Make sure you're using latest LTS version of node and run npm install.
Each day, download your puzzle input and save it in ./src/data with the name
dayX.txt where X is the relevant day, such as day1.txt.
Then make a module in ./src/puzzle named dayX.ts where X is the relevant
day.
In that module, export functions called part1 and part2 which take an input
string and return a string.
Add tests by making a dayX.test.ts file in ./src/puzzle.
Once you've solved a puzzle, it's good to add a test for the correct answer so you can refactor and know your code is still working.
Run npm run dev to get an interactive app which dynamically includes whichever
days you have modules for in ./src/puzzle.
The button labeled Set to official text in the app will load the text you
saved to the dayX.txt file.
Just run npm run test.