.gitignore

package-lock.json
node_modules

index.d.ts

/**Joins two strings.
 * @param {string} a The first string to join
 * @param {string} [b] The second string to join
 * @returns {string} text
 */
export default function join(a: string, b?: string): string;

index.js

/**Joins two strings.
 * @param {string} a The first string to join
 * @param {string} [b] The second string to join
 * @returns {string} text
 */
export default function join(a, b) {
  if (b) return a + b;
  return a;
}

package.json

{
  "name": "tsc-checkjs-dts-demo",
  "main": "index.js",
  "scripts": {
    "build": "tsc -d --emitDeclarationOnly --allowJs --checkJs index.js"
  },
  "devDependencies": {
    "typescript": "^3.7.0-beta"
  }
}