Allow JSON with trailing commas

I’m looking for a way to support the following style:

• JSON-based
• Always quote keys
• Always include trailing commas

This would typically be not a problem – I should be able to set parser: json and trailingComma: all. However, #2308* disables this specific combination and I can’t get around it.

Here’s my use case: I want Prettier to format my VSCode settings files, which I keep in a git repo.

• VSCode requires keys to be quoted, so I can’t use the json5 parser.
• I would like trailing commas to make diffs more readable.

Note: I originally filed this bug in another repo https://github.com/prettier/prettier-vscode/issues/589

* I am aware that #2308 was created because trailing commas broke VSCode. However, times have changed and VSCode settings files now support trailing commas.

Thanks!

Prettier 1.15.3 Playground link

--parser json
--trailing-comma all

Input:

{
  //
  "a": 1,
  "b": 2,
}

Output:

{
  //
  "a": 1,
  "b": 2
}

Expected behavior: I’d like a set of input options that doesn’t make changes to the input; i.e., I want the output to also be

{
  //
  "a": 1,
  "b": 2,
}

[difr]

.prettierrc

{
...
  "overrides": [
    {
      "files": ["*.json", ".*rc"],
      "options": {
        "parser": "json5",
        "quoteProps": "preserve",
        "singleQuote": false,
        "trailingComma": "all",
      },
    },
  ],
}

😃

[ikatyang]

Personally, I think jsonc (the language named JSON with Comments in VSCode, which is also used by TypeScript tsconfig.json) is better than JSON5 since it only adds missing features that are necessary so it’s relatively simple.

I was thinking of adding a --parser jsonc to address this issue, but then I found that its name (JSON with Comments) is very confusing since it looks like JSON + comments but it’s actually JSON + comments + trailing commas while our --parser json already handles JSON + comments, so the jsonc description may look something like “JSON with Comments: same parser as json, but trailing commas are allowed”, which is super confusing.