Is JavaScript's `for...in` Loop Order Guaranteed, and Should You Rely On It?

Order of Elements in a "for (… in …)" Loop

In JavaScript, "for...in" loops iterate through the enumerable properties of an object. While these properties are generally accessed in the order in which they were defined, it's important to note that this behavior is implementation-dependent.

According to John Resig, the author of jQuery, all major browsers currently loop over object properties in definition order, except for a few cases in Chrome. However, the ECMAScript specification explicitly states that this behavior is undefined.

In practice, all modern ECMAScript implementations iterate over object properties in the order in which they are defined. It's worth noting that the exceptions are Chrome and Opera, which do this for every non-numeric property name. Both browsers pull in properties sequentially, leading to the first non-numeric property (this has to do with how they implement arrays). Object.keys also follows the same order.

The following example clearly illustrates what is happening:

var obj = {
  "first": "first",
  "2": "2",
  "34": "34",
  "1": "1",
  "second": "second"
};
for (var i in obj) { console.log(i); };

Output sequence:

• "1"
• "2"
• "34"
• "first"
• "second"

It is important to note that this behavior may change at any time. Therefore, it is not recommended to rely on the current order.

Conclusion: If order is important to you, use an array.

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