Swap certain element from end and start of array - JavaScript

We need to write a JavaScript function that accepts an array of numbers and a position k, then swaps the kth element from the beginning with the kth element from the end of the array.

Understanding the Problem

For an array with indices 0 to n-1, the kth element from start is at index k-1, and the kth element from end is at index n-k. We swap these two elements.

Example Implementation

const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

const swapNth = (arr, k) => {
    const { length: l } = arr;
    let temp;
    const ind = k - 1;
    temp = arr[ind];
    arr[ind] = arr[l - k];
    arr[l - k] = temp;
};

console.log("Original array:", arr);

swapNth(arr, 4);
console.log("After swapping 4th elements:", arr);

swapNth(arr, 8);
console.log("After swapping 8th elements:", arr);

Original array: [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
After swapping 4th elements: [ 0, 1, 2, 6, 4, 5, 3, 7, 8, 9 ]
After swapping 8th elements: [ 0, 1, 7, 6, 4, 5, 3, 2, 8, 9 ]

How It Works

The function calculates two indices:

• Start index: k-1 (4th element is at index 3)
• End index: length-k (4th from end in 10-element array is at index 6)

It then uses a temporary variable to swap the values at these positions.

Alternative Using Destructuring

const swapNthDestructuring = (arr, k) => {
    const startIndex = k - 1;
    const endIndex = arr.length - k;
    [arr[startIndex], arr[endIndex]] = [arr[endIndex], arr[startIndex]];
};

const testArray = [10, 20, 30, 40, 50, 60];
console.log("Before swap:", testArray);

swapNthDestructuring(testArray, 2);
console.log("After swapping 2nd elements:", testArray);

Before swap: [ 10, 20, 30, 40, 50, 60 ]
After swapping 2nd elements: [ 10, 50, 30, 40, 20, 60 ]

Conclusion

This swap operation is useful for array manipulation tasks. The destructuring approach provides cleaner syntax while the traditional method offers better clarity for the swapping logic.