The first code example on that page claims to solve "the 3SUM problem".

According to [1], "the 3SUM problem asks if a given set of n real numbers contains three elements that sum to zero."

It's not clear to me what problem the Janet code solves but it's clearly not that 3SUM problem.

On the example input of

    @[2 4 1 3 8 7 -3 -1 12 -5 -8]

    @[@[6 1 7] @[2 5 10] @[1 2 9] @[4 6 9] @[3 0 9] @[6 2 0]]

    (defun 3sum (a)
      (let ((h (make-hash-table))
            (len (length a)))
        (dotimes (i len)
          (setf (gethash (aref a i) h) t))
        (dotimes (i len)
          (dotimes (j i)
            (when (gethash (- (+ (aref a i) (aref a j))) h)
              (return-from 3sum t))))))
    
    (3sum #(2 4 1 3 8 7 -3 -1 12 -5 -8))
    ;; => t

It outputs the indices corresponding to the solution. E.g.

    @[6 1 7]  == A[6], A[1], A[7] == -3, 4, -1
    @[2 5 10] ==                  ==  1, 7, -8

If you want a one line code, in J, for the 42 solutions:

   _ 11 11 #:,I. 0=,a+/,+/~  a=: 2 4 1 3 8 7 _3 _1 12 _5 _8

   2 12 $, ~. (/:~)"1 ({&a)  _ 11 11 #:,I. 0=,a+/,+/~  a=: 2   4 1 3 8 7 _3 _1 12 _5 _8

   _3 1 2 _5  2 3 _1 _1 2 _8  4 4
   _5 1 4 _3 _1 4 _8  1 7 _5 _3 8