I am using Firebase console for preparing data for a demo app. One of the data item is attendees. Attendees is an array. I want to add a few attendees as an array in Firebase. I understand Firebase does not have arrays, but object with keys (in chronological order). How do I do that for preparing sample data? My current Firebase data looks like the below.

The Firebase Database doesn't store arrays. It stores dictionaries/associate arrays. So the closest you can get is:

attendees: {
  0: "Bill Gates",
  1: "Larry Page",
  2: "James Tamplin"
}

You can build this structure in the Firebase Console. And then when you read it with one of the Firebase SDKs, it will be translated into an array.

firebase.database().ref('attendees').once('value', function(snapshot) {
  console.log(snapshot.val());
  // ["Bill Gates", "Larry Page", "James Tamplin"]
});

So this may be the result that you're look for. But I recommend reading this blog post on why Firebase prefers it if you don't store arrays: https://firebase.googleblog.com/2014/04/best-practices-arrays-in-firebase.html.

Don't use an array, when you actually need a set

Most developers are not actually trying to store an array and I think your case might be one of those. For example: can "Bill Gates" be an attendee twice?

attendees: {
  0: "Bill Gates",
  1: "Larry Page",
  2: "James Tamplin",
  3: "Bill Gates"
}

If not, you're going to have to check whether he's already in the array before you add him.

if (!attendees.contains("Bill Gates")) {
  attendees.push("Bill Gates");
}

This is a clear sign that your data structure is sub-optimal for the use-case. Having to check all existing children before adding a new one is going to limit scalability.

In this case, what you really want is a set: a data structure where each child can be present only once. In Firebase you model sets like this:

attendees: {
  "Bill Gates": true,
  "Larry Page": true,
  "James Tamplin": true
}

And now whenever you try to add Bill Gates a second time, it's a no-op:

attendees["Bill Gates"] = true;

So instead of having to code for the uniqueness requirement, the data structure implicitly solves it.