Usage

1. $ npm install downlevel-dts
2. $ npx downlevel-dts . ts3.4
3. To your package.json, add

"typesVersions": {
  "<=3.4.0-0": { "*": ["ts3.4/*"] }
}

4. $ cp tsconfig.json ts3.4/tsconfig.json

These instructions are modified and simplified from the Definitely Typed.