Talk Lazy Loading JS Modules on ForwardJS 2017
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Lazy loading JS modules in the browser

Tiago Garcia @ ForwardJS
Jul 27th, 2017

Tiago Garcia

Avenue Code

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Part I


  • Part I: JavaScript Lazy-loading for dummies
    • SPA for performance?
    • Lazy-loading 101
    • Do I need Lazy-loading?
    • How to Lazy load?

SPA for performance?

SPA for performance?

  • Moving you server-side app to SPA would improve the page performance:
    • Requests returning JSON instead of HTML
    • Rendering a new view instead of a page reload
    • Routing and state management on the client-side
  • But that may not be enough!
    • Have you ever checked if you application is downloading more stuff that is being actually used?
    • All those performance gains can fall short if you download all your stuff at once!

Lazy-loading 101

  • E-commerce A (server-side rendered) has 5 pages: home, browse, product, cart, checkout
    • Each page has 300 KB HTML + 100 KB JS
    • The complete flow will download:
      • *400 KB* for each page
      • *2 MB* total

Lazy-loading 101

  • E-commerce B (SPA) has the same 5 pages
    • 1 actual page + 4 views
    • Home page has 300 KB HTML + 100 KB JS
    • Each of the views has 50 KB JSON + 150 KB JS
    • The complete flow will download:
      • *1.2 MB* total (vs *2 MB*)
      • 80% just on home page: *1 MB* (vs *400 KB*)
    • However, if you use Lazy-loading:
      • 400 KB on home page
      • 200 KB each view after

What is Lazy-loading?

  • Lazy loading is a design pattern about deferring the initialization (loading/fetching/allocation) of a resource (code/data/asset) until the point at which it is needed.
  • Its main goal is to improve efficiency when a significant amount of resources is not needed at first.
  • Lazy-loading is targeted to increase performance and save on memory consumption and processing power.
  • It's an EAA pattern from Martin Fowler.

Do I need Lazy-loading?

  • Above the fold
    • what you see first when you open a page
    • part of the Critical Rendering Path
    • must be rendered during the page load time
    • thus, it can't be lazy loaded
  • Below the fold
    • everything else, needs scrolling or user interaction
    • won't be displayed during the page load time
    • it doesn't need to be rendered with the page load
    • thus, it can be lazy loaded

Do I need Lazy-loading?

  • Ask yourself: is there any chunk of code/library that only runs:
    • below the fold (as a reviews panel)?
    • after some event (as a button click)?
    • upon a certain condition (as an uncommon widget)?
  • If you answered yes, you may profit from lazy load and potentially improve your page performance.
  • Just defer the downloading of those chunks of code/libraries until the trigger is executed.

Do I need Lazy-loading?

  • Lazy loading isn't recommended for certain scenarios:
    • Supporting network limitations (offline mode)
    • Web-based mobile apps (Web Views)
    • Apps that can't be paused (games)
    • Specific UX requirements (single loading screen)

How to Lazy load?

  • DON'T include all your scripts in the page at once.
  • DON'T import all your modules on the top of your file.
  • Carefully decide WHEN to import or require your modules and libraries:
    • Below the fold (scroll listener)
    • Event callbacks (user interactions / network calls)
    • Conditionally (for uncommon scenarios)
    • After some time (chat overlays)
  • This talk is about JS but you can also lazy load images, fonts and CSS.

Part II


  • Part II: Blazing loading
    • Modules
    • Lazy-loading in CommonJS
    • Lazy-loading in ES Modules
    • Webpack 2


  • Structural design pattern
  • Purpose: to define reusable components with private/public variables and functions
  • Pre-condition: a chunk of code with a return point
  • Post-condition: a definition that represents that chunk of code as a module

Global object

  • Encapsulation through closures -> function scope
  var Kennel = (function() {
    var getBarkStyle = function(isHowler) {
      return isHowler? 'woooooow!': 'woof, woof!';
    return {
      Dog: function(name, breed) { = name;
        this.bark = getBarkStyle(breed === 'husky');
      Wolf: function(name) { = name;
        this.bark = getBarkStyle(true);
  })(); // IIFE
var myDog = new Kennel.Dog('Sherlock', 'beagle');
console.log( + ': ' + myDog.bark); // Sherlock: woof, woof!

