[javascript/en] Added for/in loop JavaScript

more explanation about for/in java script.

1 files changed, 14 insertions, 1 deletions

diff --git a/javascript.html.markdown b/javascript.html.markdown
index 3308b08d..f7a662a4 100644
--- a/javascript.html.markdown
+++ b/javascript.html.markdown
@@ -218,13 +218,26 @@ for (var i = 0; i < 5; i++){
     // will run 5 times
 }
 
-//The For/In statement loops through the properties of an object:
+//The For/In statement loops iterates over every property across the entire prototype chain
 var description = "";
 var person = {fname:"Paul", lname:"Ken", age:18}; 
 for (var x in person) {
     description += person[x] + " ";
 }
 
+//If only want to consider properties attached to the object itself, 
+//and not its prototypes use hasOwnProperty() check
+var description = "";
+var person = {fname:"Paul", lname:"Ken", age:18}; 
+for (var x in person) {
+    if( person.hasOwnProperty( x ) ) {
+        description += person[x] + " ";
+    }
+}
+
+//for/in should not be used to iterate over an Array where the index order is important.
+//There is no guarantee that for/in will return the indexes in any particular order
+
 // && is logical and, || is logical or
 if (house.size == "big" && house.colour == "blue"){
     house.contains = "bear";