Question

In php, if you have the following code:

$map = array(   'first' => 1,   'second' => 2 );  $map['third'] = 3;  foreach($map as $key => $value) {   // code } 

You know the entries will be listed in the order they have been added to the array.

Now, can I assume the same rule applies to the Javascript equivalent below?

map = {   'first': 1,   'second': 2 };  map['third'] = 3;  for (key in map) {   // code } 

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This is a duplicate of: Elements order – for (… in …) loop in javascript

Answer 1

Most browsers will loop through the properties in the order they were added to the object, but the Javascript standard says the order is undefined — so you shouldn’t rely on this behavior. For example, I read a blog post a while back about how Google Chrome didn’t always exhibit this behavior.

If you need the ordered functionality, you should create a new class for yourself that can use both object or numeric keys.