diff --git a/mw/Map.d.ts b/mw/Map.d.ts
index 14e64c9..707dcb0 100644
--- a/mw/Map.d.ts
+++ b/mw/Map.d.ts
@@ -1,7 +1,64 @@
-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 {
@@ -40,9 +97,9 @@ 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 V>(): S;
+			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;
 
 			/**
 			 * Set the value of one or more keys.