https://techdevguide.withgoogle.com/paths/foundational/maxspan-problem-return-largest-span-array/#!

maxSpan.js

"use strict";

const assert = require("assert");

/**
 * Consider the leftmost and righmost appearances of some value in an array. We'll say that the "span" is the number of elements between the two inclusive. A single value has a span of 1. Returns the largest span found in the given array. (Efficiency is not a priority.)

maxSpan([1, 2, 1, 1, 3]) → 4
maxSpan([1, 4, 2, 1, 4, 1, 4]) → 6
maxSpan([1, 4, 2, 1, 4, 4, 4]) → 6
 */

(() => {
  const inouts = [
    {
      in: [1, 2, 1, 1, 3],
      out: 4
    },
    {
      in: [1, 4, 2, 1, 4, 1, 4],
      out: 6
    },
    {
      in: [1, 4, 2, 1, 4, 4, 4],
      out: 6
    }
  ];

  inouts.forEach(e => {
    assert.equal(e.out, maxSpan(e.in), "fail");
  });
})();

function maxSpan(arr) {
  const spans = {};
  let mSpan = 0;
  for (let i = 0; i < arr.length; i++) {
    const n = arr[i];
    if (!(n in spans)) {
      spans[n] = i;
    } else {
      const span = i - spans[n] + 1;
      if (span > mSpan) {
        mSpan = span;
      }
    }
  }

  return mSpan;
}