reverseflat = function(array){
if(array.length==1){return array[0]};
const f=(arr,acc=[])=>arr.length>1?f(arr.slice(0,-1),acc.concat(arr.slice(-1)[0])):
acc.concat(arr[0]);
return f(array)}
..........................
return f(array)=f(arr,acc=[])=>arr.length>1?f(arr.slice(0,-1),acc.concat(arr.slice(-1)[0])):
acc.concat(arr[0]);}