Does JavaScript's `for...in` Loop Iterate Over Object Properties in Declaration Order?

Influence of Declaration Order on "for (… in …)" Loop Behavior

The "for…in" loop in JavaScript iterates over the properties of an object. However, the order in which the loop traverses the properties has been a subject of discussion. Does the loop adhere to the declaration order of the properties?

Quoting John Resig, the father of jQuery, "All major browsers currently loop over the properties of an object in the order in which they were defined." However, this behavior is not explicitly defined by the ECMAScript specification.

All modern implementations of ECMAScript, including the majority of browsers, respect the definition order of properties during iteration. This means that if you have an object with properties declared as:

var myObject = { A: "Hello", B: "World" };

The loop will reliably traverse the properties in the same order: first property "A" followed by property "B".

While most browsers adhere to this rule, Chrome and Opera exhibit a slight deviation. These browsers prioritize numerical property names over non-numerical property names. Therefore, if you have a mix of numerical and non-numerical properties, the non-numerical properties will be pulled in-order ahead of the first non-numerical property. This quirk stems from the way arrays are implemented in these browsers.

It's important to note that this behavior may change with future updates or revisions to browser engines. Relying on this order for critical functionality is discouraged.

In conclusion, while the "for…in" loop generally preserves the declaration order of properties in most browsers, it's prudent to use an array if the order is of paramount importance. This approach ensures reliability and consistency across different browsers and versions.

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