Can you tell me please, why the final array does not have the last item of the initial array; it returns [3, 6, 9, 2, 4, 6], and we are missing '8'.
let item = [123,456,789,12,34,56,78];
const tailAndHead = arr => arr.slice(1).reduce((a,v,i) => (a.push(arr[i]%10), a), []);
Why not use Array#map?
tailAndHead = arr.map(a => a % 10);
a instead of arr[i]Remove slice(1).
let item = [123,456,789,12,34,56,78];
const tailAndHead = arr => arr.reduce((a,v,i) => (a.push(arr[i]%10), a), []);
You should push the sliced array item. Please modify you code like this.
let item = [123,456,789,12,34,56,78];
const tailAndHead = arr => arr.slice(1).reduce((a,v,i,b) => (a.push(b[i]%10), a), []);
console.log(tailAndHead(item));