Should I return null, an empty object or an empty array for json with no data?

Question

I am sending an ajax request that will search a database. If no results are found, is there a reason to choose to return null, {} or []? Is one considered "standard" or "best practice"?

Answer 1

The differences are pretty negligible.

null is the usual way to denote "no value", but your client side code will need to explicitly check for that.

If the response actually means "here are the results, but they're empty", [] or {} might be more suitable, as long as you match the format of non-empty responses (i.e. if the client is expecting an array, don't send {}, and vice versa).

Basically:

• If an empty result set is a special case and you want to write explicit behavior for it, null seems more appropriate.
• If your client expects an array (e.g. it will loop through the results), then use [].
• If your client expects a map of key-value pairs (e.g. it will loop through keys or will use result[someKey]), then use {}.
• If your client expects a single object (i.e. it expects the result to have well-known properties and will do something like result.someProperty), then null is better than returning an empty object.

Or more clearly: If the client thinks of the result as a single value, use null. If the client thinks of it as a list, use []. If the client thinks of it as a map, use {}.