Repeat even number inside the same array - JavaScript

We are required to write a JavaScript function that should repeat the even number inside the same array.

Therefore, for example given the following array ?

const arr = [1, 2, 5, 6, 8];

We should get the output ?

const output = [1, 2, 2, 5, 6, 6, 8, 8];

Example

Following is the code ?

const arr = [1, 2, 5, 6, 8];
const repeatEvenNumbers = arr => {
    let end = arr.length - 1;
    for(let i = end; i >= 0; i--){
        if(arr[i] % 2 === 0){
            arr.splice(i, 0, arr[i]);
        };
    };
    return arr;
};
console.log(repeatEvenNumbers(arr));

Output

This will produce the following output on console ?

[
    1, 2, 2, 5,
    6, 6, 8, 8
]

How It Works

The function uses array.splice() method to insert duplicate even numbers. We iterate backwards through the array to avoid index shifting issues when inserting elements.

Alternative Method: Using map and flatMap

const arr = [1, 2, 5, 6, 8];
const repeatEvenNumbers = arr => {
    return arr.flatMap(num => num % 2 === 0 ? [num, num] : [num]);
};
console.log(repeatEvenNumbers(arr));

[ 1, 2, 2, 5, 6, 6, 8, 8 ]

Conclusion

Both approaches work effectively. The splice method modifies the original array, while flatMap creates a new array. Use flatMap for a functional programming approach or splice when you need to modify the array in place.