How to create a url string with query parameters from an object in Angular 5+?

You can use just Router and UrlSerializer:

constructor(private router: Router, private serializer: UrlSerializer) {
  const tree = router.createUrlTree([], { queryParams: { foo: 'a', bar: 42 } });
  console.log(serializer.serialize(tree)); // "/?foo=a&bar=42"
}

See demo: https://stackblitz.com/edit/angular-basic-template-3hx9at?file=src/app/hello.component.ts