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

Simple, cheap, easy to read, and always works regardless of the specs.

$delimiter = '';
for ....  {
    print $delimiter.$whatever
    $delimiter = ',';
}

The redundant assignment to $delim is a very small price to pay. Also works just as well if there is no explicit loop but separate code fragments.

Answer 2

Unfortunately the JSON specification does not allow a trailing comma. There are a few browsers that will allow it, but generally you need to worry about all browsers.

In general I try turn the problem around, and add the comma before the actual value, so you end up with code that looks like this:

s.append("[");
for (i = 0; i < 5; ++i) {
  if (i) s.append(","); // add the comma only if this isn't the first entry
  s.appendF("\"%d\"", i);
}
s.append("]");

That extra one line of code in your for loop is hardly expensive...

Another alternative I've used when output a structure to JSON from a dictionary of some form is to always append a comma after each entry (as you are doing above) and then add a dummy entry at the end that has not trailing comma (but that is just lazy ;->).

Doesn't work well with an array unfortunately.

Answer 3

PHP coders may want to check out implode(). This takes an array joins it up using a string.

From the docs...

$array = array('lastname', 'email', 'phone');
echo implode(",", $array); // lastname,email,phone

Answer 4

Use JSON5. Don't use JSON.

• Objects and arrays can have trailing commas
• Object keys can be unquoted if they're valid identifiers
• Strings can be single-quoted
• Strings can be split across multiple lines
• Numbers can be hexadecimal (base 16)
• Numbers can begin or end with a (leading or trailing) decimal point.
• Numbers can include Infinity and -Infinity.
• Numbers can begin with an explicit plus (+) sign.
• Both inline (single-line) and block (multi-line) comments are allowed.

http://json5.org/

https://github.com/aseemk/json5

Answer 5

As it's been already said, JSON spec (based on ECMAScript 3) doesn't allow trailing comma. ES >= 5 allows it, so you can actually use that notation in pure JS. It's been argued about, and some parsers did support it (http://bolinfest.com/essays/json.html, http://whereswalden.com/2010/09/08/spidermonkey-json-change-trailing-commas-no-longer-accepted/), but it's the spec fact (as shown on http://json.org/) that it shouldn't work in JSON. That thing said...

... I'm wondering why no-one pointed out that you can actually split the loop at 0th iteration and use leading comma instead of trailing one to get rid of the comparison code smell and any actual performance overhead in the loop, resulting in a code that's actually shorter, simpler and faster (due to no branching/conditionals in the loop) than other solutions proposed.

E.g. (in a C-style pseudocode similar to OP's proposed code):

s.append("[");
// MAX == 5 here. if it's constant, you can inline it below and get rid of the comparison
if ( MAX > 0 ) {
    s.appendF("\"%d\"", 0); // 0-th iteration
    for( int i = 1; i < MAX; ++i ) {
        s.appendF(",\"%d\"", i); // i-th iteration
    }
}
s.append("]");

Answer 6

With Relaxed JSON, you can have trailing commas, or just leave the commas out. They are optional.

There is no reason at all commas need to be present to parse a JSON-like document.

Take a look at the Relaxed JSON spec and you will see how 'noisy' the original JSON spec is. Way too many commas and quotes...

http://www.relaxedjson.org

You can also try out your example using this online RJSON parser and see it get parsed correctly.

http://www.relaxedjson.org/docs/converter.html?source=%5B0%2C1%2C2%2C3%2C4%2C5%2C%5D