Can TypeScript import both ESM and CJS dependencies?

I am writing a TypeScript application that uses dependencies installed with NPM:

some of these dependencies export via CJS
some of these dependencies export via ESM

Is there a tsconfig.json configuration that would let me transparently use all these dependencies?

Solution

Yes it can!

For example, this is a fresh ES Module project with an ESM dependency (chalk) and a common.js dependency (aes-js):

package.json:

{
  "type": "module",
  "private": true,
  "scripts": {
    "build": "tsc",
    "start": "yarn build && node dist/index.js"
  },
  "license": "ISC",
  "devDependencies": {
    "@types/aes-js": "^3.1.1",
    "typescript": "^4.8.4"
  },
  "dependencies": {
    "aes-js": "^3.1.2",
    "chalk": "^5.1.2"
  }
}

index.ts:

import chalk from "chalk"
import aesjs from "aes-js"

console.log(chalk.blue(aesjs.utils.utf8.toBytes("this is a string")))

tsconfig.json

{
  "compilerOptions": {
    "target": "es2022",
    "module": "es2022",
    "esModuleInterop": true,
    "moduleResolution": "node",
    "forceConsistentCasingInFileNames": true,
    "strict": true,
    "skipLibCheck": true,
    "outDir": "dist"
  }
}

Explanation: esModuleInterop is your friend. It makes sure that module.exports = gets turned into a default export (CommonJS -> ES6) and vice versa, making sure default exports are the same as module.exports = (ES6 -> CommonJS)