tsconfig.json

{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,

    // if you'd like to warn if you're using modern features, change these
    // both to e.g., "es2017"
    "module": "NodeNext",
    "target": "ESNext",
    "moduleResolution": "NodeNext",

    // proper es6 support
    "esModuleInterop": true,
    "allowSyntheticDefaultImports": true,

    // configure as you like: these are my preferred defaults!
    "strict": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,

    // "strict" implies this, and it's good for new projects, but difficult to retrofit to old
    // untyped projects
    "noImplicitAny": true,

    // modern TS stuff that allows lit-element to work
    "experimentalDecorators": true,
    "useDefineForClassFields": false,
    "allowImportingTsExtensions": true
  },
  "include": ["*.ts", "**/*.ts"]
}