maxKAdjacentSum.js

function maxKAdjacentSum(array, k) {
    if (array.length < k)
        return null
    
    let runningSum = 0
    for (let i = 0; i < k; i++)
        runningSum += array[i]

    let highestSum = runningSum
    for (let j = 0; j < array.length - k; j++) {
        runningSum += array[j + k] - array[j]
        highestSum = Math.max(highestSum, runningSum)
    }

    return highestSum
}