Fix using multiple decorators on the same instance (#78)

index.ts

@@ -3,8 +3,6 @@ import mapAgeCleaner from 'map-age-cleaner';
 
 type AnyFunction = (...arguments_: any) => any;
 
-const decoratorInstanceMap = new WeakMap();
-
 const cacheStore = new WeakMap<AnyFunction, CacheStorage<any, any>>();
 
 interface CacheStorageContent<ValueType> {
@@ -169,6 +167,8 @@ export function memDecorator<
 >(
 	options: Options<FunctionToMemoize, CacheKeyType> = {},
 ) {
+	const instanceMap = new WeakMap();
+
 	return (
 		target: any,
 		propertyKey: string,
@@ -184,13 +184,13 @@ export function memDecorator<
 		delete descriptor.writable;
 
 		descriptor.get = function () {
-			if (!decoratorInstanceMap.has(this)) {
+			if (!instanceMap.has(this)) {
 				const value = mem(input, options) as FunctionToMemoize;
-				decoratorInstanceMap.set(this, value);
+				instanceMap.set(this, value);
 				return value;
 			}
 
-			return decoratorInstanceMap.get(this) as FunctionToMemoize;
+			return instanceMap.get(this) as FunctionToMemoize;
 		};
 	};
 }

test.ts

@@ -239,19 +239,24 @@ test('prototype support', t => {
 
 test('.decorator()', t => {
 	let returnValue = 1;
+	const returnValue2 = 101;
 
 	class TestClass {
 		@memDecorator()
 		counter() {
-			return returnValue;
+			return returnValue++;
+		}
+
+		@memDecorator()
+		counter2() {
+			return returnValue2;
 		}
 	}
 
 	const alpha = new TestClass();
 	t.is(alpha.counter(), 1);
 	t.is(alpha.counter(), 1, 'The method should be memoized');
-
-	returnValue++;
+	t.is(alpha.counter2(), 101, 'The method should be memoized separately from the other one');
 
 	const beta = new TestClass();
 	t.is(beta.counter(), 2, 'The method should not be memoized across instances');