Tried and True; Troubleshooting.
The key to becoming a great developer is learning how to troubleshoot effectively.
Every language and platform has built-in methods for troubleshooting. For example — Javascript has console.log() for printing things into the console within dev tools ...
blog.millertchris.com2 min read
Nate Sire
Coding in C++ Since Eight, Be Great
I'd use xdebug for PHP. Or Node inspector for JS. Clojure has one of the best tools... Cider for SpaceMacs evaluates your code as you type.