Get JSON keys in order

Question

I have the below in a file and read as

var input = require("./mydata.json");

"User": {
        "properties": {
        "firstName": {
          "type": "string",
          "minLength": 1,
          "maxLength": 50
        },
        "lastName": {
          "type": "string",
          "maxLength": 50
        },
        "middleName": {
          "type": "string"
        },
        "title": {
          "type": "string"
        },
        "language": {
          "type": "string",
          "default": "en-US"
        }
      }
    }

I am using the below code to loop through the keys

var item = _.get(input, 'User');
var properties = item.properties;
var allKeys = _.keys(properties);
_.each(allKeys, function(key) {

});

Inside the each loop, I get the firstname, lastname etc, in the same sequence as in the input file. I want to know if I will get it in order always?

I believe, you can't rely on that order. Hash table must not to preserve it. It's just lucky implementation. — vp_arth, Jul 06 '16 at 05:42
Property order is a [complex subject](http://stackoverflow.com/a/38218582/6445533) in Javascript — , Jul 06 '16 at 07:49

score 2 · Answer 1 · answered Jul 06 '16 at 05:45

Properties order in objects is not guaranteed in JavaScript; you need to use an Array to preserve it.

Definition of an Object from ECMAScript Third Edition (pdf):

4.3.3 Object

An object is a member of the type Object. It is an unordered collection of properties each of which contains a primitive value, object, or function. A function stored in a property of an object is called a method.

Since ECMAScript 2015, using the Map object could be an alternative. A Map shares some similarities with an Object and guarantees the keys order:

A Map iterates its elements in insertion order, whereas iteration order is not specified for Objects.

score 0 · Answer 2 · answered Jul 06 '16 at 05:42

0

Yes, If you want to change the order then you will need to sort or arrange the keys in a separate array and then loop on rearranged array.

answered Jul 06 '16 at 05:42

Mahesh

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Abdennour TOUMI · Answer 3 · 2016-07-06T05:53:40.583

0

Object.keys(input.User).sort();
// Will return all keys in sorted array.

Then , if you want to read value of each key following that order , just loop through that array :

Object.keys(input.User).sort().forEach((key,index)=>{

    console.log('key :'+key+' , value :'+input.User[key]);
})

edited Jul 06 '16 at 05:53

answered Jul 06 '16 at 05:48

Abdennour TOUMI

64,884
28
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3

I think the OP means the order they were defined, not some arbitrary order for the sake of it. – Marty Jul 06 '16 at 05:55

Get JSON keys in order

3 Answers3

4.3.3 Object