Function functionalUpdate

packages/router-core/src/utils.ts:204–213

(
  updater: Updater<TPrevious, TResult> | NonNullableUpdater<TPrevious, TResult>,
  previous: TPrevious,
)

 * Accepts either a literal value or a function of the previous value.
 */
export function functionalUpdate<TPrevious, TResult = TPrevious>(
  updater: Updater<TPrevious, TResult> | NonNullableUpdater<TPrevious, TResult>,
  previous: TPrevious,
): TResult {
  if (isFunction(updater)) {
    return updater(previous)
  }

  return updater
}

const hasOwn = Object.prototype.hasOwnProperty
const isEnumerable = Object.prototype.propertyIsEnumerable