Having been through this many times before, my one piece of advice - is breathe. Take that deep breath before getting yourself all worked up - remind yourself that you are a junior developer and you are a junior for a reason - even the best senior developers don't know everything.
If you haven't written it and are seeing it for the first time, you need to learn the syntax before you can jump in and start doing something with it. Hopefully, you have a manager where you are not the first junior they are leading where the focus should be on growth and learning and not that you know everything.
What helped me most was to then break it out into small tasks into what I needed to learn. If I didn't know it, I might create a little test app to replicate functionality so they can I could better handle myself. Looking back, many tasks are "thrown" to juniors because the person assigning it knows they are time intensive and no one else wants to look at them (good to remember). This has a big benefit for you because if you become knowledgeable in that area, all of a sudden you're the expert!
As a Manager now, the only time I get worried about a junior developer is if they are looking at their own code and go - "I don't understand what I wrote" :) But that's a different issue/problem to solve.
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