In the page source, we see a comment with the base64 encoded string
c3RvcHRoYXRqcw==

When we decrypt the string using python, we get
>>> import base64
>>> b64.decode('c3RvcHRoYXRqcw==')
>>> stopthatjs

We use "stopthatjs" as a password, and see the flag appear and quickly disappear.
However, the flag still appears in the page source:
ABCTF{no(d3)_js_is_s3cur3_dasjkhadbkjfbjfdjbfsdajfasdl}