Function assignDefaultsAndValidate

packages/next/src/server/config.ts:275–1453

* Assigns defaults to the user config and validates the config. * * @param dir - The directory of the project. * @param userConfig - The user config. * @param silent - Whether to suppress warnings. * @returns The complete config.

(
  dir: string,
  userConfig: NextConfig & { configFileName: string },
  silent: boolean,
  phase: PHASE_TYPE
)

 * @returns The complete config.
 */
function assignDefaultsAndValidate(
  dir: string,
  userConfig: NextConfig & { configFileName: string },
  silent: boolean,
  phase: PHASE_TYPE
): NextConfigComplete {
  const configFileName = userConfig.configFileName
  if (typeof (userConfig as any).exportTrailingSlash !== 'undefined') {
    if (!silent) {
      Log.warn(
        `The "exportTrailingSlash" option has been renamed to "trailingSlash". Please update your ${configFileName}.`
      )
    }
    if (typeof userConfig.trailingSlash === 'undefined') {
      userConfig.trailingSlash = (userConfig as any).exportTrailingSlash
    }
    delete (userConfig as any).exportTrailingSlash
  }

  const config = Object.keys(userConfig).reduce<{ [key: string]: any }>(
    (currentConfig, key) => {
      const value = (userConfig as any)[key]

      if (value === undefined || value === null) {
        return currentConfig
      }

      if (key === 'distDir') {
        if (typeof value !== 'string') {
          throw new Error(
            `Specified distDir is not a string, found type "${typeof value}"`
          )
        }
        const userDistDir = value.trim()

        // don't allow public as the distDir as this is a reserved folder for
        // public files
        if (userDistDir === 'public') {
          throw new Error(
            `The 'public' directory is reserved in Next.js and can not be set as the 'distDir'. https://nextjs.org/docs/messages/can-not-output-to-public`
          )
        }
        // make sure distDir isn't an empty string as it can result in the provided
        // directory being deleted in development mode
        if (userDistDir.length === 0) {
          throw new Error(
            `Invalid distDir provided, distDir can not be an empty string. Please remove this config or set it to undefined`
          )
        }
      }

      if (key === 'pageExtensions') {
        if (!Array.isArray(value)) {
          throw new Error(
            `Specified pageExtensions is not an array of strings, found "${value}". Please update this config or remove it.`
          )
        }