index.js

'use strict';

const BYTE_UNITS = [
	'B',
	'kB',
	'MB',
	'GB',
	'TB',
	'PB',
	'EB',
	'ZB',
	'YB'
];

const BIT_UNITS = [
	'b',
	'kbit',
	'Mbit',
	'Gbit',
	'Tbit',
	'Pbit',
	'Ebit',
	'Zbit',
	'Ybit'
];

/*
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.
*/
const toLocaleString = (number, locale) => {
	let result = number;
	if (typeof locale === 'string') {
		result = number.toLocaleString(locale);
	} else if (locale === true) {
		result = number.toLocaleString();
	}

	return result;
};

module.exports = (number, options) => {
	if (!Number.isFinite(number)) {
		throw new TypeError(`Expected a finite number, got ${typeof number}: ${number}`);
	}

	options = Object.assign({bits: false}, options);
	const UNITS = options.bits ? BIT_UNITS : BYTE_UNITS;

	if (options.signed && number === 0) {
		return ' 0 ' + UNITS[0];
	}

	const isNegative = number < 0;
	const prefix = isNegative ? '-' : (options.signed ? '+' : '');

	if (isNegative) {
		number = -number;
	}

	if (number < 1) {
		const numberString = toLocaleString(number, options.locale);
		return prefix + numberString + ' ' + UNITS[0];
	}

	const exponent = Math.min(Math.floor(Math.log10(number) / 3), UNITS.length - 1);
	// eslint-disable-next-line unicorn/prefer-exponentiation-operator
	number = Number((number / Math.pow(1000, exponent)).toPrecision(3));
	const numberString = toLocaleString(number, options.locale);

	const unit = UNITS[exponent];

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