JavaScript moduli: Vodič za početnike

Ako ste tek početnik u JavaScript-u, žargoni poput "paketi modula protiv učitavanja modula", "Webpack vs Browserify" i "AMD vs. CommonJS" mogu brzo postati neodoljivi.

Sustav JavaScript modula možda zastrašuje, ali njegovo je razumijevanje vitalno za web programere.

U ovom postu raspakirat ću ove modne riječi na običnom engleskom jeziku (i nekoliko uzoraka koda). Nadam se da će vam biti od pomoći!

Napomena: radi jednostavnosti, ovo će biti podijeljeno u dva odjeljka: Dio 1 zaronit će u objašnjenje što su moduli i zašto ih koristimo. Dio 2 (objavljen sljedeći tjedan) provest će kroz značenje grupiranja modula i različite načine za to.

1. dio: Može li mi netko objasniti što su to moduli?

Dobri autori svoje knjige dijele na poglavlja i odjeljke; dobri programeri dijele svoje programe na module.

Poput poglavlja iz knjige, moduli su samo nakupine riječi (ili koda, ovisno o slučaju).

Dobri su moduli, međutim, vrlo samostalni s izrazitom funkcionalnošću, što omogućava njihovo miješanje, uklanjanje ili dodavanje po potrebi, bez ometanja sustava u cjelini.

Zašto koristiti module?

Puno je prednosti korištenja modula u korist raširene, međusobno ovisne baze koda. Po meni su najvažniji:

1) Održavanje: Po definiciji, modul je samostalan. Dobro dizajnirani modul ima za cilj što je moguće manje smanjiti ovisnosti o dijelovima baze podataka, tako da može samostalno rasti i poboljšavati se. Ažuriranje pojedinog modula mnogo je lakše kada je modul odvojen od ostalih dijelova koda.

Vratimo se našem primjeru knjige, ako biste željeli ažurirati poglavlje u svojoj knjizi, bila bi noćna mora ako bi mala promjena u jednom poglavlju zahtijevala da podesite i svako drugo poglavlje. Umjesto toga, trebali biste napisati svako poglavlje na takav način da se mogu napraviti poboljšanja bez utjecaja na druga poglavlja.

2) Prostor imena: U JavaScript-u su varijable izvan opsega funkcije najviše razine globalne (što znači da im svi mogu pristupiti). Zbog toga je uobičajeno imati "zagađenje prostora imena", gdje potpuno nepovezani kod dijeli globalne varijable.

Dijeljenje globalnih varijabli između nepovezanog koda veliko je ne-ne u razvoju.

Kao što ćemo vidjeti kasnije u ovom postu, moduli nam omogućuju izbjegavanje onečišćenja prostora imena stvaranjem privatnog prostora za naše varijable.

3) Ponovna upotrebljivost: Budimo iskreni ovdje: svi smo u jednom trenutku kopirali kod koji smo prethodno napisali u nove projekte. Na primjer, zamislimo da ste kopirali neke korisne metode koje ste napisali iz prethodnog projekta u svoj trenutni projekt.

To je sve u redu, ali ako pronađete bolji način da napišete neki dio tog koda, morat ćete se vratiti i prisjetiti se da ga ažurirate svugdje drugdje gdje ste ga napisali.

To je očito ogromno gubljenje vremena. Ne bi li bilo puno lakše da postoji - pričekajte - modul koji možemo iznova i iznova koristiti?

Kako možete ugraditi module?

Postoji mnogo načina za ugradnju modula u vaše programe. Prođimo kroz nekoliko njih:

Uzorak modula

Uzorak Modula koristi se za oponašanje koncepta klasa (budući da JavaScript izvorno ne podržava klase), tako da možemo pohraniti i javne i privatne metode i varijable u jedan objekt - slično kao što se klase koriste u drugim programskim jezicima poput Jave ili Python. To nam omogućuje da stvorimo API koji se suočava s javnošću za metode koje želimo izložiti svijetu, a da pritom inkapsuliramo privatne varijable i metode u opseg zatvaranja.

Postoji nekoliko načina za postizanje uzorka modula. U ovom prvom primjeru upotrijebit ću anonimno zatvaranje. To će nam pomoći da postignemo svoj cilj stavljajući sav naš kôd u anonimnu funkciju. (Zapamtite: u JavaScriptu su funkcije jedini način za stvaranje novog opsega.)

