@nestjs/config/dist/interfaces/config-change-event.interface.d.ts

/**
 * Represents a change in the configuration object.
 * Dispatched when one updates the configuration object through the `ConfigService#set` method.
 * @publicApi
 */
export interface ConfigChangeEvent<OldValue = any, NewValue = any> {
    path: string;
    oldValue: OldValue;
    newValue: NewValue;
}