Dangling commas in JSONC

[hildjj]

Tell us about your environment

• Momoa Version:
  3.2.2

• Node Version:
  v22.9.0

• npm Version:
  10.9.0

Which function(s) is causing a problem?

[ ] interpret()
[ ] iterator()
[ ] parse()
[x] print()
[ ] traverse()
[ ] tokenize()

Example JSON code that demonstrates the problem:

{
  "foo": 1,
}

What did you do?

parse('{"foo": 1,}', {mode: "jsonc"})

What did you expect to happen?

{
  type: 'Document',
  body: {
    type: 'Object',
    members: [
      {
        type: 'Member',
        name: {
          type: 'String',
          value: 'foo',
          loc: {
            start: { line: 1, column: 2, offset: 1 },
            end: { line: 1, column: 7, offset: 6 }
          }
        },
        value: {
          type: 'Number',
          value: 1,
          loc: {
            start: { line: 1, column: 9, offset: 8 },
            end: { line: 1, column: 10, offset: 9 }
          }
        },
        loc: {
          start: { line: 1, column: 2, offset: 1 },
          end: { line: 1, column: 10, offset: 9 }
        }
      }
    ],
    loc: {
      start: { line: 1, column: 1, offset: 0 },
      end: { line: 1, column: 11, offset: 10 }
    }
  },
  loc: {
    start: { line: 1, column: 1, offset: 0 },
    end: { line: 1, column: 11, offset: 10 }
  }
}

(with end positions slightly different. I just parsed without the trailing comma.)

What actually happened?

Uncaught:
UnexpectedToken [Error]: Unexpected token RBrace found. (1:11)
    at assertTokenType (file:///Users/hildjj/track/@eslint/json/node_modules/.pnpm/@humanwhocodes+momoa@3.2.2/node_modules/@humanwhocodes/momoa/dist/momoa.js:1716:19)
    at parseProperty (file:///Users/hildjj/track/@eslint/json/node_modules/.pnpm/@humanwhocodes+momoa@3.2.2/node_modules/@humanwhocodes/momoa/dist/momoa.js:1855:13)
    at parseObject (file:///Users/hildjj/track/@eslint/json/node_modules/.pnpm/@humanwhocodes+momoa@3.2.2/node_modules/@humanwhocodes/momoa/dist/momoa.js:1918:30)
    at parseValue (file:///Users/hildjj/track/@eslint/json/node_modules/.pnpm/@humanwhocodes+momoa@3.2.2/node_modules/@humanwhocodes/momoa/dist/momoa.js:2063:24)
    at Module.parse (file:///Users/hildjj/track/@eslint/json/node_modules/.pnpm/@humanwhocodes+momoa@3.2.2/node_modules/@humanwhocodes/momoa/dist/momoa.js:2075:21) {
  line: 1,
  column: 11,
  offset: 10
}

What do you think the solution is?

JSONC doesn't have a formal definition that I could find, but the one that @eslint/json points to is Microsoft's implementation. Both typescript and vscode accept trailing commas in their config files, so I believe allowTrailingComma should at least be an option for jsonc mode.

[nzakas]

I hadn't realized this, but it appears you are correct:
https://github.com/microsoft/node-jsonc-parser/blob/3c9b4203d663061d87d4d34dd0004690aef94db5/src/test/json.test.ts#L308

I'll take a look at adding this.

[hildjj]

Are you likely to want to make it an option, or just always accept trailing commas? I'd prefer always-accept, or for the option to default to always-accept, so that I don't have to ask @eslint/json for extra work. I would understand the opposite preference, however.

[nzakas]

I think always on is the right choice as that is probably what most people expect.

[nzakas]

Actually, it looks like the default is to not allow trailing commas. 🤔
https://github.com/microsoft/node-jsonc-parser/blob/3c9b4203d663061d87d4d34dd0004690aef94db5/src/impl/parser.ts#L22

I tested this in VS Code by switching a normal JSON file to JSONC and it flagged the trailing comma as a problem.

So, I think I should match that behavior and have an option that is disabled by default.