Pretty print number of bytes

Converting a numeric byte value into a human-readable string is a pretty useful tool to have in your utility belt. While seemingly easy at first sight, it takes a few steps to get right.

First and foremost, we need to define an array of units that we can attach to the resulting string, based on the exponent. We then need to calculate the exponent of the number, which will determine the unit we use. After that, we need to truncate the number to a certain number of digits, which can be done using Number.prototype.toPrecision().

Finally, we need to build the resulting string, taking into account the options we've been given and whether the number is negative or not. By default, we add a space between the number and the unit, but this can be omitted, using the addSpace argument.

const prettyBytes = (num, precision = 3, addSpace = true) => {
  const UNITS = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
  if (Math.abs(num) < 1) return num + (addSpace ? ' ' : '') + UNITS[0];

  const exponent = Math.min(
    Math.floor(Math.log10(num < 0 ? -num : num) / 3),
    UNITS.length - 1
  );

  const n = Number(
    ((num < 0 ? -num : num) / 1000 ** exponent).toPrecision(precision)
  );

  return (num < 0 ? '-' : '') + n + (addSpace ? ' ' : '') + UNITS[exponent];
};

prettyBytes(1000); // '1 KB'
prettyBytes(-27145424323.5821, 5); // '-27.145 GB'
prettyBytes(123456789, 3, false); // '123MB'