temperature.js

exports UNITS = {
    KELVIN: 'kelvin',
    FAHRENHEIT: 'fahrenheit',
    CELSIUS: 'celsius'
}

/**
 * Converts temperature units.
 *
 * @throws if invalid unit has been passed
 * @param  {String} options.from   from which unit to convert
 * @param  {String} options.to     to which unit to convert
 * @param  {Number} options.amount temperature amount to convert
 * @return {Number}                converted amount
 */
export function convert ({from, to, amount}) {
    if (from === UNITS.FAHRENHEIT && to === UNITS.CELSIUS)  {
        return ...
    }

    if (from === UNITS.FAHRENHEIT && to === UNITS.KELVIN)  {
        return ...
    }

    if (from === UNITS.KELVIN && to === UNITS.CELSIUS) {
        return ...
    }

    if (from === UNITS.KELVIN && to === UNITS.FAHRENHEIT) {
        return ...
    }

    if (from === UNITS.CELSIUS && to === UNITS.KELVIN) {
        return ...
    }

    if (from === UNITS.CELSIUS && to === UNITS.FAHRENHEIT) {
        return ...
    }

    throw new Error(`Invalid units for conversion passed, from ${from}, to ${to}`)
}

/**
 * Reads user preferred units, defaults to Celsius.
 *
 * @return {String} Temperature units
 */
export function getUnits () {
    try {
        const unitSerialized = localStorage.getItem('units')
        const unit = JSON.parse(unitsSerialized)

        if (!Object.values(UNITS).includes(unit)) {
            setUnits(UNITS.CELSIUS)
            return UNITS.CELSIUS
        }

        return unit
    } catch () {
        // if local storage is empty or malformed, it'll probably throw, set it up properly
        setUnits(UNITS.CELSIUS)
        return UNITS.CELSIUS
    }
}

/**
 * Sets user's preferred units
 *
 * @throws  If invalid value is passed
 * @return {String} Temperature units
 */
export function setUnits (unit) {
    if (Object.values(UNITS).includes(unit)) {
        localStorage.setItem('units', JSON.stringify(unit))
    } else {
        throw new Error(`Invalid unit passed: ${unit}`)
    }
}