Find the index of the longest array in an array of arrays

Question

If you have an array containing an indefinite amount of arrays

ex:

var masterArray = [ [1,2,3,4,5],
                    [1,2], 
                    [1,1,         


        

Answer 1

One-liner is:

masterArray
  .map(a=>a.length)
  .indexOf(Math.max(...masterArray.map(a=>a.length)));

But better to cache masterArray.map(a=>a.length) results.

const lengths = masterArray.map(a=>a.length);
lengths.indexOf(Math.max(...lengths));

Note, this code still iterate array at least* 3 times(map, max, indexOf separately).

*Spread operator is for readability and can be omitted

---

For more efficiency you should manual iterate array.

let max = -Infinity;
let index = -1;
masterArray.forEach(function(a, i){
  if (a.length > max) {
    max = a.length;
    index = i;
  }
});

---

Reduce method:

masterArray.reduce((maxI,el,i,arr) => 
    (el.length>arr[maxI].length) ? i : maxI, 0);