User types in field & outputs certain text in new line (JavaScript)

Background on questions - I am trying to create a fighting game notation converter with javascript but I am getting no where. I cannot find online anywhere with something like this which I feel like there probably is plenty of stuff like this! But I am new to javascript so I am probably searching poorly.

Question - How would you code/write in javascript something that would have a user write/copy paste some text into a field for example 2H, 6H, 236S and it would output on a line below the following Down B, Forward B, Quarter Circle Forward A.

I have ideas for this like creating conditionals like

if (input === 2H) { return 'Down B'};

But I just don't know where to start and how to write it correctly. I don't mind trying stuff on my own so if you know a guide that would be similar to this so I can learn feel free to send me one or if you want to write some sample code that could point me in the right direction! Anything would help!