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

instead of generating an object like {5: 2, 2: 2, 1: 1}

generate an array to the effect of

[
   {key: 5, val: 2},
   {key: 2, val: 2},
   {key: 1, val: 1}
]

or... keep track of the sort order in a separate value or key

Answer 2

I came across this same problem, and after search a lot about that, i found out that the solution to prevent this behavior is make key as string.

Like that:

{"a": 2, "b": 2}

Answer 3

1. you can use Map() in javascript ES6 which will keep the order of the keys insertion.

2. just trying to solve your problem in an alternative solution, recently like to practise leetcode-like question

function solution(arr) {
  const obj = {};
  const record = {
    value: null,
    count: 0
  };

  for (let i = 0; i < arr.length; i++) {
    let current = arr[i];
    if (!obj[current]) {
      obj[current] = 0;
    }

    obj[current]++;

    if (obj[current] > record.count) {
      record.value = current;
      record.count = obj[current];
    }
  }

  console.log("mode number: ", record.value);
  console.log("mode number count: ", record.count);
}

Answer 4

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 

Answer 5

simply do that while you're working with a numeric array index

data      = {}
data[key] = value

Answer 6

The simplest and the best way to preserve the order of the keys in the array obtained by Object.keys() is to manipulate the Object keys a little bit.

insert a "_" in front of every key name. then run the following code!

myObject = {
  _a: 1,
  _1: 2,
  _2: 3
}

const myObjectRawKeysArray = Object.keys(myObject); 
console.log(myObjectRawKeysArray)
//["_a", "_1", "_2"]

const myDesiredKeysArray = myObjectRawKeysArray.map(rawKey => {return rawKey.slice(1)}); 
console.log(myDesiredKeysArray)
//["a", "1", "2"]

You get the desired order in the array with just a few lines of code. hApPy CoDiNg :)