This also won't work if you run calculator("String one", " String two", "+")

I suggest you change the description as you give the answer directly in it!

Thanks, realised my mistake, have solved it now :)
Code was ok, but mathematics was lacking!

I am receiving an error when passing all the tests:

FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory
Aborted (core dumped)
Exit code 134

There doesn't seem to be much explanation at all on google
Any suggestions?

Is there an advantage to having the i variable outside of the for()? I have learned that it'd be best to create the variable as part of the loop, to avoid scope pollution (i.e., for(let i=0; i<len; i++))

Pretty pathetic dragons if they only need 2 bullets to be killed...