Running the same JS on client and server

gistfile1.js

var serverSide = (typeof module !== "undefined");

if (serverSide) {
  var Canvas = require('canvas')
    , Image = Canvas.Image
    , fs = require('fs');
}

function HeroImage() {
  var height = 620;
  var width = 560;
  var xOffset = 200;
  var yOffset = 100;

  this.updateFunction;
  this.sex = "male";
  this.stance = "standing";
  this.attributes = {
    skin: rand(5),
    hairColour: rand(5),
    hairStyle: rand(3),
    top: rand(5),
    bottom: rand(5),
    boots: rand(5)
  }
  function rand(x){
    return Math.floor((Math.random()*x)+0.5);
  }

  this.setAttribute = function(attribute, value) {
    this.attributes[attribute] = value;
    this.updateFunction && this.updateFunction();
  }

  this.loadSprites = function(callback) {
    var spritesLoaded = 0;
    var path = serverSide ? 'public' : '';
    var onerror = function(err) { console.log('ERROR',err); };
    var onload = function() {
      if (++spritesLoaded == 2) callback && callback()
    };

    this.sprite = new Image();
    this.hairSprite = new Image();

    this.sprite.onload = this.hairSprite.onload = onload;
    this.sprite.onerror = this.hairSprite.onerror = onerror;

    this.sprite.src = path+'/images/'+this.sex+'-'+this.stance+'.png';
    this.hairSprite.src = path+'/images/'+this.sex+'-hair.png';
  }

  this.drawPart = function(ctx, down, across) {
    var partWidth = 200;
    var partHeight = 400;

    var x = across * partWidth;
    var y = down * partHeight;
    ctx.drawImage(this.sprite, x, y, partWidth, partHeight, xOffset, yOffset, partWidth, partHeight);
  }

  this.drawHair = function(ctx, down, across) {
    var hairWidth = 150;
    var hairHeight = 150;

    var x = across * hairWidth;
    var y = down * hairHeight;
    ctx.drawImage(this.hairSprite, x, y, hairWidth, hairWidth, xOffset, yOffset, hairWidth, hairWidth);
  }
  
  this.drawOnContext = function(ctx) {
    ctx.clearRect(0, 0, width, height);

    this.drawPart(ctx, 0, this.attributes.skin);
    this.drawPart(ctx, 1, this.attributes.top);
    this.drawPart(ctx, 2, this.attributes.bottom);
    this.drawHair(ctx, this.attributes.hairStyle, this.attributes.hairColour);
    this.drawPart(ctx, 3, this.attributes.hair);
    this.drawPart(ctx, 4, this.attributes.shoes);
    this.drawPart(ctx, 5, this.attributes.accessory);
  };
};

if (serverSide) {  
	module.exports = HeroImage;
}