Parameters (base)

• Selector: parameter
• Allowed: camelCase
• Leading underscore: allowed

Why This Rule

Function parameters should follow standard JavaScript/TypeScript conventions:

• camelCase: Standard JavaScript/TypeScript convention for parameters you define
• Leading underscore: Conventional signal for intentionally unused parameters (e.g., _event, _req)

Why not snake_case?

Unlike destructured parameters (which preserve external naming), regular parameters are part of your function's API. They should follow idiomatic JavaScript naming:

// ❌ Bad - snake_case isn't idiomatic for JS/TS parameters
function processUser(user_id: string, first_name: string) {
  // WHY: snake_case parameters are non-idiomatic and inconsistent
  return `${first_name} (${user_id})`;
}

// ✅ Good - camelCase is the JavaScript standard
function processUser(userId: string, firstName: string) {
  // WHY: camelCase parameters are idiomatic and readable
  return `${firstName} (${userId})`;
}

What about external APIs?

Use destructured parameters instead - they preserve the original naming from external sources:

// ✅ Good - destructuring preserves API shape
function processUser({ user_id, first_name }: ApiUser) {
  // WHY: destructured names preserve external contract and avoid noisy renaming
  return `${first_name} (${user_id})`;
}

The leading underscore is especially valuable in TypeScript where the noUnusedLocals compiler option would error on genuinely unused parameters (like in interface implementations).

References:

• ESLint no-unused-vars - Ignoring with underscore
• TypeScript Handbook - Unused Parameters

✅ Good

function fn(userId: string, _unused: number, firstName: string) {
  // WHY: underscores allowed for intentionally unused params
  return userId + firstName;
}

❌ Bad

// PascalCase not allowed
function fn(UserId: string) {
  // WHY: PascalCase for parameter looks like a type/class, not a value
  return UserId;
}

// snake_case not allowed for base parameters
function fn(user_id: string) {
  // WHY: snake_case breaks JS/TS convention for parameter names
  return user_id;
}