Longest subarray which only contains strictly increasing numbers JavaScript

We are required to write a JavaScript function that takes in an array of numbers as the first and the only argument.

The function should then return the length of the longest continuous subarray from the array that only contains elements in a strictly increasing order.

A strictly increasing sequence is the one in which any succeeding element is greater than all its preceding elements.

Example

const arr = [5, 7, 8, 12, 4, 56, 6, 54, 89];

const findLongest = (arr) => {
    if(arr.length == 0) {
        return 0;
    };
    let max = 0;
    let count = 0;
    for(let i = 1; i < arr.length; i++) {
        if(arr[i] > arr[i-1]) {
            count++;
        } else {
            count = 0;
        }
        if(count > max) {
            max = count;
        }
    }
    return max + 1;
};

console.log(findLongest(arr));

Output

And the output in the console will be ?

4

How It Works

Let's trace through the algorithm with our example array [5, 7, 8, 12, 4, 56, 6, 54, 89]:

1. Start with count = 0 and max = 0
2. Compare adjacent elements: 7 > 5, so count becomes 1
3. 8 > 7, count becomes 2
4. 12 > 8, count becomes 3, max becomes 3
5. 4 < 12, count resets to 0
6. 56 > 4, count becomes 1
7. 6 < 56, count resets to 0
8. Continue until end...

The longest strictly increasing subarray is [5, 7, 8, 12] with length 4.

Alternative Approach

Here's a more explicit version that tracks the actual subarray positions:

const findLongestDetailed = (arr) => {
    if(arr.length <= 1) return arr.length;
    
    let maxLength = 1;
    let currentLength = 1;
    let bestStart = 0;
    let currentStart = 0;
    
    for(let i = 1; i < arr.length; i++) {
        if(arr[i] > arr[i-1]) {
            currentLength++;
        } else {
            if(currentLength > maxLength) {
                maxLength = currentLength;
                bestStart = currentStart;
            }
            currentLength = 1;
            currentStart = i;
        }
    }
    
    // Check final sequence
    if(currentLength > maxLength) {
        maxLength = currentLength;
        bestStart = currentStart;
    }
    
    console.log(`Longest subarray: [${arr.slice(bestStart, bestStart + maxLength)}]`);
    return maxLength;
};

const testArray = [5, 7, 8, 12, 4, 56, 6, 54, 89];
console.log(`Length: ${findLongestDetailed(testArray)}`);

Longest subarray: [5,7,8,12]
Length: 4

Conclusion

The algorithm efficiently finds the longest strictly increasing subarray in O(n) time complexity by tracking consecutive increases and maintaining the maximum length found so far.