index.js

'use strict';

var UNITS = ['B', 'kB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];

/*
Formats the given number using `Number#toLocaleString`.
- If locale is a string, the value is expected to be a locale-key (for example: `de`).
- If locale is true, the system default locale is used for translation.
- If no value for locale is specified, the number is returned unmodified.
*/
function toLocaleString(number, locale) {
	var result = number;
	if (typeof locale === 'string') {
		result = number.toLocaleString(locale);
	} else if (locale === true) {
		result = number.toLocaleString();
	}

	return result;
}

export default function (number, options) {
	if (!Number.isFinite(number)) {
		throw new TypeError('Expected a finite number, got ' + typeof number + ': ' + number);
	}

	options = Object.assign({}, options);

	if (options.signed && number === 0) {
		return ' 0 B';
	}

	var isNegative = number < 0;
	var prefix = isNegative ? '-' : options.signed ? '+' : '';

	if (isNegative) {
		number = -number;
	}

	if (number < 1) {
		var _numberString = toLocaleString(number, options.locale);
		return prefix + _numberString + ' B';
	}

	var exponent = Math.min(Math.floor(Math.log10(number) / 3), UNITS.length - 1);
	number = Number((number / Math.pow(1000, exponent)).toPrecision(3));
	var numberString = toLocaleString(number, options.locale);

	var unit = UNITS[exponent];

	return prefix + numberString + ' ' + unit;
};