -358,7 +360,6 @@ declare global {
              * @see https://www.mediawiki.org/wiki/Manual:Interface/JavaScript#wgDiffNewId
              */
             wgDiffNewId?: number;
-            [key: string]: unknown; // more config keys can be added by extensions
         }>;
     }
 }