Sadly most junior developers I've interviewed lack of proper foundations. That said my advice is to study and understand SOLID, and prepare for the stack required in the job you're applying to. For eg if a job asks for NodeJs, Express, Postgres and Angular knowledge/expertise/experience, you should be prepared to be questioned on those topics. If your expertises are in PHP o .Net and not NodeJs you shouldn't lie to yourself thinking you may succeed on that application.
Perhaps as a junior candidate you may not have all the expertise required, and that's ok. Just keep in mind that some other candidates who do know the stack will be over your application.