Does JavaScript's `for...in` Loop Guarantee Property Order?

Loop Element Order in "for (... in ...)" Statement

The "for (... in ...)" loop in JavaScript is used to iterate over the properties of an object. The question arises whether these properties are processed in the order they are declared.

According to John Resig, all major browsers maintain the definition order of object properties during loop iterations. However, Chrome and Opera exhibit an exception only for non-numerical property names. These properties are processed ahead of the first numerical property in both browsers.

To simplify:

• Properties with numerical names are processed in order.
• Properties with non-numerical names are processed before numerical properties in Chrome and Opera.

For example:

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

In most browsers, the output will be:

1
2
34
first
second

In Chrome and Opera, the output will be:

1
2
34
first
second

Despite the current consistency in browser behavior, the ECMAScript specification leaves the loop mechanics undefined. Therefore, relying on the current order is not recommended.

Best Practice:

If order is crucial for your application, it's advisable to use an array instead of an object with a "for (... in ...)" loop. Arrays preserve the order of elements more reliably.

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