Sorting Array of Number by Increasing Frequency using JavaScript

To sort an array of given numbers based on the frequency of occurrence of each number first sort the array such that the elements repeated for the least number of times appear first followed by the elements with increasing frequency.

Example:

Input:
array = [3, 3, 1, 1, 1, 8, 3, 6, 8, 8] 

Output:
[6, 1, 1, 1, 3, 3, 3, 8, 8, 8]

Explanation:
Number 6 occurs 1 time
Number 1 occurs 3 times
Number 3 occurs 3 times
Number 8 occurs 3 times

Below are the approaches to sort an array of Numbers by increasing frequency using JavaScript:

Using an Object

Initialize an empty object to store the frequency of each number. Iterate through the array and count the frequency of each number. Sort the array based on increasing frequency using a custom comparator function. If two numbers have different frequencies, sort them based on their frequencies in ascending order. If two numbers have the same frequency, sort them based on their values in ascending order. Return the sorted array.

Example: Demonstration of Sorting array of Numbers by increasing frequency in JavaScript using an Object.

function sortIncreasFreq(arr) {
    const freqMap = {};
    arr.forEach(num => {
        freqMap[num] = 
                (freqMap[num] || 0) + 1;
    });

    return arr.sort((a, b) => {
        if (freqMap[a] !== freqMap[b]) {
            return freqMap[a] - freqMap[b];
        } else {
            return a - b;
        }
    });
}

const arr = [3, 3, 1, 1, 1, 8, 3, 6, 8, 8];
console.log("Sorted by increasing frequency:", 
                    sortIncreasFreq(arr));

Sorted by increasing frequency: [
  6, 1, 1, 1, 3,
  3, 3, 8, 8, 8
]

Time Complexity: O(n log n)

Space Complexity: O(n)

Using Map

Initialize an empty Map to store the frequency of each number. Iterate through the array and update the frequency of each number. Sort the array based on increasing frequency using a custom comparator function. If two numbers have different frequencies, sort them based on their frequencies in ascending order. If two numbers have the same frequency, sort them based on their values in ascending order. Return the sorted array.

Example: Demonstration of Sorting array of Numbers by increasing frequency in JavaScript using Map.

function sortInFreq(arr) {
    const freqMap = new Map();
    arr.forEach(num => {
        freqMap.set(num, 
                (freqMap.get(num) || 0) + 1);
    });

    return arr.sort((a, b) => {
        if (freqMap.get(a) !== freqMap.get(b)) {
            return freqMap.get(a) 
                        - freqMap.get(b);
        } else {
            return a - b;
        }
    });
}

const arr = [3, 3, 1, 1, 1, 8, 3, 6, 8, 8];
console.log("Sorted by increasing frequency using Map :",
                        sortInFreq(arr));

Sorted by increasing frequency using Map : [
  6, 1, 1, 1, 3,
  3, 3, 8, 8, 8
]

Time Complexity: O(n log n)

Space Complexity: O(n)