Test JavaScript Rounding

accurate-rounding-in-javascript.js

const maximumNumberOfDecimals = 5;
const targetDecimals = 2;
const roundingAdjustment = 3;
// The more maximum number of decimals, the higher this needs to go for accurate results
// It looks like targetDecimals + roundingAdjustment = maximumNumberOfDecimals for fully accurate results

var results = [];
for (
        let value = 1 / Math.pow(10, maximumNumberOfDecimals);
        value <= 100;
        value = (Math.round(value * Math.pow(10, maximumNumberOfDecimals)) + 1) / Math.pow(10, maximumNumberOfDecimals)
    ) {
    if (value.toString().length > "100".length + ".".length + maximumNumberOfDecimals)
        throw `Invalid step, ${value}`

    results.push({
        roundedValue: // round(value, targetDecimals, maximumNumberOfDecimals)
            Math.round(
                Math.round(
                    value * Math.pow(10, targetDecimals + roundingAdjustment)
                ) / Math.pow(10, roundingAdjustment)
            ) / Math.pow(10, targetDecimals),
        initialValue: value
    });
}

results.forEach(
    (result, resultIndex) => {
        if (resultIndex > 1
            && results[resultIndex - 1].roundedValue !== result.roundedValue) {
                console.log(result, results[resultIndex - 1]);
        }
    }
);

// A relatively goood round function which can be used to get much more accurate
// results by specifying a value for `maximumNumberOfDecimalsForValue`.
function round(value, decimals, maximumNumberOfDecimalsForValue) {
    if (typeof maximumNumberOfDecimalsForValue !== "number" || maximumNumberOfDecimalsForValue < decimals)
        maximumNumberOfDecimalsForValue = decimals + 1;
    return (
        Math.round(
            Math.round(
                value * Math.pow(10, maximumNumberOfDecimalsForValue)
            ) / Math.pow(10, maximumNumberOfDecimalsForValue - decimals)
        ) / Math.pow(10, decimals);
}