# Trailing Commas

Like [comments](https://json.nlohmann.me/features/comments/index.md), this library does not support trailing commas in arrays and objects *by default*.

You can set parameter `ignore_trailing_commas` to `true` in the [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) function to allow trailing commas in arrays and objects. Note that a single comma as the only content of the array or object (`[,]` or `{,}`) is not allowed, and multiple trailing commas (`[1,,]`) are not allowed either.

This library does not add trailing commas when serializing JSON data.

For more information, see [JSON With Commas and Comments (JWCC)](https://nigeltao.github.io/blog/2021/json-with-commas-comments.html).

Example

Consider the following JSON with trailing commas.

```
{
    "planets": [
        "Mercury",
        "Venus",
        "Earth",
        "Mars",
        "Jupiter",
        "Uranus",
        "Neptune",
    ]
}
```

When calling `parse` without additional argument, a parse error exception is thrown. If `ignore_trailing_commas` is set to `true`, the trailing commas are ignored during parsing:

```
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    std::string s = R"(
    {
        "planets": [
            "Mercury",
            "Venus",
            "Earth",
            "Mars",
            "Jupiter",
            "Uranus",
            "Neptune",
        ]
    }
    )";

    try
    {
        json j = json::parse(s);
    }
    catch (json::exception& e)
    {
        std::cout << e.what() << std::endl;
    }

    json j = json::parse(s,
                         /* callback */ nullptr,
                         /* allow exceptions */ true,
                         /* ignore_comments */ false,
                         /* ignore_trailing_commas */ true);
    std::cout << j.dump(2) << '\n';
}
```

Output:

```
[json.exception.parse_error.101] parse error at line 11, column 9: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
{
  "planets": [
    "Mercury",
    "Venus",
    "Earth",
    "Mars",
    "Jupiter",
    "Uranus",
    "Neptune"
  ]
}
```