Consistent field naming convention

Mixed casing in API response fields — userId alongside created_at in the same response object — forces every consumer to handle two naming conventions simultaneously. This is the classic sign of database column names leaking directly into the API layer without a serialization pass. It creates client-side bugs when consumers assume one convention and get the other, and it makes the API feel unfinished. iso-25010:2011 maintainability.modifiability is the direct impact: every client integration must special-case the inconsistency.

Pick one casing convention and enforce it with a serialization layer that transforms database field names before they leave the API. Add a transformer function in your handler files:

// Before -- mixed casing leaking from database:
{
  userId: row.user_id,
  created_at: row.created_at,  // forgot to transform
  firstName: row.first_name
}

// After -- consistent camelCase via transformer:
function toUserResponse(row) {
  return {
    userId: row.user_id,
    createdAt: row.created_at,
    firstName: row.first_name
  }
}

For larger codebases, use camelcase-keys or a serialization framework that enforces the convention globally.