Is JavaScript's `for...in` Loop Property Order Consistent Across Browsers?

For-In Loops and Object Property Order in JavaScript

When iterating through an object's properties using a "for...in" loop, it's natural to wonder if the order of traversal matches the original declaration order.

According to John Resig, past implementations of JavaScript in major browsers executed this loop in the order of property definition. However, Chrome had an exception with non-numerical property names being pulled in-order ahead of the first numerical property.

Today, this behavior varies among browsers. Modern browsers adhere to the definition order, with the exception of Chrome and Opera, which continue to shuffle non-numerical properties. This applies to both the "for...in" loop and the "Object.keys" method.

An example illustrates this:

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

Output in Chrome and Opera:

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

Output in other browsers:

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

Despite this variability, it's essential to note that browser implementations can change, and relying on the preservation of order can be risky.

Therefore, if the order of the properties is crucial in your application, it's prudent to use arrays as they provide a predictable and stable ordering mechanism.

The above is the detailed content of Is JavaScript's `for...in` Loop Property Order Consistent Across Browsers?. For more information, please follow other related articles on the PHP Chinese website!