tsconfig.json

{
    "compilerOptions": {
      /* Basic Options */
      "target": "es6",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017','ES2018' or 'ESNEXT'. */
      "module": "commonjs",                     /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */

      /* Strict Type-Checking Options */
      "strict": true,                           /* Enable all strict type-checking options. */

      /* Module Resolution Options */
      "esModuleInterop": true,                  /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */

      /* https://github.com/apollographql/graphql-subscriptions/issues/83 */
      "lib": [
        "es6",
        "esnext.asynciterable"
      ]
    },
    "files": [
      "index.ts"
    ]
  }