floating point - BigDecimal in JavaScript

Question

I'm very new to JavaScript (I come from a Java background) and I am trying to do some financial calculations with small amounts of money.

My original go at this was:

<script type="text/javascript">
    var normBase = ("[price]").replace("$", "");
    var salesBase = ("[saleprice]").replace("$", "");
    var base;
    if (salesBase != 0) {
        base = salesBase;
    } else {
        base = normBase;
    }
    var per5  = (base - (base * 0.05));
    var per7  = (base - (base * 0.07));
    var per10 = (base - (base * 0.10));
    var per15 = (base - (base * 0.15));
    document.write
        (
        '5% Off: $'  + (Math.ceil(per5  * 100) / 100).toFixed(2) + '<br/>' +
        '7% Off: $'  + (Math.ceil(per7  * 100) / 100).toFixed(2) + '<br/>' +
        '10% Off: $' + (Math.ceil(per10 * 100) / 100).toFixed(2) + '<br/>' +
        '15% Off: $' + (Math.ceil(per15 * 100) / 100).toFixed(2) + '<br/>'
    );
</script>

This worked well except it always rounded up (Math.ceil). Math.floor has the same issue, and Math.round is also no good for floats.

In Java, I would have avoided the use of floats completely from the get-go, however in JavaScript there does not seem to be a default inclusion of something comparable.

The problem is, all the libraries mentioned are either broken or for a different purpose. The jsfromhell.com/classes/bignumber library is very close to what I need, however I'm having bizarre issues with its rounding and precision... No matter what I set the Round Type to, it seems to decide on its own. So for example, 3.7107 with precision of 2 and round type of ROUND_HALF_UP somehow winds up as 3.72 when it should be 3.71.

I also tried @JasonSmith BigDecimal library (a machined port from Java's BigDecimal), but it seems to be for node.js which I don't have the option of running.

How can I accomplish this using vanilla JavaScript (and be reliable) or is there a modern (ones mentioned above are all years old now) library that I can use that is maintained and is not broken?

Answer 1

Since we have native support for BigInt, it doesn't require much code any more to implement BigDecimal.

Here is a BigDecimal class based on BigInt with the following characteristics:

• The number of decimals is configured as a constant, applicable to all instances.
• Whether excessive digits are truncated or rounded is configured as a boolean constant.
• An instance stores the decimal number as a BigInt, multiplied by a power of 10 so to include the decimals.
• All calculations happen with those BigInt values.
• The arguments passed to add, subtract, multiply and divide can be numeric, string, or instances of BigDecimal
• These methods return new instances, so a BigDecimal is treated as immutable.
• The toString method reintroduces the decimal point.
• A BigDecimal can coerce to a number (via implicit call to toString), but that will obviously lead to loss of precision.

class BigDecimal {
    // Configuration: constants
    static DECIMALS = 18; // number of decimals on all instances
    static ROUNDED = true; // numbers are truncated (false) or rounded (true)
    static SHIFT = BigInt("1" + "0".repeat(BigDecimal.DECIMALS)); // derived constant
    constructor(value) {
        if (value instanceof BigDecimal) return value;
        let [ints, decis] = String(value).split(".").concat("");
        this._n = BigInt(ints + decis.padEnd(BigDecimal.DECIMALS, "0")
                                     .slice(0, BigDecimal.DECIMALS)) 
                  + BigInt(BigDecimal.ROUNDED && decis[BigDecimal.DECIMALS] >= "5");
    }
    static fromBigInt(bigint) {
        return Object.assign(Object.create(BigDecimal.prototype), { _n: bigint });
    }
    add(num) {
        return BigDecimal.fromBigInt(this._n + new BigDecimal(num)._n);
    }
    subtract(num) {
        return BigDecimal.fromBigInt(this._n - new BigDecimal(num)._n);
    }
    static _divRound(dividend, divisor) {
        return BigDecimal.fromBigInt(dividend / divisor 
            + (BigDecimal.ROUNDED ? dividend  * 2n / divisor % 2n : 0n));
    }
    multiply(num) {
        return BigDecimal._divRound(this._n * new BigDecimal(num)._n, BigDecimal.SHIFT);
    }
    divide(num) {
        return BigDecimal._divRound(this._n * BigDecimal.SHIFT, new BigDecimal(num)._n);
    }
    toString() {
        const s = this._n.toString().padStart(BigDecimal.DECIMALS+1, "0");
        return s.slice(0, -BigDecimal.DECIMALS) + "." + s.slice(-BigDecimal.DECIMALS)
                .replace(/.?0+$/, "");
    }
}

// Demo
var a = new BigDecimal("123456789123456789876");
var b = a.divide("10000000000000000000");
var c = b.add("9.000000000000000004");
console.log(b.toString());
console.log(c.toString());
console.log(+c); // loss of precision when converting to number