Cache function reference when it's a single argument (#35)

Co-Authored-By: Sindre Sorhus <[email protected]>

index.d.ts

@@ -20,7 +20,7 @@ declare namespace mem {
 		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.
+		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 (if it's a `function`, its reference will be used as 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)`.
 		*/

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

@@ -101,7 +101,7 @@ Milliseconds until the cache expires.
 
 Type: `Function`
 
-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.
+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 (if it's a `function`, its reference will be used as 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)`.
 

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 => {