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

generateRoutes.forEach((route) => {
      // route is either a string or like { route : '/my_route/1', payload: {} }
      const path = isString(route) ? route : route.route
      routeMap[path] = {
        route: path,
        payload: route.payload || null
      }
    })
    return Object.values(routeMap)

.map((serverMiddleware) => {
        if (isString(serverMiddleware)) {
          return serverMiddleware
        }
        if (isPureObject(serverMiddleware) && isString(serverMiddleware.handler)) {
          return serverMiddleware.handler
        }
      })
      .filter(Boolean)

.map((serverMiddleware) => {
        if (isString(serverMiddleware)) {
          return serverMiddleware
        }
        if (isPureObject(serverMiddleware) && isString(serverMiddleware.handler)) {
          return serverMiddleware.handler
        }
      })
      .filter(Boolean)