javascript - Retrieving a property of a JSON object by index?

Question

Assuming this JSON object:

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

The "set2" property may be retrieved like so:

obj["set2"]

Is there a way to retrieve the "set2" property by index? It is the second property of the JSON object. This does not work (of course):

obj[1]  

So, let's say that I want to retrieve the second property of the JSON object, but I don't know its name - how would I do it then?

Update: Yes, I understand that objects are collections of unordered properties. But I don't think that the browsers mess with the "original" order defined by the JSON literal / string.

Answer 1

Objects in JavaScript are collections of unordered properties. Objects are hashtables.

If you want your properties to be in alphabetical order, one possible solution would be to create an index for your properties in a separate array. Just a few hours ago, I answered a question on Stack Overflow which you may want to check out:

• Iterating over a JavaScript object in sort order based on particular key value of a child object

Here's a quick adaptation for your object¹:

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

var index = [];

// build the index
for (var x in obj) {
   index.push(x);
}

// sort the index
index.sort(function (a, b) {    
   return a == b ? 0 : (a > b ? 1 : -1); 
}); 

Then you would be able to do the following:

console.log(obj[index[1]]);

The answer I cited earlier proposes a reusable solution to iterate over such an object. That is unless you can change your JSON to as @Jacob Relkin suggested in the other answer, which could be easier.

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¹ You may want to use the hasOwnProperty() method to ensure that the properties belong to your object and are not inherited from Object.prototype.