It's not intended to be understood. Readable code would be

function(acc,val) {
  if ( val in acc )
    acc[val] += 1;
  else
    acc[val] = 1;
  return acc;
}

And there you see why it was written unreadably - it's shorter. Note that acc[val] may be undefined.

Neat and clever

What is the use of having plus sign in the year function?

This is the first time for me to recognize that concat() method can take more than one array as it's parameters!

It's hard to have test cases because it depends on random values.
just skip the test code step and try the attempt option directly

Yeah same question,I would like to hear explanation

Good job!