percentage helper. calculate amount, total, percentage from 2 of these

iaPercenter.js

/**
 *
 * @param initOptions object {percentage:0.16,decimals_amount:2,decimals_percent:2} defaults
 * @returns {{calcTotal: *, calcAmount: *, options: *, calcPercent: *, option: *}}
 *
 * @example
 *  var percer = new iaPercenter( {percentage:0.16,decimals_amount:2,decimals_percent:2} );
 *
 *  percer.calcPercent(100,116) => 0.16
 *
 *  percer.calcTotal(100) => 116
 *  percer.calcTotal(100, 0.2) => 120
 *
 *  percer.calcAmount(116) => 116
 *  percer.calcAmount(120, 0.2) => 120
 *
 * percer.options() => {percentage:0.16,decimals_amount:2,decimals_percent:2}
 * percer.options('decimals_amount', 4) => sets deciamls_amount to 4, returns this
 * percer.options({decimals_percent:3}) => sets decimals_percent to 3 returns this
 *
 *
 * @author Raúl José Santos
 * @author Informática Asociada
 * @copyright 2017
 * @license MIT
 * @version 1.0.0
 */
function iaPercenter(initOptions) {
	if (!(this instanceof iaPercenter))
		return new iaPercenter(initOptions);

    var settings = {percentage:0.16, decimals_amount:2, decimals_percent:2};
    if(typeof initOptions === 'object' && initOptions !== null)
        for(var o in initOptions)
            if(initOptions.hasOwnProperty(o))
                settings[o] = initOptions[o];


  // functionality ________________________________________
    function percentage(amount, total) {
        return eq0(total) || eq0(amount) ? 0.00 : round(total/amount-1.00, settings.decimals_percent);
    }

    function total(amount, percentage) {
        if(typeof percentage === 'undefined')
            percentage = settings.percentage;
        return round(amount*(1+percentage), settings.decimals_amount);
    }

    function amount(total, percentage) {
        if(typeof percentage === 'undefined')
            percentage = settings.percentage;
        return eqMinus1(percentage) === -1 ? 0.00 : round(total/(1+percentage), settings.decimals_amount);
    }

    function opt(key,value) {
        if(typeof key === 'undefined' || key === null)
            return settings;
        if(typeof key === 'object')
            for(var o in initOptions)
                if(initOptions.hasOwnProperty(o))
                    settings[o] = initOptions[o];
        if(typeof value === 'undefined')
            return settings[key];
        settings[key] = value;
        return this;
    }

    // helpers ___________________________________________
        function eq0(n) {return round(n, settings.decimals_amount) === 0;}
        function eqMinus1(n) {return round(n, settings.decimals_percent) === -1;}

        function round(n, decimals) {
            var r=parseFloat('1E'+decimals);
            return Math.round(n*r)/r;
        }

    var exportThis = {
            calcPercent: percentage,
            calcTotal: total,
            calcAmount: amount,
            option: opt,
            options: opt
        };
    for(var e in exportThis)
        if(exportThis.hasOwnProperty(e))
            Object.defineProperty(exportThis, e, { writable: false,configurable:false });
    return exportThis;
}