JavaScript Object organizes its elements by keys in an automatic sorting process

How does a JavaScript object sort itself in ascending order of keys?

This is a new discovery for me.

/* Yes, it maintains an order. The order is based on the sequence of insertion. If there are no specific orders being maintained, please provide an example to help clarify the concept for me.

Thanks in advance.
*/

var a = {"13" : "1", "15" : "2", "14" : "3", "12" : "4"};

console.log(JSON.stringify(a)); /* This is not related to any for loop */ 

for(let i in a) {
console.log(i + " = " + a[i]);
}
/* However, keys that are not numbers are not sorted. */
var b = {"c" : "1", "a" : "2", "b" : "3", "d" : "4"};

for(let i in b) {
console.log(i + " = " + b[i]);
}

The first object organizes itself by default. I am using string keys for both objects. Surprisingly, the second object does not sort alphabetically as expected.

Answer №1

This specific behavior is now officially incorporated into the JavaScript protocol:

  1. Every individual property key P of object O which happens to be an integer index is included in the keys list, arranged in ascending numerical order

    a. Insert P as the final element of keys.

  2. Each property key P of object O that is a String but not an integer index is added at the end of keys in the sequence in which the properties were created

    a. Append P as the last item in keys.

A more detailed explanation can be found here:

To summarize, the reasoning behind this decision is as follows:

  • Logically speaking, the order should be insignificant and developers shouldn't rely on it.
  • Yet most web browsers display a predictable order, causing certain programs to depend on this order. If browsers alter their sequencing, some applications might encounter issues.
  • There are two alternative approaches to handling this dilemma:
    • Recognize in the standard that the order remains consistent --- allowing developers to confidently rely on it.
    • Oblige programmers to comprehend this aspect by generating a unique order each time the program runs.
  • Traditionally, the latter solution was simpler to execute (at least, based on my experience with Perl). However, nowadays, the former option is more convenient.
  • Therefore, the regulatory organization opted to include the deterministic order within the official protocol.

Answer №2

I stumbled upon this insightful response that directly addresses the topic you're inquiring about.

Here is an excerpt from the reply:

The reordering of object keys in Chrome is due to how v8 handles associative arrays. Although it's a documented issue (Issue 164), it aligns with specifications and thus is considered 'working as intended'. With associative arrays, there isn't a set order for iteration.

A practical workaround involves prefixing numeric values with letters, such as: 'size_7':['9149','9139'] and so forth.

The upcoming changes in the ECMAScript specification will require [Chrome] developers to modify this behavior.

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