How to migrate Javascript webpack configuration to Typescript

It turned out that the newest versions (at least 5+) of webpack support Typescript out of the box. So the algorithm is next:

• Create tsconfig.json at the root level. Content:

  {
    "compilerOptions": {
      "module": "CommonJS",
      "target": "ES5",
      "esModuleInterop": true,
      "checkJs": false,
      "strict": true,
    }
  }
  

• Rename all *.js files to *.ts
• Type all of them:
  • no more ugly require, use import.
  • webpack package has typings out of the box.
    • You may find these types useful: Configuration & RuleSetRule
    • To enable devServer write this: 

      interface Configuration extends WebpackConfiguration {
        devServer?: WebpackDevServerConfiguration;
      }
      

  • Some of the popular plugins have types too.
  • Some of them don’t have types at all:
    • Create a *.d.ts file
    • Put there something like this:

      declare module 'postcss-assets' {
        export default function postcssAssets(opts: {
          basePath: string;
          relative: boolean;
        }): unknown;
      }
      

• Make sure your webpack.config.ts file is placed at the root level. I mean exactly at the same spot where node_modules is. Otherwise, you won’t be able to build it. No compilerOptions helped me. 
• Run webpack. It should work.