BigDecimal in JavaScript

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

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