sebastienros.jint/Jint/Native/Number/NumberIntlHelper.cs

// Ideally, internacionalization formats implemented through the ECMAScript standards would follow this:
// https://tc39.es/ecma402/#sec-initializedatetimeformat
// https://tc39.es/ecma402/#sec-canonicalizelocalelist
// Along with the implementations of whatever is subsequenlty called.

// As this is not in place (See TODOS in NumberFormatConstructor and DateTimeFormatConstructor) we can arrange
// values that will match the JS behavior using the host logic. This bypasses the ECMAScript standards but can
// do the job for the most common use cases and cultures meanwhile.

namespace Jint.Native.Number
{
    internal class NumberIntlHelper
    {
        // Obtined empirically. For all cultures tested, we get a maximum of 3 decimal digits.
        private const int JS_MAX_DECIMAL_DIGIT_COUNT = 3;

        /// <summary>
        /// Checks the powers of 10 of number to count the number of decimal digits. 
        /// Returns a clamped JS_MAX_DECIMAL_DIGIT_COUNT count.
        /// JavaScript will use the shortest representation that accurately represents the value
        /// and clamp the decimal digits to JS_MAX_DECIMAL_DIGIT_COUNT.
        /// C# fills the digits with zeros up to the culture's numberFormat.NumberDecimalDigits
        /// and does not provide the same max (numberFormat.NumberDecimalDigits != JS_MAX_DECIMAL_DIGIT_COUNT).
        /// This function matches the JS behaviour for the decimal digits returned, this is the actual decimal
        /// digits for a number (with no zeros fill) clamped to JS_MAX_DECIMAL_DIGIT_COUNT. 
        /// </summary>
        public static int GetDecimalDigitCount(double number)
        {
            for (int i = 0; i < JS_MAX_DECIMAL_DIGIT_COUNT; i++)
            {
                var powOf10 = number * System.Math.Pow(10, i);
                bool isInteger = powOf10 == ((int) powOf10);
                if (isInteger)
                {
                    return i;
                }
            }

            return JS_MAX_DECIMAL_DIGIT_COUNT;
        }
    }
}