Ask HN: Reading someone else's code and bug fixing

Hey guys, What is your process of reading a code someone else wrote (particularly in JavaScript) and applying fixes?

I am working on a JavaScript SPA product that is more than a decade old with huge code base and a lot of service integrations.

I am new in the team, sometimes a bug fix (compromising of just 1-2 lines) takes me a couple of hours to finish. Starting from understanding the issue, reading the code pertaining to that area, applying the fix and then testing different use case to make sure the bug is fixed before handing it over to QA.

I want to be efficient and as quick as possible and wondering if you guys share your process of debugging and fixing bugs for big products.