From e8260674988be12080b3c75d3af84c64ed35af86 Mon Sep 17 00:00:00 2001
From: NoNameSheep <HerringtonDarkholme@users.noreply.github.com>
Date: Sat, 4 Apr 2015 13:54:49 +0800
Subject: [PATCH] advice against side effect in default parameter

---
 README.md | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/README.md b/README.md
index f68fc1d4..869f8a05 100644
--- a/README.md
+++ b/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
+  ```
+
 
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