Backport memory leak fixes

Commit da4e439 resolved memory
leaking issues by introducing a dependency that was not compatible
with Node.js v4 which mem v1 supported. This commit adds timers
for ensuring the cache Map is cleaned up without using additional
dependencies.

Bug ref: sindresorhus#14

index.js

@@ -20,20 +20,30 @@ module.exports = (fn, opts) => {
 	const memoized = function () {
 		const cache = cacheStore.get(memoized);
 		const key = opts.cacheKey.apply(null, arguments); // eslint-disable-line prefer-rest-params
+		const maxAgeDefined = typeof opts.maxAge === 'number';
 
 		if (cache.has(key)) {
 			const c = cache.get(key);
 
-			if (typeof opts.maxAge !== 'number' || Date.now() < c.maxAge) {
+			if (Date.now() < c.maxAge) {
 				return c.data;
 			}
+
+			if (typeof c.maxAgeTimeout !== 'undefined') {
+				clearTimeout(c.maxAgeTimeout);
+			}
+
+			cache.delete(key);
 		}
 
 		const ret = fn.apply(null, arguments); // eslint-disable-line prefer-spread, prefer-rest-params
 
 		cache.set(key, {
 			data: ret,
-			maxAge: Date.now() + (opts.maxAge || 0)
+			maxAge: maxAgeDefined ? Date.now() + opts.maxAge : Infinity,
+			maxAgeTimeout: maxAgeDefined ? setTimeout(() => {
+				cache.delete(key);
+			}, opts.maxAge).unref() : undefined
 		});
 
 		return ret;
@@ -50,6 +60,12 @@ module.exports.clear = fn => {
 	const cache = cacheStore.get(fn);
 
 	if (cache && typeof cache.clear === 'function') {
+		cache.forEach(c => {
+			if (typeof c.maxAgeTimeout !== 'undefined') {
+				clearTimeout(c.maxAgeTimeout);
+			}
+		});
+
 		cache.clear();
 	}
 };