hill.js

const hill =  [4, 2, 4, 1, 5, 3, 16, 6, 17, 19, 4, 13, 5, 3, 10, 10, 13, 6, 2, 1, 5, 15,
    13, 19, 16, 9, 13, 1, 7, 18, 20, 13, 9, 7, 2, 10, 8, 18, 4, 7, 5, 8, 10, 13, 
    7, 18, 19, 2, 19, 8, 10, 10, 17, 6, 6, 20, 20, 11, 10, 11, 13, 9, 7, 1, 10, 
    5, 12, 16, 10, 7, 15, 13, 12, 10, 1, 1, 4, 2, 16, 10, 20, 17, 11, 19, 19, 20, 
    9, 10, 17, 9, 18, 8, 10, 18, 8, 19, 16, 17, 3, 1]

// now traverse

const previousPoints = []
var waterUnits = 0

for (var currentIndex = 0; currentIndex < hill.length; currentIndex++) {

    let currentSpot = hill[currentIndex],
        previousSpot = hill[currentIndex - 1],
        nextSpot = hill[currentIndex + 1];

    function findPreviousMatchingPoint(height) {
        for (var previousPointsIt = 0; previousPointsIt < previousPoints.length; previousPointsIt++) {
            const previousPointIndex = previousPoints[previousPointsIt]
            const previousPointHeight = hill[previousPointIndex]

            if (previousPointHeight >= height) {
                return previousPointIndex
            }
        }
    }

    // if the spot previous was lower than us, we need to calculate the captured water
    if (previousSpot != null && previousSpot < currentSpot) {

        // for each unit of increase, find the most recent point that matches that height
        // and calculate the water units based on distance of that point
        for (let unitHeight = previousSpot + 1; unitHeight <= currentSpot; unitHeight++) {

            const matchingPointIndex = findPreviousMatchingPoint(unitHeight)

            if (matchingPointIndex != null) {
                waterUnits += ((currentIndex - matchingPointIndex) - 1)
            }
        }
    }

    // detect if this is a point and store it for future use if it is
    if (previousSpot != null && previousSpot < currentSpot || 
        nextSpot != null && nextSpot < currentSpot) {
        previousPoints.unshift(currentIndex)
    }
}

console.log(previousPoints, waterUnits)