Question

I’ve read through both the RFC and JSON.org and couldn’t see a reference to whether array indexes were 0-based or 1-based. Being originally ‘discovered’ in JS, does everyone just assume it is 0-based?

For example:

{ 
  "selectedItem": 1,
  "Items": ["boots", "gloves", "hat"]
}

Is the selected item "boots" or "gloves"?

If this is the case (assumed 0-based), isn’t this assumption potentially troublesome, particular if JSON support is implemented in languages that by default use 1-based indexes?

Answer 1

I don’t think array index is a concept covered by JSON at all. A JSON array is just an ordered set written in a particular fashion:

An array is an ordered collection of values. An array begins with [ (left bracket) and ends with ] (right bracket). Values are separated by , (comma).

And that’s all there is to it. Indexing questions are left to the end-points, the first element could be index 0, 1, -10, or "pancakes" and it wouldn’t make any difference at all to JSON as long as the indexes matched the order of the elements in the array.

Back to specifics. Whether "selectedItem": 1 refers to "boots" or "gloves" is up to the JSON producer.