Custom error classes extend Error

Throwing strings, numbers, or plain objects instead of Error instances destroys the stack trace entirely — the thrown value arrives at the catch handler with no indication of where in the call stack the problem originated. ISO 25010:2011 §6.5.4 classifies this as a reliability defect. Beyond diagnostics, plain-object throws break instanceof checks that callers use to distinguish error types: a catch handler checking if (err instanceof NotFoundError) silently falls through when the throw was { code: 404, text: '...' }.

Always throw Error instances. For typed error handling, define custom classes that extend Error and set the prototype correctly for correct instanceof behavior after TypeScript compilation:

export class UserNotFoundError extends Error {
  constructor(public readonly userId: string) {
    super(`User not found: ${userId}`)
    this.name = 'UserNotFoundError'
    // Required for instanceof to work correctly when compiled to ES5:
    Object.setPrototypeOf(this, new.target.prototype)
  }
}

// Throw:
throw new UserNotFoundError(id)

// Catch with discrimination:
if (error instanceof UserNotFoundError) {
  return Response.json({ error: 'User not found' }, { status: 404 })
}