const positionArray = [500, 1500, 2200, 3100]
const derivePositionIndex = (scrollPosition) => {
    let positionIndex = null // Insert default value
    positionArray.some((d, i) => {
        if (scrollPosition <= d) {
            positionIndex = i
            return true
        }
        return false
    })
    return positionIndex
}

const scrollPositionY = 1700 // Test value. Replace with actual value
const derivedPositionIndex = derivePositionIndex(scrollPositionY)
console.log("Position index:", derivedPositionIndex)
console.log("Element position:", positionArray[derivedPositionIndex])