How to revert a JSON object with numerical keys

Question

I am new to JavaScript programming, and I am having a headache trying to figure it out how to reverse keys of a single JSON object. This is the object that I'm trying to reverse:

{70: "a",
276: "b ", 
277: "c ", 
688: "d", 
841: "e", 
842: "f", 
843: "g", 
1078: "h", 
1079: "i"}

I am running this code on localhost. I am analysing the console.logs with google chrome developer tools( F12 ). I can also use Jquery in this script.

This is what I've tried so far:

//The object is stored in a variable called json
var json = 
{70: "a",
276: "b ", 
277: "c ", 
688: "d", 
841: "e", 
842: "f", 
843: "g", 
1078: "h", 
1079: "i"}; 

var entries = Object.entries(json);

entries.reverse();

var newjson = Object.fromEntries(entries);

console.log(newjson);

When I run this code, the output is the same as the old json variable, is inaltered.

And this is the expected console.log()

{ 1079: "i",
  1078: "h", 
  843: "g", 
  842: "f",
  841: "e", 
  688: "d",  
  277: "c",
  276: "b",
  70: "a",
}

Much thanks to anyone who tries to help me.

The order of object properties isn't guaranteed to be used when printing the object. Use an array if order is important. — Barmar
– Barmar, Commented Jun 11, 2019 at 21:30
Use any array instead. Object.values(o).sort((x, y) => y - x) — Aluan Haddad
– Aluan Haddad, Commented Jun 11, 2019 at 21:31
There's no such thing as a JSON Object. JSON is always a String. You have a plain Object. — Mulan
– Mulan, Commented Jun 11, 2019 at 21:36
@Barmar, I forgot to say that i need those keys. I will try to implement the code using map(), thanks for the help man. — Marco Antonio Occhialini Filho
– Marco Antonio Occhialini Filho, Commented Jun 11, 2019 at 21:45

Luillyfe · Accepted Answer · 2019-06-11 22:54:14Z

1

I hope this helps. I'm not even sure about the performance. Feel free to vote down as long you give me feedback.

Let's say you have the json var:

const reversedKeys = Object.keys(json).reverse();
const reversedJson = reversedKeys.map(e => ({[e]: json[Number(e)]}) );

answered Jun 11, 2019 at 22:54

Luillyfe

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Comments

Nick Louloudakis · Accepted Answer · 2019-06-11 21:55:05Z

Consider that the order of the elements in an object is NOT preserved. In practice, JavaScript object is a dictionary, where, internally, data are not stored in a sequential manner such as in an Array, and values are accessed using a hash function that takes a key as input and - in a simplified sense, gives the "position" of the value inside the structure.

Dictionary is a Data Structure that does NOT preserve ordering (yet, it provides fast data retrieval via indexing), not in JavaScript, or in any other programming language.

I suggest that you use an array for preserving order.

Maybe an array containing different objects could be a solution for you. You can then sort the array based on the key of the object. Here is a "dummy" example:

var arr = [{1: "Test1"}, {2: "Test2"}]
arr.sort((obj1, obj2) => {
  return Object.keys(obj2)[0] - Object.keys(obj1)[0];
});

Meziane · Accepted Answer · 2019-06-12 09:01:40Z

0

As adam-lassek said in this post Javascript associative arrays are unordered. You must write your own function to reverse your array as did yossi-neiman in this post

answered Jun 12, 2019 at 9:01

Meziane

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