Cache function reference when it's a single argument

index.js

@@ -12,11 +12,9 @@ const defaultCacheKey = (...arguments_) => {
 
 	if (arguments_.length === 1) {
 		const [firstArgument] = arguments_;
-		if (
-			firstArgument === null ||
-			firstArgument === undefined ||
-			(typeof firstArgument !== 'function' && typeof firstArgument !== 'object')
-		) {
+		const isObject = typeof firstArgument === 'object' && firstArgument !== null;
+		const isPrimitive = !isObject;
+		if (isPrimitive) {
 			return firstArgument;
 		}
 	}

readme.md

@@ -2,7 +2,7 @@
 
 > [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
 
-Memory is automatically released when an item expires.
+The input is cached by value (if supported by `JSON.stringify`) unless it’s a single argument of type `function`, in which case it’s stored by reference. Memory is automatically released when an item expires.
 
 
 ## Install

test.js

@@ -15,6 +15,20 @@ test('memoize', t => {
 	t.is(memoized('foo', 'bar'), 2);
 	t.is(memoized('foo', 'bar'), 2);
 	t.is(memoized('foo', 'bar'), 2);
+	t.is(memoized(1), 3);
+	t.is(memoized(1), 3);
+	t.is(memoized(null), 4);
+	t.is(memoized(null), 4);
+	t.is(memoized(undefined), 5);
+	t.is(memoized(undefined), 5);
+	t.is(memoized(fixture), 6);
+	t.is(memoized(fixture), 6);
+	t.is(memoized(true), 7);
+	t.is(memoized(true), 7);
+
+	// Ensure that functions are stored by reference and not by "value" (e.g. their `.toString()` representation)
+	t.is(memoized(() => i++), 8);
+	t.is(memoized(() => i++), 9);
 });
 
 test('memoize with multiple non-primitive arguments', t => {