DSA using JavaScript

Initialization: largest = 10 , secondLargest = -Infinity .
Loop: Compare 20 with 10 : secondLargest = 10 , largest = 20 . Compare 4 with 20 : No change. Compare 45 with 20 : secondLargest = 20 , largest = 45 . Compare 99 with 45 : secondLargest = 45 , largest = 99 .

Array

function findLargestAndSecondLargest(arr) {
    let largest = arr[0];
    let secondLargest = -Infinity;

    // Finding the largest element
    for (let i = 1; i < arr.length; i++) {
        if (arr[i] > largest) {
            largest = arr[i];
        }
    }

    // Finding the second largest element
    for (let i = 1; i < arr.length; i++) {
        if (arr[i] > secondLargest && arr[i] < largest) {
            secondLargest = arr[i];
        }
    }

    return { largest, secondLargest: secondLargest === -Infinity ? 'Not Found' : secondLargest };
}

const arr = [10,20,4,45,99];
console.log(findLargestAndSecondLargest(arr));

function findLargestAndSecondLargestEfficient(arr) {
    let largest = arr[0];
    let secondLargest = -Infinity;

    for (let i = 1; i < arr.length; i++) {
        if (arr[i] > largest) {
            secondLargest = largest;
            largest = arr[i];
        } else if (arr[i] > secondLargest && arr[i] !== largest) {
            secondLargest = arr[i];
        }
    }

    return { largest, secondLargest: secondLargest === -Infinity ? 'Not Found' : secondLargest };
}

const arr = [10,20,4,45,99];
console.log(findLargestAndSecondLargestEfficient(arr));