utctf - Fast Fox
Initially attempted to log the window variable which can give an indication of how limited the code execution is.
console.log( window );
errors: 510343821206210853.js:1:1 ReferenceError: window is not defined Stack: @510343821206210853.js:1:1
Picking this function at random to attempt to test and check that whatever is executing input actually IS the JSShell which this page is documentation for. If there is an error when running gc() then the assumption was wrong otherwise the documentation page will definitely have some useful functions
The function didn't throw an error! So remember the goal is to read a "/flag.txt" file. With that in mind the function shown below could be useful.
console.log( read( "/flag.txt" ) )