Primjer 1: Anonimno zatvaranje

(function () { // We keep these variables private inside this closure scope var myGrades = [93, 95, 88, 0, 55, 91]; var average = function() { var total = myGrades.reduce(function(accumulator, item) { return accumulator + item}, 0); return 'Your average grade is ' + total / myGrades.length + '.'; } var failing = function(){ var failingGrades = myGrades.filter(function(item) { return item < 70;}); return 'You failed ' + failingGrades.length + ' times.'; } console.log(failing()); }()); // ‘You failed 2 times.’

Ovom konstrukcijom naša anonimna funkcija ima svoje okruženje za procjenu ili „zatvaranje“, a mi je odmah procijenjujemo. To nam omogućuje skrivanje varijabli iz nadređenog (globalnog) prostora imena.

Ono što je lijepo kod ovog pristupa je to što možete koristiti lokalne varijable unutar ove funkcije bez slučajnog prepisivanja postojećih globalnih varijabli, ali ipak pristupite globalnim varijablama, na primjer:

var global = 'Hello, I am a global variable :)'; (function () { // We keep these variables private inside this closure scope var myGrades = [93, 95, 88, 0, 55, 91]; var average = function() { var total = myGrades.reduce(function(accumulator, item) { return accumulator + item}, 0); return 'Your average grade is ' + total / myGrades.length + '.'; } var failing = function(){ var failingGrades = myGrades.filter(function(item) { return item < 70;}); return 'You failed ' + failingGrades.length + ' times.'; } console.log(failing()); console.log(global); }()); // 'You failed 2 times.' // 'Hello, I am a global variable :)'

Imajte na umu da su potrebne zagrade oko anonimne funkcije, jer se izrazi koji počinju s ključnom riječi function uvijek smatraju deklaracijama funkcija (sjetite se, ne možete imati neimenovane deklaracije funkcija u JavaScriptu.) Slijedom toga, okolne zagrade kreiraju izraz funkcije umjesto toga. Ako ste znatiželjni, više možete pročitati ovdje.

Primjer 2: Globalni uvoz

Još jedan popularan pristup koji koriste knjižnice poput jQuery-a je globalni uvoz. Slično je anonimnom zatvaranju koje smo upravo vidjeli, osim što sada kao parametre unosimo globale:

(function (globalVariable) { // Keep this variables private inside this closure scope var privateFunction = function() { console.log('Shhhh, this is private!'); } // Expose the below methods via the globalVariable interface while // hiding the implementation of the method within the // function() block globalVariable.each = function(collection, iterator) { if (Array.isArray(collection)) { for (var i = 0; i < collection.length; i++) { iterator(collection[i], i, collection); } } else { for (var key in collection) { iterator(collection[key], key, collection); } } }; globalVariable.filter = function(collection, test) { var filtered = []; globalVariable.each(collection, function(item) { if (test(item)) { filtered.push(item); } }); return filtered; }; globalVariable.map = function(collection, iterator) { var mapped = []; globalUtils.each(collection, function(value, key, collection) { mapped.push(iterator(value)); }); return mapped; }; globalVariable.reduce = function(collection, iterator, accumulator) { var startingValueMissing = accumulator === undefined; globalVariable.each(collection, function(item) { if(startingValueMissing) { accumulator = item; startingValueMissing = false; } else { accumulator = iterator(accumulator, item); } }); return accumulator; }; }(globalVariable)); 

U ovom je primjeru globalVariable jedina globalna varijabla. Prednost ovog pristupa nad anonimnim zatvaranjem je u tome što unaprijed deklarirate globalne varijable, čineći ih kristalno jasnima ljudima koji čitaju vaš kôd.

Primjer 3: Sučelje objekta

Još jedan pristup je stvaranje modula pomoću samostalnog objektnog sučelja, na sljedeći način:

var myGradesCalculate = (function () { // Keep this variable private inside this closure scope var myGrades = [93, 95, 88, 0, 55, 91]; // Expose these functions via an interface while hiding // the implementation of the module within the function() block return { average: function() { var total = myGrades.reduce(function(accumulator, item) { return accumulator + item; }, 0); return'Your average grade is ' + total / myGrades.length + '.'; }, failing: function() { var failingGrades = myGrades.filter(function(item) { return item < 70; }); return 'You failed ' + failingGrades.length + ' times.'; } } })(); myGradesCalculate.failing(); // 'You failed 2 times.' myGradesCalculate.average(); // 'Your average grade is 70.33333333333333.'

Kao što vidite, ovaj pristup omogućuje nam da odlučimo koje varijable / metode želimo zadržati privatnima (npr. MyGrades ) i koje varijable / metode želimo izložiti stavljanjem u povratnu izjavu (npr. Prosjek i neuspjeh ).

