level/index.js

const US_LIQUID_GALLONS_TO_CUBIC_INCHES = 231
const CUBIC_INCHES_TO_US_LIQUID_GALLONS = 1 / US_LIQUID_GALLONS_TO_CUBIC_INCHES

const toLiquid = cubicInches => cubicInches * CUBIC_INCHES_TO_US_LIQUID_GALLONS
const fromLiquid = gallons => gallons * US_LIQUID_GALLONS_TO_CUBIC_INCHES

class Container {
    constructor({ l, w, h }, level = 0) {
        this.l = l
        this.w = w
        this.h = h
        this.level = level
    }
    get liquidVolumeCapacity() {
        return toLiquid(this.l * this.w * this.h)
    }
    get liquidVolumeFilled() {
        return toLiquid(this.l * this.w * this.level)
    }
    get liquidVolumeRemaining() {
        return this.liquidVolumeCapacity - this.liquidVolumeFilled
    }
}

/** Functional Core */
const _changeLevel = (amount, container) => {
    const alteredLevel =
        fromLiquid(container.liquidVolumeFilled + amount) /
        container.l /
        container.w
    return createContainer(container, alteredLevel)
}
const createContainer = ({ l, w, h }, level) => {
    if (Math.sign(l) + Math.sign(w) + Math.sign(h) !== 3)
        return console.error(
            `[Error] All dimensions must be positive and greater than zero!`
        )
    return new Container({ l, w, h }, level)
}
const fill = ({ container, amount }) => _changeLevel(amount, container)
const drain = ({ container, amount }) => _changeLevel(-1 * amount, container)

myTank = createContainer({ l: 12, w: 12, h: 12 })
console.log('capacity:', myTank.liquidVolumeCapacity)
console.log('filled:', myTank.liquidVolumeFilled)

myTank = fill({ amount: myTank.liquidVolumeCapacity, container: myTank })
console.log('\nremaining liquid volume:', myTank.liquidVolumeRemaining)
console.log('remain h:', myTank.h - myTank.level)

myTank = drain({ amount: 4, container: myTank })
console.log('\nremaining liquid volume:', myTank.liquidVolumeRemaining)
console.log('remain h:', myTank.h - myTank.level)
console.log('filled:', myTank.liquidVolumeFilled)

myTank = fill({ amount: 3, container: myTank })
console.log('\nremaining liquid volume:', myTank.liquidVolumeRemaining)
console.log('remain h:', myTank.h - myTank.level)
console.log('filled:', myTank.liquidVolumeFilled)