reverse json javascript

Question

Is there an inexpensive way to reverse:

{
    "10": "..."
    "11": "...",
    "12": "...",
    "13": "...",
    "14": "...",
}

so that I get:

{
    "14": "...",
    "13": "...",
    "12": "..."
    "11": "...",
    "10": "...",
}

reverse() doesn't seem to work on json objects. The only way I can think of is to loop through all the elements and create an array. feels like there should be a better way.

Edit: thanks for all the help UPDATE:

What about let's say if each key has chronological data. When I use $.each on the object, it runs through the objects from top to bottom, I didn't realize that was unreliable.

Here's what I'm trying to do:

$.each(object, function (key, value) {
  function foo (key, value);
});

I want to not run foo on all but the last 3 pairs, that is I only want to use the last 3 pairs. I figured if I can reverse them I can just run the first three and stop.

Is there any way I can just do the last 3? If the last 3 ordering is unreliable, is there a safer way to grab the last 3. The last 3 will have the largest numerical keys.

Thanks.

Edit 2: I'm basically deciding finally to do the manipulations on the server side. I'm reorganizing my database so that the relevant subdocuments are now full on documents that could be queried with mongodb. Thanks.

I really have to question what you're trying to accomplish by doing this. Most languages that have a JSON parser will not care what order the pairs are in. — Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams, Commented Nov 28, 2010 at 23:46
@Mark reverse() is an Array method - and you got objects. Also, You want to create "another array", but what you got are not arrays but plain objects. — Šime Vidas
– Šime Vidas, Commented Nov 28, 2010 at 23:46
@Mark I think that both code-snippets will create objects with the same characteristics. Once the object literals are evaluated, the information about the order of the properties in the literal is lost. — Šime Vidas
– Šime Vidas, Commented Nov 28, 2010 at 23:52
@Šime Vidas thanks I updated the question @Ignacio Vazquez-Abrams thank you too. — Mark
– Mark, Commented Nov 29, 2010 at 0:05
Order is never guaranteed in JSON objects. I don't understand why you would want this anyway. No matter what order the properties are in, you will still access the elements in the same fashion. By their key name. — Alex
– Alex, Commented Nov 29, 2010 at 0:06

Adam Lassek · Accepted Answer · 2010-11-29 01:56:53Z

8

Javascript associative arrays are unordered. You cannot depend on the properties being in any particular order.

From Mozilla Developer Network:

Although ECMAScript makes iteration order of objects implementation-dependent, it may appear that all major browsers support an iteration order based on the earliest added property coming first (at least for properties not on the prototype). However, in the case of Internet Explorer, when one uses delete on a property, some confusing behavior results, preventing other browsers from using simple objects like object literals as ordered associative arrays. In Explorer, while the property value is indeed set to undefined, if one later adds back a property with the same name, the property will be iterated in its old position--not at the end of the iteration sequence as one might expect after having deleted the property and then added it back.

So if you want to simulate an ordered associative array in a cross-browser environment, you are forced to either use two separate arrays (one for the keys and the other for the values), or build an array of single-property objects, etc.

edited Nov 29, 2010 at 1:56

answered Nov 28, 2010 at 23:46

Adam Lassek

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5 Comments

Adam Lassek Over a year ago

@Ignacio this isn't a PHP question.

Šime Vidas Over a year ago

@Ignacio He was talking about JavaScript objects.

Orbling Over a year ago

This is because Javascript "associative arrays" are actually objects with properties, rather than actual arrays.

Mark Over a year ago

Is the problem only with IE? Pretty cool. This is very good to know, since I'm not really doing any such moving about with the client.

gsnedders Over a year ago

That quote is no longer quite true. Both Chrome and Opera (from 10.50) order properties whose names are unsigned 32-bit integers before all others (which then appear in insertion order).

Rasheed · Accepted Answer · 2013-05-21 09:55:45Z

3

Use this on json objects arrays

jsonObjectArray.reverse();
$.each(jsonObjectArray, function(i, item) {
    //do something with the item
});

Hope this helps

answered May 21, 2013 at 9:55

Rasheed

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Comments

zoli · Accepted Answer · 2010-11-29 01:46:46Z

2

This might help. Get all the keys from the json objects into an array, which you can sort.

var a = { 1 : 'x', 3 : 'y', 2 : 'z' };
var keys = []
for (i in a) { keys.push(i); }
keys.sort();

then you can use reverse() and slice() to just iterate over the keys you need.

$.each(keys, function(idx, key) { 
  // do whatever with a[key]
});

answered Nov 29, 2010 at 1:46

zoli

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Comments

Calete · Accepted Answer · 2012-05-15 15:31:48Z

1

Follow the json in reverse order,

for(json.length;i>=0;i--)
{
    console.log(json.[i]);
}

answered May 15, 2012 at 15:31

Calete

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