Primjer 4: Otkrivanje uzorka modula

Ovo je vrlo slično gornjem pristupu, osim što osigurava da sve metode i varijable budu privatne dok se izričito ne izlože:

var myGradesCalculate = (function () { // Keep this variable private inside this closure scope var myGrades = [93, 95, 88, 0, 55, 91]; var average = function() { var total = myGrades.reduce(function(accumulator, item) { return accumulator + item; }, 0); return'Your average grade is ' + total / myGrades.length + '.'; }; var failing = function() { var failingGrades = myGrades.filter(function(item) { return item < 70; }); return 'You failed ' + failingGrades.length + ' times.'; }; // Explicitly reveal public pointers to the private functions // that we want to reveal publicly return { average: average, failing: failing } })(); myGradesCalculate.failing(); // 'You failed 2 times.' myGradesCalculate.average(); // 'Your average grade is 70.33333333333333.'

To se možda čini mnogo za uzeti, ali to je samo vrh ledenog brijega kada su u pitanju uzorci modula. Evo nekoliko resursa koji su mi se učinili korisnim u mojim vlastitim istraživanjima:

  • Učenje obrazaca dizajna JavaScript-a, Addy Osmani: riznica detalja u impresivno jezgrovitom čitanju
  • Adekvatno dobro, Ben Cherry: koristan pregled s primjerima napredne upotrebe uzorka modula
  • Blog Carla Danleyja: pregled uzorka modula i resursi za druge JavaScript obrasce.

CommonJS i AMD

Pristupi prije svega imaju jedno zajedničko: korištenje jedne globalne varijable za umotavanje koda u funkciju, stvarajući tako privatni prostor imena za sebe pomoću opsega zatvaranja.

While each approach is effective in its own way, they have their downsides.

For one, as a developer, you need to know the right dependency order to load your files in. For instance, let’s say you’re using Backbone in your project, so you include the script tag for Backbone’s source code in your file.

However, since Backbone has a hard dependency on Underscore.js, the script tag for the Backbone file can’t be placed before the Underscore.js file.

As a developer, managing dependencies and getting these things right can sometimes be a headache.

Another downside is that they can still lead to namespace collisions. For example, what if two of your modules have the same name? Or what if you have two versions of a module, and you need both?

So you’re probably wondering: can we design a way to ask for a module’s interface without going through the global scope?

Fortunately, the answer is yes.

There are two popular and well-implemented approaches: CommonJS and AMD.

CommonJS

CommonJS is a volunteer working group that designs and implements JavaScript APIs for declaring modules.

A CommonJS module is essentially a reusable piece of JavaScript which exports specific objects, making them available for other modules to require in their programs. If you’ve programmed in Node.js, you’ll be very familiar with this format.

With CommonJS, each JavaScript file stores modules in its own unique module context (just like wrapping it in a closure). In this scope, we use the module.exports object to expose modules, and require to import them.

When you’re defining a CommonJS module, it might look something like this:

function myModule() { this.hello = function() { return 'hello!'; } this.goodbye = function() { return 'goodbye!'; } } module.exports = myModule;

We use the special object module and place a reference of our function into module.exports. This lets the CommonJS module system know what we want to expose so that other files can consume it.

Then when someone wants to use myModule, they can require it in their file, like so:

var myModule = require('myModule'); var myModuleInstance = new myModule(); myModuleInstance.hello(); // 'hello!' myModuleInstance.goodbye(); // 'goodbye!'

There are two obvious benefits to this approach over the module patterns we discussed before:

1. Avoiding global namespace pollution

2. Making our dependencies explicit

Moreover, the syntax is very compact, which I personally love.

Another thing to note is that CommonJS takes a server-first approach and synchronously loads modules. This matters because if we have three other modules we need to require, it’ll load them one by one.

Now, that works great on the server but, unfortunately, makes it harder to use when writing JavaScript for the browser. Suffice it to say that reading a module from the web takes a lot longer than reading from disk. For as long as the script to load a module is running, it blocks the browser from running anything else until it finishes loading. It behaves this way because the JavaScript thread stops until the code has been loaded. (I’ll cover how we can work around this issue in Part 2 when we discuss module bundling. For now, that’s all we need to know).

AMD

CommonJS is all well and good, but what if we want to load modules asynchronously? The answer is called Asynchronous Module Definition, or AMD for short.

Loading modules using AMD looks something like this:

define(['myModule', 'myOtherModule'], function(myModule, myOtherModule) { console.log(myModule.hello()); });

What’s happening here is that the define function takes as its first argument an array of each of the module’s dependencies. These dependencies are loaded in the background (in a non-blocking manner), and once loaded define calls the callback function it was given.

