The fullstack stock ticker charting app allows the user to search for stock prices by ticker symbols. It uses ChartJS to chart the results and uses socket.io to update in real time any browsers viewing the application. I used a stock market API called AlphaAdvantage to query for stock prices.
This was my favorite of all of the freeCodeCamp backend projects. I think mostly because of how fun web sockets are to work with. There was also no need to store anything in a database, so that simplified things quite a bit.