Javascript: Find Duplicate using hashMap

Find duplicate in an array using hashmap.

Code:

 const arr = [4, 2, 7, 4, 2, 8, 4, 9];

 const hashMap = new Map();

 for (const num of arr) {

     if (hashMap.has(num)) {

           let count = hashMap.get(num);

           hashMap.set(num, count + 1);

     } else {

            hashMap.set(num, 1);

     }

 }

console.log("Ocuurance of numbers:", hashMap);

const duplicates = Array.from(hashMap)

    .filter(([key, value]) => value > 1)

    .map(([key, value]) => key);

console.log("Duplicates occur for :",duplicates);

output:

Ocuurance of numbers: Map(5) { 4 => 3, 2 => 2, 7 => 1, 8 => 1, 9 => 1 }

Duplicates occur for : [ 4, 2 ]    

Explanation:

1. We first initialize an empty hashmap using the `Map()` constructor.

2. We then loop through the array and check if the current number is

already present in the hashmap using the `has()` method. If it is present,

we increment its count by 1 using the `get()` and `set()` methods.

If it is not present, we add it to the hashmap with a count of 1 using the `set()` method.

3. After processing the entire array, we log the hashmap to the console, which contains

the occurrence of each number in the array.

4. We then use the `Array.from()` method to convert the hashmap into an array of key-value

 pairs, and filter out the pairs where the value (count) is greater than 1, indicating that

 the number is a duplicate. We then use the `map()` method to  extract the keys (numbers)

 from the filtered pairs and store them in a new array called `duplicates`.

5. Finally, we log the `duplicates` array to the console, which contains all the duplicate

values found in the original array.