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

import webpack from 'webpack'
import { RawSource } from 'webpack-sources'
import { join, relative, dirname } from 'path'
import {IS_BUNDLED_PAGE_REGEX} from '../../../lib/constants'

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 {
  constructor (options) {
    this.options = options
  }
  apply (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) => {
      compilation.mainTemplate.hooks.localVars.intercept({
        register (tapInfo) {
          if (tapInfo.name === 'MainTemplate') {
            const originalFn = tapInfo.fn
            tapInfo.fn = (source, chunk) => {
              // 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 (!IS_BUNDLED_PAGE_REGEX.exec(chunk.name)) {
                return originalFn(source, chunk)
              }
              const pagePath = join(outputPath, dirname(chunk.name))
              const 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.Template.asString([
                source,
                '// The module cache',
                `var installedModules = require('${relativePathToBaseDirNormalized}');`
              ])
            }
          }
          return tapInfo
        }
      })
    })
  }
}