Type ‘Set’ can only be iterated through when using the ‘–downlevelIteration’ flag or with a ‘–target’ of ‘es2015’ or higher

The problem is that you want to use a feature that is only available in the ES2015 JavaScript version onwards.

The solution is to update the tsconfig.json file, changing "target" to at least "es2015" (also called "es6").

{
  "compilerOptions": {
    "target": "es2015",
    ...
  }
}

Source: TSConfig Target option