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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.

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    The order of object properties isn't guaranteed to be used when printing the object. Use an array if order is important. Commented Jun 11, 2019 at 21:30
  • Use any array instead. Object.values(o).sort((x, y) => y - x) Commented Jun 11, 2019 at 21:31
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    There's no such thing as a JSON Object. JSON is always a String. You have a plain Object. Commented Jun 11, 2019 at 21:36
  • Or use a Map. Commented Jun 11, 2019 at 21:37
  • @Barmar, I forgot to say that i need those keys. I will try to implement the code using map(), thanks for the help man. Commented Jun 11, 2019 at 21:45

3 Answers 3

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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)]}) );
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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]; });
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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

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