rsnext/packages/next/build/webpack/plugins/nextjs-ssr-module-cache.ts

import webpack from 'webpack'
import { RawSource } from 'webpack-sources'
import { join, relative, dirname } from 'path'
import getRouteFromEntrypoint from '../../../next-server/server/get-route-from-entrypoint'
const SSR_MODULE_CACHE_FILENAME = 'ssr-module-cache.js'

// By default webpack keeps initialized modules per-module.
// This means that if you have 2 entrypoints loaded into the same app
// they will *not* share the same instance
// This creates many issues when developers / libraries rely on the singleton pattern
// As this pattern assumes every module will have 1 instance
// This plugin overrides webpack's code generation step to replace `installedModules`
// The replacement is a require for a file that's also generated here that only exports an empty object
// Because of Node.js's single instance modules this makes webpack share all initialized instances
// Do note that this module is only geared towards the `node` compilation target.
// For the client side compilation we use `runtimeChunk: 'single'`
export default class NextJsSsrImportPlugin {
  private options: { outputPath: string }

  constructor(options: { outputPath: string }) {
    this.options = options
  }
  apply(compiler: webpack.Compiler) {
    const { outputPath } = this.options
    compiler.hooks.emit.tapAsync(
      'NextJsSSRModuleCache',
      (compilation, callback) => {
        compilation.assets[SSR_MODULE_CACHE_FILENAME] = new RawSource(`
      /* This cache is used by webpack for instantiated modules */
      module.exports = {}
      `)
        callback()
      }
    )
    compiler.hooks.compilation.tap(
      'NextJsSSRModuleCache',
      (compilation: any) => {
        compilation.mainTemplate.hooks.localVars.intercept({
          register(tapInfo: any) {
            if (tapInfo.name === 'MainTemplate') {
              const originalFn = tapInfo.fn
              tapInfo.fn = (source: any, chunk: any) => {
                // If the chunk is not part of the pages directory we have to keep the original behavior,
                // otherwise webpack will error out when the file is used before the compilation finishes
                // this is the case with mini-css-extract-plugin

                if (!getRouteFromEntrypoint(chunk.name)) {
                  return originalFn(source, chunk)
                }
                const pagePath = join(outputPath, dirname(chunk.name))
                let relativePathToBaseDir = relative(
                  pagePath,
                  join(outputPath, SSR_MODULE_CACHE_FILENAME)
                )

                // Make sure even in windows, the path looks like in unix
                // Node.js require system will convert it accordingly
                const relativePathToBaseDirNormalized = relativePathToBaseDir.replace(
                  /\\/g,
                  '/'
                )
                return (webpack as any).Template.asString([
                  source,
                  '// The module cache',
                  `var installedModules = require('${relativePathToBaseDirNormalized}');`,
                ])
              }
            }
            return tapInfo
          },
        })
      }
    )
  }
}