Can you use a trailing comma in a JSON object?

Question

When manually generating a JSON object or array, it's often easier to leave a trailing comma on the last item in the object or array. For example, code to output from an array of strings might look like (in a C++ like pseudocode):

s.append("[");
for (i = 0; i < 5; ++i) {
    s.appendF("\"%d\",", i);
}
s.append("]");

giving you a string like

[0,1,2,3,4,5,]

Is this allowed?

Answer 1

No. The JSON spec, as maintained at http://json.org, does not allow trailing commas. From what I've seen, some parsers may silently allow them when reading a JSON string, while others will throw errors. For interoperability, you shouldn't include it.

The code above could be restructured, either to remove the trailing comma when adding the array terminator or to add the comma before items, skipping that for the first one.

Answer 2

According to the Class JSONArray specification:

• An extra , (comma) may appear just before the closing bracket.
• The null value will be inserted when there is , (comma) elision.

So, as I understand it, it should be allowed to write:

[0,1,2,3,4,5,]

But it could happen that some parsers will return the 7 as item count (like IE8 as Daniel Earwicker pointed out) instead of the expected 6.

---

Edited:

I found this JSON Validator that validates a JSON string against RFC 4627 (The application/json media type for JavaScript Object Notation) and against the JavaScript language specification. Actually here an array with a trailing comma is considered valid just for JavaScript and not for the RFC 4627 specification.

However, in the RFC 4627 specification is stated that:

2.3. Arrays

An array structure is represented as square brackets surrounding zero or more values (or elements). Elements are separated by commas.

array = begin-array [ value *( value-separator value ) ] end-array

To me this is again an interpretation problem. If you write that Elements are separated by commas (without stating something about special cases, like the last element), it could be understood in both ways.

P.S. RFC 4627 isn't a standard (as explicitly stated), and is already obsolited by RFC 7159 (which is a proposed standard) RFC 7159

Answer 3

Interestingly, both C & C++ (and I think C#, but I'm not sure) specifically allow the trailing comma -- for exactly the reason given: It make programmaticly generating lists much easier. Not sure why JavaScript didn't follow their lead.