bigIntToFloatString.ts

function bigIntToFloatString(value: bigint, decimalPlaces: number) {
    if (typeof decimalPlaces !== 'number' || decimalPlaces < 0 || !Number.isInteger(decimalPlaces)) {
        throw new TypeError('The second argument must be a non-negative integer');
    }

    const divisor = BigInt(10 ** decimalPlaces); // 1e{decimalPlaces}

    const integerPart = value / divisor;
    const fractionalPart = value % divisor;

    const integerString = integerPart.toString();
    const fractionalString = fractionalPart.toString().padStart(decimalPlaces, '0');

    return `${integerString}.${fractionalString}`;
}

floatStringToBigInt.ts

function floatStringToBigInt(floatString: string, decimalPlaces: number) {
    if (typeof decimalPlaces !== 'number' || decimalPlaces < 0 || !Number.isInteger(decimalPlaces)) {
        throw new TypeError('The second argument must be a non-negative integer');
    }

    const [integerPart, fractionalPart = ''] = floatString.split('.');

    const combinedString = integerPart + fractionalPart.padEnd(decimalPlaces, '0');

    return BigInt(combinedString);
}