How can I add a terminal in web page to test user's code?That just like codecademy that provide a interactive way to learn programing

Now, I am trying to build my website. I want to make it to help user learn programing, and the webpage can get user's code and show the output. It works just like "codecademy.com". I have tried to find answer by google, but I just find repl which is not free. Is there any open-source method can help me? THANKS for your answer!