vim.cmd [[autocmd BufRead,BufNewFile tsconfig.json set filetype=jsonc]]

-- ftplugin/json.lua
vim.keymap.set('n', 'o', function()
  local line = vim.api.nvim_get_current_line()

  local should_add_comma = string.find(line, '[^,{[]$')
  if should_add_comma then
    return 'A,<cr>'
  else
    return 'o'
  end
end, { buffer = true, expr = true })