BigDecimal in JavaScript

There are several implementations of BigDecimal in js:

  • js-big-decimal
  • big.js
  • bignumber.js
  • decimal.js

The last 3 come from the same author: see the differences.


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

I like using accounting.js for number, money and currency formatting.

Homepage - https://openexchangerates.github.io/accounting.js/

Github - https://github.com/openexchangerates/accounting.js

Tags:

Javascript

Floating Point

Bigdecimal

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