advice against side effect in default parameter

README.md

@@ -550,6 +550,23 @@
       // ...
     }
     ```
+    
+  - Don't put side effect into default parameter
+  
+  > it introduces confusion and subtlety. Arguments in function call are evaluated at call site, but default parameters are not evaluated at define site. 
+
+  ```javascript
+  var b = 1
+  // bad
+  function count(a = b++) {
+    console.log(a)
+  }
+  count()  // 1
+  count()  // 2
+  count(3) // 3
+  count()  // 3
+  ```
+
 
 **[⬆ back to top](#table-of-contents)**