feat: hw4

Author: YingMuo

hw4/README.md

@@ -0,0 +1,15 @@
+# HW4
+
+You can install package through:
+
+```shell
+npm i
+```
+
+After finish your test code in `tests/calculator_test.js`, you can run `Stryker` by:
+
+```shell
+npm run mutate
+```
+
+to get your mutation testing result.