index.js

//   (Non-Breaking Space)
var NBSP = String.fromCharCode(160);

function parseNumber(number) {
	var isNegative = number < 0;
	var numberString = String(number);
	if (isNegative) {
		numberString = numberString.slice(1);
	}
	var decimal = numberString.split('.');
	return {
		integer: decimal[0],
		fraction: decimal[1] || '',
		sign: isNegative ? '-' : ''
	};
}

function format(number, separator) {
	number = String(number);

	while (number.length % 3) {
		number = '#' + number;
	}

	var result = number.substr(0, 3);
	result = result.replace(/#/g, '');
	var i;
	var length = number.length;
	for (i = 3; i < length; i += 3) {
		result = result + separator + number.substr(i, 3);
	}

	return result;
}

/**
 * @param {Number} number
 * @param {Object|String} [options=' ']
 * @param {String} [options.separator=' ']
 * @param {Boolean} [options.formatFourDigits=true]
 * @returns {String}
 */
module.exports = function (number, options) {
	var result = '';
	var separator = NBSP;
	var formatFourDigits = true;

	if (!number && number !== 0) {
		return result;
	}

	var numberObject = parseNumber(number);
	var numberString = String(number);

	if (typeof options === 'object') {
		if (options.separator) {
			separator = options.separator;
		}

		if (typeof options.formatFourDigits === 'boolean') {
			formatFourDigits = options.formatFourDigits;
		}
	} else if (typeof options !== 'undefined') {
		separator = options;
	}

	if (
		numberObject.integer.length <= 3 ||
		(numberObject.integer.length === 4 && !formatFourDigits)
	) {
		result = numberString;
	} else {
		result += numberObject.sign;
		result += format(numberObject.integer, separator);
		if (numberObject.fraction) {
			result += '.';
			result += numberObject.fraction;
		}
	}

	return result;
};