purdy.js

var PurdyPercent = function (num, options) {
  this.settings = {
    hide_decimal_on_whole : true,
    decimals : 2,
    truncate : false, // (soon)
    match_decimals: true, // forces floats to match defined decimal count
    rounding : 'default', //default, up, down (soon)
    postfix : '%'
  };

  // helper methods
  this.extend = function(destination, source) {
    for (var property in source) {
      if (source.hasOwnProperty(property)) {
        destination[property] = source[property];
      }
    }

    return destination;
  };

  this.is_float = function () {
    this.modulus = this.num % 1;
    this.root = parseInt(this.num, 10);
    return this.modulus % 1 != 0;
  };

  this.set_decimals = function () {
    var factor = Math.pow(10,this.settings.decimals);

    this.value = Math.round(this.num*factor)/factor;
    if (this.settings.match_decimals) this.match_decimals();
  };

  this.match_decimals = function () {
    var fraction = this.fraction(),
        length = fraction.length,
        diff = this.settings.decimals - length;

    if (diff > 0) {
      for (var i = diff - 1; i >= 0; i--) {
        fraction = fraction + '0';
      }
    } else if (diff < 0) {
      diff = Math.abs(diff);
      for (var i = diff - 1; i >= 0; i--) {
        fraction = fraction.slice(0, -1);
      }
    }

    this.value = [this.root, fraction].join('.');

  };

  this.fraction = function () {
    return this.num.toString().split('.')[1];
  };

  // logic
  if (num instanceof Array) {
    this.numerator = parseFloat(num[0]);
    this.denominator = parseFloat(num[1]);
    this.num = (this.numerator / this.denominator) * 100.0;
  } else {
    this.num = parseFloat(num);
  }

  this.extend(this.settings, this.options || options || {});

  if (this.is_float()) {
    this.set_decimals();
  } else {
    if (this.settings.hide_decimal_on_whole) {
      this.value = parseInt(this.value, 10);
    } else {
      this.value = this.num;
    }
  }

  return [this.value, this.settings.postfix].join('');
};