Array of objects vs Object of Objects

The issue is to decide the trade offs between the following notations:

JSON based:

"users": {
    "id1": {
        "id": "id1",
        "firstname": "firstname1",
        "lastname": "lastname1"
    },
    "id2": {
        "id": "id2",
        "firstaame": "firstname2",
        "lastname": "lastname2"
    }
}

Array Based:

users: [
    {
        "id": "id",
        "key2": "value2",
        "key3": "value3"
    },
    {
        "id": "id",
        "key2": "value2",
        "key3": "value3"
    }
]

Relating to this post on the same issue, I have decided (on front end) to use the JSON object notation instead of an array of objects as it suits my requirements, provides better performance, and results in less code in the browser.

But the problem is that the list itself is not static. By this I mean the list is being generated i.e. fetched/stored from DB (NoSQL) and created for new entries through a Java API on the server. I am not able to decide on which notation I should use at the back end (which eventually will also affect the UI too).

Any thoughts/suggestions about performance, maintainability or scalability is appreciated.

This is a totally subjective question. There might be many other points, but let me point out a few below:

JSON based approach: If I am not wrong then this will be implemented using Map on server side.

Advantage: In JavaScript you can directly use users.id1, users.id2, i.e. no need of iteration

Disadvantage: On the client side, somehow you will need the ids to be present in your JSON, i.e. either hard coding them or using some dynamic approach which will tell you which id is present in your JSON.

Array Based approach: If I am not wrong then this will be implemented using Array/List on server side.

Advantage:

1. On client side, you can directly iterate through array, without worrying in advance about which id is present inside it i.e. no hard coding.
2. As pointed out by @JBNizet, the array based approach will maintain the order.

Disadvantage: If you want to fetch single id then you will need to iterate through the array.

Generally we don't send much information on the client side, so array based approach will not create any problem. And transforming an array into a map is possible on both sides (server and client) if you want an id based approach.