Tools: 🐛 formatter should not remove generic trailing comma in tsx

[yeonjuan]

Environment

$ rome rage --summary
Environment Variables
  Object {
    COLORTERM: "truecolor"
    LANG: "en_US.UTF-8"
    TERM: "xterm-256color"
    TERM_PROGRAM: "vscode"
  }

User Config
  unset

Rome Version
  10.0.4-beta-dev

Node Version
  12.16.3

Platform
  darwin x64 18.7.0

Terminal Features
  Object {
    background: "unknown"
    colorDepth: 24
    columns: 170
    cursor: true
    hyperlinks: true
    isTTY: true
    progress: true
    unicode: true
  }

Client Flags
  Object {
    clientName: "cli"
    silent: false
    cwd: ___R$project$rome$$internal$path$index_ts$AbsoluteFilePath: /Users/yeonjuan/Desktop/open-source/tools
    realCwd: ___R$project$rome$$internal$path$index_ts$AbsoluteFilePath: /Users/yeonjuan/Desktop/open-source/tools
  }

Server Status
  Object {
    projects: Array [
      Object {id: 0}
      Object {id: 1}
    ]
    server: Object {
      heapTotal: 27_672_576
      pid: 84_558
      uptime: 0.781767559
    }
    workers: Array [
      Object {
        astCacheSize: 0
        heapTotal: 27_672_576
        ownedBytes: 0n
        ownedFileCount: 0
        pid: 84_558
        uptime: 0.78147324
      }
    ]
  }

Steps To Reproduce

1. write below example in a file with .tsx extension

// test.tsx
const func = <T,>() => {
}

1. run formatter

$ ./rome format test.tsx

• output

const func = <T>() => {}; // ts error: JSX Element `T` has no corresponding closing tag

Expected Results

The result makes ts parsing error so it should not remove trailing comma.

const func = <T,>() => {}; 

[ematipico]

At the beginning I thought it was a typescript bug it's not! Although it seems strange having a trailing comma

[antonk52]

it's the only way for typescript to differentiate the initial type assertion syntax in tsx file with jsx ie

// *.ts
const a = <string>bar;
const b = bar as string;

// *.tsx

// error
const a = <string>bar;

// fine
const a = <string,>bar;
const b = bar as string;

They've introduced the as syntax in v1.6 to preserve assertions in tsx files, the comma trick was introduced later on as far as I know