Convert range of decibel to percentage and back again

scale_db.js

// I wrote this to convert dB to percentage with a weird scale.
// For instance, a user may set an amplifier's min-max dB limits to -50 - 10.
// The UI they use only increments by 1 and dB does not (properly) scale linearly.
// Having to work around these issues I wrote this based on the script I found here www.redwirez.com/pcalc.jsp
// =-=-=-=-=-=-=-=

// Calcs dB to percent with a non-standard dB range
// it does this by getting a new percentage range using min/max dB
// defaults to the standard range of -80 ~ 0 if no min or max is provided
const calcDb = (perc, minDb, maxDb, minP, maxP) => {
    const getPerc = (db) => (Math.round(1000000000 * Math.pow(10, db / 20)) / 10000000)
    const maxPerc = maxP || (_.isNumber(maxDb) && getPerc(maxDb)) || 100
    const minPerc = minP || (_.isNumber(minDb) && getPerc(minDb)) || 0
    const range = maxPerc - minPerc
    const newPerc = (range / 100) * perc

    return Math.round(1000000000 * 20 * (Math.log(newPerc * 0.01) / Math.log(10))) / 1000000000
}

// Calcs percentage to dB with a non-standard dB range
// leverages calcDb to scale to the new range
// defaults to 0 - 100 min or max percent/dB if they are not provided
const calcPerc = (dB, minDb, maxDb, minP, maxP) => {
    const getPerc = (db) => (Math.round(1000000000 * Math.pow(10, db / 20)) / 10000000)
    const maxPerc = maxP || (_.isNumber(maxDb) && getPerc(maxDb)) || 100
    const minPerc = minP || (_.isNumber(minDb) && getPerc(minDb)) || 0
    const range = maxPerc - minPerc
    const newDb = calcDb((getPerc(dB) / range) * 100, null, null, minPerc, maxPerc)

    return Math.round(1000000000 * Math.pow(10, newDb / 20)) / 10000000
}