Implementation of Ruby on Rails ActiveSupport::NumberHelper::NumberToHumanSizeConverter in javascript < ES5

number-to-human-size-converter.js

var NumberToHumanSizeConverter = {
  STORAGE_UNITS: ['bytes', 'kb', 'mb', 'gb', 'tb', 'pb', 'eb'],
  base: 1024,
  parsedNum: null,
  defaults: {
    delimeter: '.',
    precision: '2'
  },

  convert: function(number) {
    this.parsedNum = parseFloat(number);

    if (this._isSmallerThanBase(this.parsedNum)) {
      var numberToFormat = parseInt(this.parsedNum);
    } else {
      var numberToFormat = this.parsedNum / (this.base ** this._exponent());
    }
    var humanSize = this._conversionFormat(numberToFormat);

    return interpolate("%s %s", [humanSize, this._unit().toUpperCase()])
  },

  _isSmallerThanBase: function() {
    if(parseInt(this.parsedNum) < this.base) return true;
    return false;
  },

  _exponent: function() {
    var max = this.STORAGE_UNITS.size - 1;
    var exp = parseInt(Math.log(this.parsedNum) / Math.log(this.base));

    if(exp > max) {
      return exp;
    }

    return exp;
  },

  _unit: function() {
    return this._isSmallerThanBase() ? "byte" : this.STORAGE_UNITS[this._exponent()]
  },

  _conversionFormat: function(converted) {
    var formatted = converted;

    if (!this._isInt(converted)) {
      formatted = formatted.toFixed(this.defaults.precision).
        toString().
        replace('.', this.defaults.delimeter);
    }

    return formatted;
  },

  _isInt: function(n) {
    return parseInt(n) === n;
  }
};


// See this for original answer https://stackoverflow.com/a/31007976/5347939
var interpolate = function(theString, argumentArray) {
  var regex = /%s/;
  var _r = function(p,c) { return p.replace(regex, c); }
  return argumentArray.reduce(_r, theString);
};

Usage

NumberToHumanSizeConverter.convert(1024 * 1000000);
=> "976.56 MB"

NumberToHumanSizeConverter.convert(1024);
=> "1 KB"

NumberToHumanSizeConverter.convert(764);
=> "764 BYTES"