added unit testing, and started implementing unit tests...phew

node_modules/mem/index.d.ts

@@ -0,0 +1,71 @@
+export interface CacheStorage<
+	KeyType extends unknown,
+	ValueType extends unknown
+> {
+	has(key: KeyType): boolean;
+	get(key: KeyType): ValueType | undefined;
+	set(key: KeyType, value: ValueType): void;
+	delete(key: KeyType): void;
+	clear?: () => void;
+}
+
+export interface Options<
+	ArgumentsType extends unknown[],
+	CacheKeyType extends unknown,
+	ReturnType extends unknown
+> {
+	/**
+	 * Milliseconds until the cache expires.
+	 *
+	 * @default Infinity
+	 */
+	readonly maxAge?: number;
+
+	/**
+	 * Determines the cache key for storing the result based on the function arguments. By default, if there's only one argument and it's a [primitive](https://developer.mozilla.org/en-US/docs/Glossary/Primitive), it's used directly as a key, otherwise it's all the function arguments JSON stringified as an array.
+	 *
+	 * You could for example change it to only cache on the first argument `x => JSON.stringify(x)`.
+	 */
+	readonly cacheKey?: (...arguments: ArgumentsType) => CacheKeyType;
+
+	/**
+	 * Use a different cache storage. Must implement the following methods: `.has(key)`, `.get(key)`, `.set(key, value)`, `.delete(key)`, and optionally `.clear()`. You could for example use a `WeakMap` instead or [`quick-lru`](https://github.com/sindresorhus/quick-lru) for a LRU cache.
+	 *
+	 * @default new Map()
+	 */
+	readonly cache?: CacheStorage<CacheKeyType, ReturnType>;
+
+	/**
+	 * Cache rejected promises.
+	 *
+	 * @default false
+	 */
+	readonly cachePromiseRejection?: boolean;
+}
+
+/**
+ * [Memoize](https://en.wikipedia.org/wiki/Memoization) functions - An optimization used to speed up consecutive function calls by caching the result of calls with identical input.
+ *
+ * @param fn - Function to be memoized.
+ */
+declare const mem: {
+	<
+		ArgumentsType extends unknown[],
+		ReturnType extends unknown,
+		CacheKeyType extends unknown
+	>(
+		fn: (...arguments: ArgumentsType) => ReturnType,
+		options?: Options<ArgumentsType, CacheKeyType, ReturnType>
+	): (...arguments: ArgumentsType) => ReturnType;
+
+	/**
+	 * Clear all cached data of a memoized function.
+	 *
+	 * @param fn - Memoized function.
+	 */
+	clear<ArgumentsType extends unknown[], ReturnType extends unknown>(
+		fn: (...arguments: ArgumentsType) => ReturnType
+	): void;
+};
+
+export default mem;