Automatically use WeakMap when possible (#56)

index.js

@@ -4,25 +4,34 @@ const mapAgeCleaner = require('map-age-cleaner');
 
 const cacheStore = new WeakMap();
 
-const mem = (fn, {
-	cacheKey = ([firstArgument]) => firstArgument,
-	cache = new Map(),
-	maxAge
-} = {}) => {
+const mem = (fn, options = {}) => {
+	// Automatically use WeakMap unless the user provided their own cache
+	const weakCache = options.cache || new WeakMap();
+	const {
+		cacheKey = ([firstArgument]) => firstArgument,
+		cache = new Map(),
+		maxAge
+	} = options;
+
 	if (typeof maxAge === 'number') {
 		mapAgeCleaner(cache);
 	}
 
 	const memoized = function (...arguments_) {
 		const key = cacheKey(arguments_);
 
-		if (cache.has(key)) {
-			return cache.get(key).data;
+		// Prefer WeakMap if the key allows it
+		const bestCache = key && (typeof key === 'object' || typeof key === 'function') ?
+			weakCache :
+			cache;
+
+		if (bestCache.has(key)) {
+			return bestCache.get(key).data;
 		}
 
 		const cacheItem = fn.apply(this, arguments_);
 
-		cache.set(key, {
+		bestCache.set(key, {
 			data: cacheItem,
 			maxAge: maxAge ? Date.now() + maxAge : Infinity
 		});

readme.md

@@ -193,9 +193,9 @@ Refer to the [caching strategies](#caching-strategy) section for more informatio
 ##### cache
 
 Type: `object`\
-Default: `new Map()`
+Default: `new Map()`, but it also intelligently uses `new WeakMap()` whenevever possible
 
-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.
+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 [`quick-lru`](https://github.com/sindresorhus/quick-lru) for a LRU cache.
 
 Refer to the [caching strategies](#caching-strategy) section for more information.