Returns the value of a float rounded using the number of decimal points provided as precision.

precisionRound.js

/**
 * Returns the value of a float rounded using the number of decimal points provided as precision.
 *
 * Fixes binary rounding issues eg. Math.round(1.005 * 100) / 100 === 1 that present
 * problems for accounting and finance-related software.
 *
 * The normal technique for rounding with decimals is to multiply the number by the log10 of the
 * precision, round then divide by the same log eg. for 2 decimal points `round(num * 100) / 100`.
 * The binary rounding issue, however, causes this:  1.005 * 100 === 100.49999999999999.
 *
 * This method plays on the use of converting scientific notation to a number,
 * eg 1.005e2 === 100.5 so casting: `round(num + "e2") + "e-2"` to a number will result
 * in the desired result.
 *
 * @param   {number}  number        The value to round
 * @param   {int}     precision     The optional number of decimal digits to round to
 *
 * @returns {number} The adjusted value
 *
 * Based on MDN function
 * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/round
 */
Math.precisionRound = function( number, precision ) {
    "use strict";

    var nativeRound = Math.round,
        castNumber = +number,
        castPrecision = +precision,
        scaledNumber, eSplit, eString;

    // If the exp is undefined or zero, just use native rounding
    if( typeof precision === "undefined" || 0 === castPrecision ) {
        return nativeRound( number );
    }

    // If the value is not a number or the exp is not an integer...
    if( isNaN( castNumber ) || !(typeof castPrecision === "number" && 0 === castPrecision % 1) ) {
        return NaN;
    }

    // In case the number is already in scientific when casted to string, split off the exponent
    eSplit = ("" + castNumber).split( "e" );

    // Increase the exponent by the given precision and create a scientific notion string
    eString = (eSplit[0] + "e" + (eSplit[1] ? (+eSplit[1] + castPrecision) : castPrecision));

    // Cast to number and round
    scaledNumber = nativeRound( +eString );

    // Do the same as before, backwards
    eSplit = ("" + scaledNumber).split( "e" );
    eString = (eSplit[0] + "e" + (eSplit[1] ? (+eSplit[1] - castPrecision) : -castPrecision));

    return +eString;
};