# Remove Parameter

> Delete a parameter the method body no longer uses, updating every caller and override so the signature states only real dependencies.

- kind: technique
- category: simplifying-method-calls
- aliases: Change Function Declaration
- canonical: https://aipatterns.dev/refactoring/techniques/remove-parameter
- inspired-by: https://refactoring.guru/remove-parameter

## Problem

A method takes a parameter it ignores. It was used once, or added "in case", and now every caller has to compute and pass a value that changes nothing. Readers assume it matters.

## Solution

**Remove the unused parameter from the method and from every call site.**

## Before

```ts
function scheduleReminder(
  appointment: Appointment,
  leadMinutes: number,
  timezone: string, // unused since reminders moved to UTC
  channel: "sms" | "email",
): Job {
  const runAt = new Date(appointment.startsAt.getTime() - leadMinutes * 60_000);
  return queue.enqueue("reminder", { appointmentId: appointment.id, channel }, runAt);
}

scheduleReminder(appt, 60, clinic.timezone, "sms");
```

## After

```ts
function scheduleReminder(
  appointment: Appointment,
  leadMinutes: number,
  channel: "sms" | "email",
): Job {
  const runAt = new Date(appointment.startsAt.getTime() - leadMinutes * 60_000);
  return queue.enqueue("reminder", { appointmentId: appointment.id, channel }, runAt);
}

scheduleReminder(appt, 60, "sms");
```

## Why

- A signature is a contract. Every parameter in it claims the method depends on that value.
- Callers stop computing values that are thrown away, which sometimes removes whole lookups.

## Steps

1. Confirm the parameter is unused in the body and in every override and implementation. A parameter unused in the base class may be used in a subclass.
2. If the method is overridden or implements an interface, change all declarations together.
3. If your tooling has a safe "change signature" refactoring, use it. Run the tests.
4. Otherwise create a new method without the parameter, make the old one delegate to it, and move callers over a few at a time. Run the tests after each batch.
5. Delete the old signature when nothing calls it.

## Watch out

- Do not remove a parameter required by an interface, callback type or framework hook (Express middleware `next`, event handler signatures). Prefix it with `_` to signal it is intentionally unused.
- Positional removal in JavaScript shifts later arguments. Callers passing values by position will send the wrong value into the wrong slot without a type error if the types match. Use the type checker, and update all callers in one change.
- Public APIs need a deprecation period. Keep an overload that accepts and ignores the old argument.
- Check whether computing the argument had side effects at the call site that callers still rely on.
- The inverse technique is [Add Parameter](https://aipatterns.dev/refactoring/techniques/add-parameter.md).

## Related

- Inverse: [Add Parameter](https://aipatterns.dev/refactoring/techniques/add-parameter.md).
- Fixes [Long Parameter List](https://aipatterns.dev/refactoring/smells/long-parameter-list.md), [Speculative Generality](https://aipatterns.dev/refactoring/smells/speculative-generality.md) and [Dead Code](https://aipatterns.dev/refactoring/smells/dead-code.md).
- When the value is still needed but derivable, use [Replace Parameter with Method Call](https://aipatterns.dev/refactoring/techniques/replace-parameter-with-method-call.md).

## Agent checklist

- [ ] The parameter is unused in every override and implementation.
- [ ] It is not required by an external interface, callback type or framework.
- [ ] All positional callers were updated in the same change.
- [ ] Argument expressions removed from call sites had no side effects callers depend on.