{
  "compilerOptions": {
    "declaration": true,
    "module": "commonjs",
    "outDir": "lib",
    "sourceMap": true,
    "target": "es6"
  },
  "files": [
    "src/index.ts"
   ]
}

{
  "compilerOptions": {
    "declaration": true,
    "module": "commonjs",
    "outDir": "lib",
    "sourceMap": true,
    "strictNullChecks": true,
    "target": "es6"
  },
  "files": [
    "src/index.ts"
   ]
}

Enable strictNullChecks TypeScript compiler option
Enable strictNullChecks TypeScript compiler option

https://www.typescriptlang.org/docs/release-notes/typescript-2.0.html#strictnullchecks
