zeppelin/backend/src/utils/getMissingPermissions.ts

import { Permissions } from "discord.js";

/**
 * @param resolvedPermissions A Permission object from e.g. GuildChannel#permissionsFor() or Member#permission
 * @param requiredPermissions Bitmask of required permissions
 * @return Bitmask of missing permissions
 */
export function getMissingPermissions(
  resolvedPermissions: Permissions | Readonly<Permissions>,
  requiredPermissions: number | bigint,
): bigint {
  const allowedPermissions = resolvedPermissions;
  const nRequiredPermissions = requiredPermissions;

  if (Boolean(allowedPermissions.bitfield & Permissions.FLAGS.ADMINISTRATOR)) {
    return BigInt(0);
  }

  return BigInt(nRequiredPermissions) & ~allowedPermissions.bitfield;
}