Function normalizeUserOnWarn

packages/vite/src/node/build.ts:1182–1195

(
  userOnWarn: WarningHandlerWithDefault,
  defaultHandler: LogOrStringHandler,
)

}

function normalizeUserOnWarn(
  userOnWarn: WarningHandlerWithDefault,
  defaultHandler: LogOrStringHandler,
): LogOrStringHandler {
  return (logLevel, logging) => {
    if (logLevel === 'warn') {
      userOnWarn(normalizeLog(logging), (log) =>
        defaultHandler('warn', typeof log === 'function' ? log() : log),
      )
    } else {
      defaultHandler(logLevel, logging)
    }
  }
}

const normalizeLog = (log: RollupLog | string): RollupLog =>
  typeof log === 'string' ? { message: log } : log