Add .decorator() method (#61)

Co-authored-by: Sindre Sorhus <[email protected]>
Co-authored-by: Federico <[email protected]>

index.ts

@@ -123,7 +123,7 @@ const mem = <
 
 		cache.set(key, {
 			data: result,
-			maxAge: maxAge ? Date.now() + maxAge : Infinity
+			maxAge: maxAge ? Date.now() + maxAge : Number.POSITIVE_INFINITY
 		});
 
 		return result;
@@ -140,6 +140,51 @@ const mem = <
 
 export = mem;
 
+/**
+@returns A TypeScript decorator which memoizes the given function.
+
+@example
+```
+import mem = require('mem');
+
+class Example {
+	index = 0
+
+	@mem.decorator()
+	counter() {
+		return ++this.index;
+	}
+}
+
+class ExampleWithOptions {
+	index = 0
+
+	@mem.decorator({maxAge: 1000})
+	counter() {
+		return ++this.index;
+	}
+}
+```
+*/
+mem.decorator = <
+	FunctionToMemoize extends AnyFunction,
+	CacheKeyType
+>(
+	options: Options<FunctionToMemoize, CacheKeyType> = {}
+) => (
+	target: any,
+	propertyKey: string,
+	descriptor: PropertyDescriptor
+): void => {
+	const input = target[propertyKey];
+
+	if (typeof input !== 'function') {
+		throw new TypeError('The decorated value must be a function');
+	}
+
+	descriptor.value = mem(input, options);
+};
+
 /**
 Clear all cached data of a memoized function.
 

package.json

@@ -45,13 +45,13 @@
 		"@ava/typescript": "^1.1.1",
 		"@sindresorhus/tsconfig": "^0.7.0",
 		"@types/serialize-javascript": "^4.0.0",
-		"ava": "^3.13.0",
+		"ava": "^3.15.0",
 		"del-cli": "^3.0.1",
 		"delay": "^4.4.0",
 		"serialize-javascript": "^5.0.1",
 		"tsd": "^0.13.1",
 		"typescript": "^4.0.3",
-		"xo": "^0.33.1"
+		"xo": "^0.38.2"
 	},
 	"ava": {
 		"files": [

readme.md

@@ -196,6 +196,38 @@ Use a different cache storage. Must implement the following methods: `.has(key)`
 
 Refer to the [caching strategies](#caching-strategy) section for more information.
 
+### mem.decorator(options)
+
+Returns a TypeScript decorator which memoizes the given function.
+
+#### options
+
+Type: `object`
+
+Same as options for `mem()`.
+
+```js
+const mem = require('mem');
+
+class Example {
+	index = 0
+
+	@mem.decorator()
+	counter() {
+		return ++this.index;
+	}
+}
+
+class ExampleWithOptions {
+	index = 0
+
+	@mem.decorator({maxAge: 1000})
+	counter() {
+		return ++this.index;
+	}
+}
+```
+
 ### mem.clear(fn)
 
 Clear all cached data of a memoized function.

test-d/index.test-d.ts

@@ -41,7 +41,7 @@ mem((text: string) => Boolean(text), {
 
 mem(() => 1, {
 	cacheKey: arguments_ => {
-		expectType<[]>(arguments_);
+		expectType<[]>(arguments_); // eslint-disable-line @typescript-eslint/ban-types
 	}
 });
 

test.ts

@@ -222,9 +222,9 @@ test('.clear()', t => {
 
 test('prototype support', t => {
 	class Unicorn {
-		i = 0;
+		index = 0;
 		foo() {
-			return this.i++;
+			return this.index++;
 		}
 	}
 
@@ -237,6 +237,24 @@ test('prototype support', t => {
 	t.is(unicorn.foo(), 0);
 });
 
+test('.decorator()', t => {
+	class TestClass {
+		index = 0;
+
+		@mem.decorator()
+		counter() {
+			return ++this.index;
+		}
+	}
+
+	const alpha = new TestClass();
+	t.is(alpha.counter(), 1);
+	t.is(alpha.counter(), 1, 'The method should be memoized');
+
+	const beta = new TestClass();
+	t.is(beta.counter(), 1, 'The method should not be memoized across instances');
+});
+
 test('mem.clear() throws when called with a plain function', t => {
 	t.throws(() => {
 		mem.clear(() => {});

tsconfig.json

@@ -1,7 +1,8 @@
 {
 	"extends": "@sindresorhus/tsconfig",
 	"compilerOptions": {
-		"outDir": "dist"
+		"outDir": "dist",
+		"experimentalDecorators": true
 	},
 	"files": [
 		"index.ts",