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],                      [1,1,1,1,2,2,2,2,4,4],                     [1,2,3,4,5] ]; 

What is an efficient way to find the index of the longest array in masterArray? (in this example index would be 2).

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

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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;   } }); 

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Reduce method:

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

Answer 2

.reduce is the nicest way to do this:

masterArray.reduce(function (pending, cur, index, ar) { ar[ pending ].length > cur.length ? pending : index }, 0); 

Or with ES6:

masterArray.reduce((p, c, i, a) => a[p].length > c.length ? p : i, 0);