Checking for the types explicitly in your code creates a brittle system. For a one-off bit of code, fine. For anything that may be extended in the future, it's a bad idea and will have to be refactored to become maintainable.
It's fine when you have related types, but in the example `someFunction` accepts both ThisClass and str, which (I assume) have very different functionality.
You see this anti-pattern a lot in dynamically-typed languages. As an example, both these examples are valid and do the same thing in `knex`, an SQL query builder for node:
knex.select('name', 'email').from('users')
knex.select(['name', 'email']).from('users')