index.js

module.exports = prettierBytes

function prettierBytes (num) {
  if (typeof num !== 'number' || isNaN(num)) {
    throw new TypeError('Expected a number, got ' + typeof num)
  }

  var neg = num < 0
  var units = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB']

  if (neg) {
    num = -num
  }

  if (num < 1) {
    return (neg ? '-' : '') + num + ' B'
  }

  var exponent = Math.min(Math.floor(Math.log(num) / Math.log(1000)), units.length - 1)
  num = Number(num / Math.pow(1000, exponent))
  var unit = units[exponent]

  if (num >= 10 || num % 1 === 0) {
    // Do not show decimals when the number is two-digit, or if the number has no
    // decimal component.
    return (neg ? '-' : '') + num.toFixed(0) + ' ' + unit
  } else {
    return (neg ? '-' : '') + num.toFixed(1) + ' ' + unit
  }
}