Can you use a trailing comma in a JSON object?

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.

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.

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

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.

Rather than engage in a debating club, I would adhere to the principle of Defensive Programming by combining both simple techniques in order to simplify interfacing with others:

• As a developer of an app that receives json data, I'd be relaxed and allow the trailing comma.

• When developing an app that writes json, I'd be strict and use one of the clever techniques of the other answers to only add commas between items and avoid the trailing comma.

There are bigger problems to be solved...

There is a possible way to avoid a if-branch in the loop.

s.append("[ "); // there is a space after the left bracket
for (i = 0; i < 5; ++i) {
  s.appendF("\"%d\",", i); // always add comma
}
s.back() = ']'; // modify last comma (or the space) to right bracket

No. The "railroad diagrams" in https://json.org are an exact translation of the spec and make it clear a , always comes before a value, never directly before ]:

or }:

It is not recommended, but you can still do something like this to parse it.

jsonStr = '[0,1,2,3,4,5,]';
let data;
eval('data = ' + jsonStr);
console.log(data)

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

As stated it is not allowed. But in JavaScript this is:

var a = Array()
for(let i=1; i<=5; i++) {
    a.push(i)
}
var s = "[" + a.join(",") + "]"

(works fine in Firefox, Chrome, Edge, IE11, and without the let in IE9, 8, 7, 5)

Since a for-loop is used to iterate over an array, or similar iterable data structure, we can use the length of the array as shown,

awk -v header="FirstName,LastName,DOB" '
  BEGIN {
    FS = ",";
    print("[");
    columns = split(header, column_names, ",");
  }
  { print("  {");
    for (i = 1; i < columns; i++) {
      printf("    \"%s\":\"%s\",\n", column_names[i], $(i));
    }
    printf("    \"%s\":\"%s\"\n", column_names[i], $(i));
    print("  }");
  }
  END { print("]"); } ' datafile.txt

With datafile.txt containing,

 Angela,Baker,2010-05-23
 Betty,Crockett,1990-12-07
 David,Done,2003-10-31

String l = "[" + List<int>.generate(5, (i) => i + 1).join(",") + "]";

I usually loop over the array and attach a comma after every entry in the string. After the loop I delete the last comma again.

Maybe not the best way, but less expensive than checking every time if it's the last object in the loop I guess.