How to prevent automatic sort of Object numeric property?

Question

Why I met this problem: I tried to solve an algorithm problem and I need to return the number which appeared most of the times in an array. Like [5,4,3,2,1,1] should return

Answer 1

This is an old topic but it is still worth mentioning as it is hard to find a straight explanation in one-minute googling.

I recently had a coding exercise that finding the first occurrence of the least/most frequent integer in an array, it is pretty much the same as your case.

I encountered the same problem as you, having the numeric keys sorted by ASC in JavaScript object, which is not preserving the original order of elements, which is the default behavior in js.

A better way to solve this in ES6 is to use a new data type called: Map

Map can preserve the original order of elements(pairs), and also have the unique key benefit from object.

let map = new Map()
map.set(4, "first") // Map(1) {4 => "first"}
map.set(1, "second") // Map(2) {4 => "first", 1 => "second"}
map.set(2, "third") // Map(3) {4 => "first", 1 => "second", 2 => "third"}
for(let [key, value] of map) {
  console.log(key, value)
}
// 4 "first"
// 1 "second"
// 2 "third"

However, using the object data type can also solve the problem, but we need the help of the input array to get back the original order of elements:

function findMostAndLeast(arr) {
  let countsMap = {};
  let mostFreq = 0;
  let leastFreq = arr.length;
  let mostFreqEl, leastFreqEl;

  for (let i = 0; i < arr.length; i++) {
    let el = arr[i];
    // Count each occurrence
    if (countsMap[el] === undefined) {
      countsMap[el] = 1;
    } else {
      countsMap[el] += 1;
    }
  }

  // Since the object is sorted by keys by default in JS, have to loop again the original array
  for (let i = 0; i < arr.length; i++) {
    const el = arr[i];

    // find the least frequent 
    if (leastFreq > countsMap[el]) {
      leastFreqEl = Number(el);
      leastFreq = countsMap[el];
    }

    // find the most frequent 
    if (countsMap[el] > mostFreq) {
      mostFreqEl = Number(el);
      mostFreq = countsMap[el];
    }
  }

  return {
    most_frequent: mostFreqEl,
    least_frequent: leastFreqEl
  }
}
const testData = [6, 1, 3, 2, 4, 7, 8, 9, 10, 4, 4, 4, 10, 1, 1, 1, 1, 6, 6, 6, 6];    
console.log(findMostAndLeast(testData)); // { most_frequent: 6, least_frequent: 3 }, it gets 6, 3 instead of 1, 2