To be productive in a new web programming language, you only need to understand four things.
How do to find, install and load the languages libraries; how to reference the library’s methods.
How do you print an output message? How do you take a substring? (related: master regular expressions because most modern programming languages use them for string matching.)
Accessing and creating JSON documents.
Most server-to-server communication is done with JSON documents these days. Make sure you know how to put things into JSON format and get things out of a JSON object (or as Stack Overflow will probably tell you “a string that just happens to be a JSON object”).
Those aforementioned JSON documents flow via REST API requests, all bundled up into HTTP requests. Know how to create them, and how to troubleshoot them (this is where knowledge of networks and security comes in useful).