Detect is a simplified version of find.
detect [ 1, 2, 3 ]
(elt % 2) == 0
# f.ret --> 2
With objects (maps), it returns the first matching entry (pair).
detect { a: 'A', b: 'B', c: 'C' }
val == 'B'
# f.ret --> [ 'b', 'B' ]
Iterating blocks are given 3 to 4 local variables.
A block iterating over an array will receive elt (the current element
of the iteration), idx (the zero-based index of the current element),
and len (the length of the array).
A block iterating over an object will receive key (the current string
key), val (the current value), idx (the zero-based index of the
current key/val), and len (the length of the object).
find.