diff --git a/grammar/basic.md b/grammar/basic.md
index 22f4b611..d0d9aa63 100644
--- a/grammar/basic.md
+++ b/grammar/basic.md
@@ -710,7 +710,7 @@ if (v){
 // ReferenceError: v is not defined
 
 // 正确的写法
-if (typeof v === "undefined"){
+if (typeof v !== "undefined"){
 	// ...
 }