How to use the @nuxt/utils.flatRoutes function in @nuxt/utils

if (this.options.router.mode !== 'hash') {
      try {
        generateRoutes = await promisifyRoute(
          this.options.generate.routes || [],
          ...args
        )
      } catch (e) {
        consola.error('Could not resolve routes')
        throw e // eslint-disable-line no-unreachable
      }
    }
    // Generate only index.html for router.mode = 'hash'
    let routes =
      this.options.router.mode === 'hash'
        ? ['/']
        : flatRoutes(this.options.router.routes)

    routes = routes.filter(route => this.options.generate.exclude.every(regex => !regex.test(route)))

    routes = this.decorateWithPayloads(routes, generateRoutes)

    // extendRoutes hook
    await this.nuxt.callHook('generate:extendRoutes', routes)

    return routes
  }