Background: it is not difficult to introduce vulnerabilities in backend/API code when not coding with security in mind. This typically leads to vulnerabilities such as XSS or various injections (or others).
It is also possible that this backend/API code is intentionally written as malicious - this is something I put on the side, concentrating on the first case.
Question: is it possible (and if so - how) to introduce vulnerabilities via a sloppy Javascript code? In other words: is there Javascript code which can be used by an attacker in a browser context, and in that browser context alone?
Vulnerabilities which lead to server-side issues (XSS, ...) are not in scope, I am wondering about a scenario where the attack would be made possible because of flaws in JS code per se.
In contrast, I know how to use malicious JS code to steal information from the DOM (in the case of, for instance, some widget portal where the JS code running is not vetted). This is not the case I am interested in.