Merge two sorted arrays to form a resultant sorted array in JavaScript

We are required to write a JavaScript function that takes in two sorted arrays of numbers. The function should merge the two arrays together to form a resultant sorted array and return that array.

For example, if the two arrays are:

const arr1 = [2, 6, 6, 8, 9];
const arr2 = [1, 4, 5, 7];

Then the output array should be:

[1, 2, 4, 5, 6, 6, 7, 8, 9]

Method 1: Using Two Pointers Technique

The most efficient approach is the two-pointers technique, which compares elements from both arrays and builds the result array in sorted order:

const arr1 = [2, 6, 6, 8, 9];
const arr2 = [1, 4, 5, 7];

const mergeSortedArrays = (arr1, arr2) => {
    let i = 0, j = 0;
    const result = [];
    
    // Compare elements from both arrays
    while (i < arr1.length && j < arr2.length) {
        if (arr1[i] <= arr2[j]) {
            result.push(arr1[i]);
            i++;
        } else {
            result.push(arr2[j]);
            j++;
        }
    }
    
    // Add remaining elements from arr1
    while (i < arr1.length) {
        result.push(arr1[i]);
        i++;
    }
    
    // Add remaining elements from arr2
    while (j < arr2.length) {
        result.push(arr2[j]);
        j++;
    }
    
    return result;
};

const merged = mergeSortedArrays(arr1, arr2);
console.log(merged);

[1, 2, 4, 5, 6, 6, 7, 8, 9]

Method 2: Using Spread Operator and Sort

A simpler but less efficient approach is to concatenate arrays and sort the result:

const arr1 = [2, 6, 6, 8, 9];
const arr2 = [1, 4, 5, 7];

const mergeSortedArrays = (arr1, arr2) => {
    return [...arr1, ...arr2].sort((a, b) => a - b);
};

const merged = mergeSortedArrays(arr1, arr2);
console.log(merged);

[1, 2, 4, 5, 6, 6, 7, 8, 9]

How the Two Pointers Method Works

The algorithm uses two pointers to traverse both arrays simultaneously:

1. Compare current elements from both arrays
2. Push the smaller element to the result array
3. Move the pointer of the array from which element was taken
4. Repeat until one array is exhausted
5. Add remaining elements from the non-exhausted array

Comparison

Conclusion

The two pointers technique is the optimal solution for merging sorted arrays with O(m + n) time complexity. Use the spread operator method only for simplicity when performance isn't critical.