next.js/packages/next/src/shared/lib/dynamic.tsx

import React from 'react'
import Loadable from './loadable.shared-runtime'

const isServerSide = typeof window === 'undefined'

type ComponentModule<P = {}> = { default: React.ComponentType<P> }

export declare type LoaderComponent<P = {}> = Promise<
  React.ComponentType<P> | ComponentModule<P>
>

export declare type Loader<P = {}> =
  | (() => LoaderComponent<P>)
  | LoaderComponent<P>

export type LoaderMap = { [module: string]: () => Loader<any> }

export type LoadableGeneratedOptions = {
  webpack?(): any
  modules?(): LoaderMap
}

export type DynamicOptionsLoadingProps = {
  error?: Error | null
  isLoading?: boolean
  pastDelay?: boolean
  retry?: () => void
  timedOut?: boolean
}

// Normalize loader to return the module as form { default: Component } for `React.lazy`.
// Also for backward compatible since next/dynamic allows to resolve a component directly with loader
// Client component reference proxy need to be converted to a module.
function convertModule<P>(mod: React.ComponentType<P> | ComponentModule<P>) {
  return { default: (mod as ComponentModule<P>)?.default || mod }
}

export type DynamicOptions<P = {}> = LoadableGeneratedOptions & {
  loading?: (loadingProps: DynamicOptionsLoadingProps) => React.ReactNode
  loader?: Loader<P> | LoaderMap
  loadableGenerated?: LoadableGeneratedOptions
  ssr?: boolean
}

export type LoadableOptions<P = {}> = DynamicOptions<P>

export type LoadableFn<P = {}> = (
  opts: LoadableOptions<P>
) => React.ComponentType<P>

export type LoadableComponent<P = {}> = React.ComponentType<P>

export function noSSR<P = {}>(
  LoadableInitializer: LoadableFn<P>,
  loadableOptions: DynamicOptions<P>
): React.ComponentType<P> {
  // Removing webpack and modules means react-loadable won't try preloading
  delete loadableOptions.webpack
  delete loadableOptions.modules

  // This check is necessary to prevent react-loadable from initializing on the server
  if (!isServerSide) {
    return LoadableInitializer(loadableOptions)
  }

  const Loading = loadableOptions.loading!
  // This will only be rendered on the server side
  return () => (
    <Loading error={null} isLoading pastDelay={false} timedOut={false} />
  )
}

/**
 * This function lets you dynamically import a component.
 * It uses [React.lazy()](https://react.dev/reference/react/lazy) with [Suspense](https://react.dev/reference/react/Suspense) under the hood.
 *
 * Read more: [Next.js Docs: `next/dynamic`](https://nextjs.org/docs/app/building-your-application/optimizing/lazy-loading#nextdynamic)
 */
export default function dynamic<P = {}>(
  dynamicOptions: DynamicOptions<P> | Loader<P>,
  options?: DynamicOptions<P>
): React.ComponentType<P> {
  let loadableFn = Loadable as LoadableFn<P>

  let loadableOptions: LoadableOptions<P> = {
    // A loading component is not required, so we default it
    loading: ({ error, isLoading, pastDelay }) => {
      if (!pastDelay) return null
      if (process.env.NODE_ENV !== 'production') {
        if (isLoading) {
          return null
        }
        if (error) {
          return (
            <p>
              {error.message}
              <br />
              {error.stack}
            </p>
          )
        }
      }
      return null
    },
  }

  // Support for direct import(), eg: dynamic(import('../hello-world'))
  // Note that this is only kept for the edge case where someone is passing in a promise as first argument
  // The react-loadable babel plugin will turn dynamic(import('../hello-world')) into dynamic(() => import('../hello-world'))
  // To make sure we don't execute the import without rendering first
  if (dynamicOptions instanceof Promise) {
    loadableOptions.loader = () => dynamicOptions
    // Support for having import as a function, eg: dynamic(() => import('../hello-world'))
  } else if (typeof dynamicOptions === 'function') {
    loadableOptions.loader = dynamicOptions
    // Support for having first argument being options, eg: dynamic({loader: import('../hello-world')})
  } else if (typeof dynamicOptions === 'object') {
    loadableOptions = { ...loadableOptions, ...dynamicOptions }
  }

  // Support for passing options, eg: dynamic(import('../hello-world'), {loading: () => <p>Loading something</p>})
  loadableOptions = { ...loadableOptions, ...options }

  const loaderFn = loadableOptions.loader as () => LoaderComponent<P>
  const loader = () =>
    loaderFn != null
      ? loaderFn().then(convertModule)
      : Promise.resolve(convertModule(() => null))

  // coming from build/babel/plugins/react-loadable-plugin.js
  if (loadableOptions.loadableGenerated) {
    loadableOptions = {
      ...loadableOptions,
      ...loadableOptions.loadableGenerated,
    }
    delete loadableOptions.loadableGenerated
  }

  // support for disabling server side rendering, eg: dynamic(() => import('../hello-world'), {ssr: false}).
  if (typeof loadableOptions.ssr === 'boolean' && !loadableOptions.ssr) {
    delete loadableOptions.webpack
    delete loadableOptions.modules

    return noSSR(loadableFn, loadableOptions)
  }

  return loadableFn({ ...loadableOptions, loader: loader as Loader<P> })
}