Revealing Module

  • Flexibility to switch items from private to public scope.
  let Kennel = (function() {
    let getBarkStyle = isHowler => isHowler ? 'woooooow!' : 'woof, woof!';
    let Dog = function(name, breed) { = name;
      this.bark = getBarkStyle(breed === 'husky');
    let Wolf = function(name) { = name;
      this.bark = getBarkStyle(true);
    return {
      Dog: Dog,
      Wolf: Wolf
  })(); // IIFE

Why to use Modules?

  • Spaghetti code is bad for reusability, readability, code organization and is very brittle (side-effects).
  • Modules can fix these all if properly implemented.
  • A module can be delivered as a dependency for another module.
  • Modules can be packaged and deployed separately from each other, mitigating the "butterfly effect".
  • Modules bring cohesion up and coupling down.

Module loaders

  • On the previous example, Kennel is a global var:
    • fragile (any posterior code can modify/redefine it)
    • not scalable (what if you define 100 modules?)
    • counter-productive (you have to manually resolve your dependencies)
  • Enter module loaders:
    • Container for module registration under aliases
    • Dependency injection
    • Modules loading on demand
  • Included with your Module standard of choice!


  • Export your module interface with module.exports
  • Import on the client using require(dependency)
  // dog.js
  var Dog = function(name, breed) { = name;
    this.breed = breed;
  Dog.prototype.bark = function() {
    return + ': ' + getBarkStyle(this.breed);
  function getBarkStyle(breed) {
    return breed === 'husky'? 'woooooow!': 'woof, woof!';
  module.exports = Dog;

Lazy-loading in CommonJS

// main.js
        .addEventListener('click', function(e) {
  // Lazy-loading dog module
  require.ensure([], function(require) {
    var Dog = require('./dog'),
        dogContainer = document.getElementById('dogContainer');

    var sherlock = new Dog('Sherlock', 'beagle');
    dogContainer.innerHTML += sherlock.bark();

    var whisky = new Dog('Whisky', 'husky');
    dogContainer.innerHTML += '<br/>' + whisky.bark();

ES Modules

  • ES2015+ offers native Modules which are quite a bit similar to CommonJS.
// dog.js
let getBarkStyle = breed => {
  return breed === 'husky'? 'woooooow!': 'woof, woof!';

export class Dog {
  constructor(name, breed) { = name;
    this.breed = breed;

  bark() {
    return `${}: ${getBarkStyle(this.breed)}`;

Using ES Modules


  • System.js is a module loader which supports AMD, CommonJS, ES and global scripts.
  • It performs asynchronous module loading using a Promises-based API.
  • Promises can be chained and combined.
  • Promises.all can load multiple modules in parallel.
  • System.js 0.2.0 ships with dynamic import() operator.

Lazy-loading ES Modules

// main.js
        .addEventListener('click', e => {
  // Lazy-loading dog module
  import('dog').then(Dog => {
    let dogContainer = document.getElementById('dogContainer');

    let sherlock = new Dog('Sherlock', 'beagle');
    dogContainer.innerHTML += sherlock.bark();

    let whisky = new Dog('Whisky', 'husky');
    dogContainer.innerHTML += `<br/>${whisky.bark()}`;
  })).catch(err => {
    console.log("Module loading failed");

Webpack 2

  • Webpack is a module bundler constituted of Entries, Outputs, Loaders and Plugins.
  • Webpack 2 offers native ES modules and System.js support.
  • Loads JS modules in AMD, CommonJS and ES modules (and also TypeScript and CoffeeScript).
  • Performs bundling (importing the dependency graph in the right order).
  • Lazy-loads JS and CSS (code splitting).
  • Performs tree-shaking.