next.js/packages/next/src/server/lib/i18n-provider.ts

import type { DomainLocale, I18NConfig } from '../config-shared'
import { getRequestMeta, type NextIncomingMessage } from '../request-meta'

/**
 * The result of matching a locale aware route.
 */
export interface LocaleAnalysisResult {
  /**
   * The pathname without the locale prefix (if any).
   */
  pathname: string

  /**
   * The detected locale. If no locale was detected, this will be `undefined`.
   */
  detectedLocale?: string

  /**
   * True if the locale was inferred from the default locale.
   */
  inferredFromDefault: boolean
}

type LocaleAnalysisOptions = {
  /**
   * When provided, it will be used as the default locale if the locale
   * cannot be inferred from the pathname.
   */
  defaultLocale?: string
}

/**
 * The I18NProvider is used to match locale aware routes, detect the locale from
 * the pathname and hostname and normalize the pathname by removing the locale
 * prefix.
 */
export class I18NProvider {
  private readonly lowerCaseLocales: ReadonlyArray<string>
  private readonly lowerCaseDomains?: ReadonlyArray<
    DomainLocale & {
      // The configuration references a domain with an optional port, but the
      // hostname is always the domain without the port and is used for
      // matching.
      hostname: string
    }
  >

  constructor(public readonly config: Readonly<I18NConfig>) {
    if (!config.locales.length) {
      throw new Error('Invariant: No locales provided')
    }

    this.lowerCaseLocales = config.locales.map((locale) => locale.toLowerCase())
    this.lowerCaseDomains = config.domains?.map((domainLocale) => {
      const domain = domainLocale.domain.toLowerCase()
      return {
        defaultLocale: domainLocale.defaultLocale.toLowerCase(),
        hostname: domain.split(':', 1)[0],
        domain,
        locales: domainLocale.locales?.map((locale) => locale.toLowerCase()),
        http: domainLocale.http,
      }
    })
  }

  /**
   * Detects the domain locale from the hostname and the detected locale if
   * provided.
   *
   * @param hostname The hostname to detect the domain locale from, this must be lowercased.
   * @param detectedLocale The detected locale to use if the hostname does not match.
   * @returns The domain locale if found, `undefined` otherwise.
   */
  public detectDomainLocale(
    hostname?: string,
    detectedLocale?: string
  ): DomainLocale | undefined {
    if (!hostname || !this.lowerCaseDomains || !this.config.domains) return

    if (detectedLocale) detectedLocale = detectedLocale.toLowerCase()

    for (let i = 0; i < this.lowerCaseDomains.length; i++) {
      const domainLocale = this.lowerCaseDomains[i]
      if (
        // We assume that the hostname is already lowercased.
        domainLocale.hostname === hostname ||
        // Configuration validation ensures that the locale is not repeated in
        // other domains locales.
        domainLocale.locales?.some((locale) => locale === detectedLocale)
      ) {
        return this.config.domains[i]
      }
    }

    return
  }

  /**
   * Pulls the pre-computed locale and inference results from the query
   * object.
   *
   * @param req the request object
   * @param pathname the pathname that could contain a locale prefix
   * @returns the locale analysis result
   */
  public fromRequest(
    req: NextIncomingMessage,
    pathname: string
  ): LocaleAnalysisResult {
    const detectedLocale = getRequestMeta(req, 'locale')

    // If a locale was detected on the query, analyze the pathname to ensure
    // that the locale matches.
    if (detectedLocale) {
      const analysis = this.analyze(pathname)

      // If the analysis contained a locale we should validate it against the
      // query and strip it from the pathname.
      if (analysis.detectedLocale) {
        if (analysis.detectedLocale !== detectedLocale) {
          throw new Error(
            `Invariant: The detected locale does not match the locale in the query. Expected to find '${detectedLocale}' in '${pathname}' but found '${analysis.detectedLocale}'}`
          )
        }

        pathname = analysis.pathname
      }
    }

    return {
      pathname,
      detectedLocale,
      inferredFromDefault:
        getRequestMeta(req, 'localeInferredFromDefault') ?? false,
    }
  }

  /**
   * Analyzes the pathname for a locale and returns the pathname without it.
   *
   * @param pathname The pathname that could contain a locale prefix.
   * @param options The options to use when matching the locale.
   * @returns The matched locale and the pathname without the locale prefix
   *          (if any).
   */
  public analyze(
    pathname: string,
    options: LocaleAnalysisOptions = {}
  ): LocaleAnalysisResult {
    let detectedLocale: string | undefined = options.defaultLocale

    // By default, we assume that the default locale was inferred if there was
    // no detected locale.
    let inferredFromDefault = typeof detectedLocale === 'string'

    // The first segment will be empty, because it has a leading `/`. If
    // there is no further segment, there is no locale (or it's the default).
    const segments = pathname.split('/', 2)
    if (!segments[1])
      return {
        detectedLocale,
        pathname,
        inferredFromDefault,
      }

    // The second segment will contain the locale part if any.
    const segment = segments[1].toLowerCase()

    // See if the segment matches one of the locales. If it doesn't, there is
    // no locale (or it's the default).
    const index = this.lowerCaseLocales.indexOf(segment)
    if (index < 0)
      return {
        detectedLocale,
        pathname,
        inferredFromDefault,
      }

    // Return the case-sensitive locale.
    detectedLocale = this.config.locales[index]
    inferredFromDefault = false

    // Remove the `/${locale}` part of the pathname.
    pathname = pathname.slice(detectedLocale.length + 1) || '/'

    return {
      detectedLocale,
      pathname,
      inferredFromDefault,
    }
  }
}