Clone the following Git repository on your machine:
https://github.com/liammclennan/JavaScript-Koans
Follow the instructions on that page on how to answer the Koans. You will need to open the main HTML page in a browser, and then fix the JS files one at a time based on what displayed on the browser.
Try to do as many Koans as you can. Notice, however, that some of them require knowledge that you might not have yet, like for example on Stacks and Queues. Just skip those.