Retrieving key of a JavaScript object using its index in JQUERY

try this:

convert the json in a array:

  var obj = {
        "set1": [1, 2, 3],
        "set2": [4, 5, 6, 7, 8],
        "set3": [9, 10, 11, 12]
    };
    var arr = []
    for (var key in obj) {
        arr.push(obj[key]);
    }
    console.log(arr[1]);

or array of objects:

var obj = {
    "set1": [1, 2, 3],
    "set2": [4, 5, 6, 7, 8],
    "set3": [9, 10, 11, 12]
};
var arr = []
for (var key in obj) {
    arr.push({key:obj[key]});
}
console.log(arr[1]);

jsfiddle:https://jsfiddle.net/3yx8a12e/

You have these options (here's JSFiddle too):

var obj = {
    "set1": [1, 2, 3],
    "set2": [4, 5, 6, 7, 8],
    "set3": [9, 10, 11, 12]
};

var sets = [];
for (var o in obj) {
  if (obj.hasOwnProperty(o)) { //Thanks Magrangs for reference
     sets.push(o);
  };
};

//Or in modern browsers
//var sets = Object.keys(obj);

console.log(sets[1]);

You can use Object.keys(obj) to get an array of the keys of the object.

In your example, Object.keys(obj)[1] will return "set2".

Note that Object.keys is not supported in older browsers (see browser compatibility guide below):

• Chrome - 5
• Firefox (Gecko) - 4.0 (2.0)
• Internet Explorer - 9
• Opera - 12
• Safari - 5

But you can polyfill it using the following code:

// From https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys
if (!Object.keys) {
  Object.keys = (function() {
    'use strict';
    var hasOwnProperty = Object.prototype.hasOwnProperty,
        hasDontEnumBug = !({ toString: null }).propertyIsEnumerable('toString'),
        dontEnums = [
          'toString',
          'toLocaleString',
          'valueOf',
          'hasOwnProperty',
          'isPrototypeOf',
          'propertyIsEnumerable',
          'constructor'
        ],
        dontEnumsLength = dontEnums.length;

    return function(obj) {
      if (typeof obj !== 'object' && (typeof obj !== 'function' || obj === null)) {
        throw new TypeError('Object.keys called on non-object');
      }

      var result = [], prop, i;

      for (prop in obj) {
        if (hasOwnProperty.call(obj, prop)) {
          result.push(prop);
        }
      }

      if (hasDontEnumBug) {
        for (i = 0; i < dontEnumsLength; i++) {
          if (hasOwnProperty.call(obj, dontEnums[i])) {
            result.push(dontEnums[i]);
          }
        }
      }
      return result;
    };
  }());
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys

function getKey(data, i){
    var n = 0;
    for(var k in data){
        if(n == i) return k;
        n++;
    }
}

data - your js object, i - index of member

You can Retrieve All Keys this code

    var obj = {
        "set1": [1, 2, 3],
        "set2": [4, 5, 6, 7, 8],
        "set3": [9, 10, 11, 12]
    };

var newary= [];
for (var x in obj) {
  if (obj.hasOwnProperty(x)) { //Thanks Magrangs for reference
     newary.push(x);
  };
};

console.log(newary[1])


/*For Modern Browser use this code*/

    // Retrieve All Keys of Object 
    Object.keys(obj);

    // Retrieve Specific Keys of Object
    Object.keys(obj)[1];