Next, the callback function takes, as arguments, the dependencies that were loaded — in our case, myModule and myOtherModule — allowing the function to use these dependencies. Finally, the dependencies themselves must also be defined using the define keyword.

For example, myModule might look like this:

define([], function() { return { hello: function() { console.log('hello'); }, goodbye: function() { console.log('goodbye'); } }; });

So again, unlike CommonJS, AMD takes a browser-first approach alongside asynchronous behavior to get the job done. (Note, there are a lot of people who strongly believe that dynamically loading files piecemeal as you start to run code isn’t favorable, which we’ll explore more when in the next section on module-building).

Aside from asynchronicity, another benefit of AMD is that your modules can be objects, functions, constructors, strings, JSON and many other types, while CommonJS only supports objects as modules.

That being said, AMD isn’t compatible with io, filesystem, and other server-oriented features available via CommonJS, and the function wrapping syntax is a bit more verbose compared to a simple require statement.

UMD

For projects that require you to support both AMD and CommonJS features, there’s yet another format: Universal Module Definition (UMD).

UMD essentially creates a way to use either of the two, while also supporting the global variable definition. As a result, UMD modules are capable of working on both client and server.

Here’s a quick taste of how UMD goes about its business:

(function (root, factory) { if (typeof define === 'function' && define.amd) { // AMD define(['myModule', 'myOtherModule'], factory); } else if (typeof exports === 'object') { // CommonJS module.exports = factory(require('myModule'), require('myOtherModule')); } else { // Browser globals (Note: root is window) root.returnExports = factory(root.myModule, root.myOtherModule); } }(this, function (myModule, myOtherModule) { // Methods function notHelloOrGoodbye(){}; // A private method function hello(){}; // A public method because it's returned (see below) function goodbye(){}; // A public method because it's returned (see below) // Exposed public methods return { hello: hello, goodbye: goodbye } }));

For more examples of UMD formats, check out this enlightening repo on GitHub.

Native JS

Phew! Are you still around? I haven’t lost you in the woods here? Good! Because we have *one more* type of module to define before we’re done.

As you probably noticed, none of the modules above were native to JavaScript. Instead, we’ve created ways to emulate a modules system by using either the module pattern, CommonJS or AMD.

Fortunately, the smart folks at TC39 (the standards body that defines the syntax and semantics of ECMAScript) have introduced built-in modules with ECMAScript 6 (ES6).

ES6 offers up a variety of possibilities for importing and exporting modules which others have done a great job explaining — here are a few of those resources:

  • jsmodules.io
  • exploringjs.com

What’s great about ES6 modules relative to CommonJS or AMD is how it manages to offer the best of both worlds: compact and declarative syntax and asynchronous loading, plus added benefits like better support for cyclic dependencies.

Probably my favorite feature of ES6 modules is that imports are live read-only views of the exports. (Compare this to CommonJS, where imports are copies of exports and consequently not alive).

Here’s an example of how that works:

// lib/counter.js var counter = 1; function increment() { counter++; } function decrement() { counter--; } module.exports = { counter: counter, increment: increment, decrement: decrement }; // src/main.js var counter = require('../../lib/counter'); counter.increment(); console.log(counter.counter); // 1

In this example, we basically make two copies of the module: one when we export it, and one when we require it.

Moreover, the copy in main.js is now disconnected from the original module. That’s why even when we increment our counter it still returns 1 — because the counter variable that we imported is a disconnected copy of the counter variable from the module.

So, incrementing the counter will increment it in the module, but won’t increment your copied version. The only way to modify the copied version of the counter variable is to do so manually:

counter.counter++; console.log(counter.counter); // 2

On the other hand, ES6 creates a live read-only view of the modules we import:

// lib/counter.js export let counter = 1; export function increment() { counter++; } export function decrement() { counter--; } // src/main.js import * as counter from '../../counter'; console.log(counter.counter); // 1 counter.increment(); console.log(counter.counter); // 2

Cool stuff, huh? What I find really compelling about live read-only views is how they allow you to split your modules into smaller pieces without losing functionality.

Then you can turn around and merge them again, no problem. It just “works.”

Looking forward: bundling modules

Wow! Where does the time go? That was a wild ride, but I sincerely hope it gave you a better understanding of modules in JavaScript.

In the next section I’ll walk through module bundling, covering core topics including:

  • Why we bundle modules
  • Different approaches to bundling
  • ECMAScript’s module loader API
  • …and more. :)

NOTE: To keep things simple, I skipped over some of the nitty-gritty details (think: cyclic dependencies) in this post. If I left out anything important and/or fascinating, please let me know in the comments!