Single-line ArgMax for JavaScript

argmax.js

/**
 * Retrieve the array key corresponding to the largest element in the array.
 *
 * @param {Array.<number>} array Input array
 * @return {number} Index of array element with largest value
 */
function argMax(array) {
  return array.map((x, i) => [x, i]).reduce((r, a) => (a[0] > r[0] ? a : r))[1];
}

console.log(argMax([ 5.2, 1.4, 9.6, 1.8, 3.2, 2.4 ]));
// Output: 2

With dictionnary-like object you can also do this :

// with [0] at the end it return only the key, without it, it return [maxKey, maxValue]
const argMax = obj => Object.entries(obj).reduce((a, r) => a[1] > r[1] ? a : r)[0] 

// or if you need key value and index:
const argMax = obj => Object.entries(obj).map(([k, v], idx) => [k, v, idx]).reduce((a, r) => a[1] > r[1] ? a : r)