Partially reversing an array - JavaScript

Suppose, we have an array of literals like this:

const arr = [3, 5, 5, 2, 23, 4, 7, 8, 8, 9];
console.log("Original array:", arr);

Original array: [3, 5, 5, 2, 23, 4, 7, 8, 8, 9]

We are required to write a JavaScript function that takes in such an array and a number, say n (n must be less than or equal to the length of array). And the function should reverse the first n elements of the array within.

For example, if for this array, the number is 4, then the first 4 elements [3, 5, 5, 2] should be reversed to [2, 5, 5, 3], while the remaining elements stay in their original positions.

Example Implementation

Let us write the code for this function:

const arr = [3, 5, 5, 2, 23, 4, 7, 8, 8, 9];

const partialReverse = (arr = [], num = 0) => {
    const partialArr = arr.slice(0, num);
    partialArr.reverse();
    arr.splice(0, num, ...partialArr);
};

console.log("Before partial reverse:", arr);
partialReverse(arr, 5);
console.log("After reversing first 5 elements:", arr);

Before partial reverse: [3, 5, 5, 2, 23, 4, 7, 8, 8, 9]
After reversing first 5 elements: [23, 2, 5, 5, 3, 4, 7, 8, 8, 9]

How It Works

The function works in three steps:

1. Extract: arr.slice(0, num) creates a copy of the first n elements
2. Reverse: partialArr.reverse() reverses the extracted portion
3. Replace: arr.splice(0, num, ...partialArr) replaces the original first n elements with the reversed ones

Alternative Approach Using In-Place Swapping

Here's a more memory-efficient approach that reverses elements in-place:

const partialReverseInPlace = (arr, num) => {
    for (let i = 0; i 

Before: [3, 5, 5, 2, 23, 4, 7, 8, 8, 9]
After reversing first 4 elements: [2, 5, 5, 3, 23, 4, 7, 8, 8, 9]


Comparison

Conclusion

Both approaches successfully reverse the first n elements of an array. The slice & splice method is more readable, while the in-place swapping approach is more memory-efficient for large arrays.