They usually require a special "trick" to solve correctly
​HackerRank​
​LeetCode​
​CareerCup​
​Interview Cake​
​InterviewBit​
JavaScript seems to be using ES5 (2009)
No use of const/let
Go to meetups​
​coderpad.io​
​collabedit​
karak.io
When using these, ask wether the interviewer if they are comfortable with pseudo code, or is correct syntax/semantics important.