If you ever open the developer console of Facebook so, you found that Facebook has written a message which alerts the user that do not attempt to run any malicious code if you doesn’t have an understanding of it.
It is quite a simple technique only and only needs 2 lines of JavaScript which can do this job for you.
This message will be shown once the page gets loaded and will not shows the line number and filename same as Facebook but if you like to show the line number and filename with the message then just use the code as mentioned below: