Zahtjevi modula u Node.js: Sve što trebate znati

Ažuriranje: Ovaj je članak sada dio moje knjige "Node.js izvan osnova".

Pročitajte ažuriranu verziju ovog sadržaja i više o Nodeu na jscomplete.com/node-beyond-basics .

Čvor koristi dva temeljna modula za upravljanje ovisnostima o modulima:

  • requireModul, koji, kako se čini dostupan na globalnom okviru - nema potrebe za require('require').
  • moduleModul, koji također čini se da je dostupan na globalnom okviru - nema potrebe za require('module').

O requiremodulu možete razmišljati kao o naredbi, a o modulemodulu kao o organizatoru svih potrebnih modula.

Zahtjev za modulom u Nodeu nije toliko kompliciran koncept.

const config = require('/path/to/file');

Glavni objekt koji requiremodul izvozi je funkcija (kao što je korišteno u gornjem primjeru). Kada Node pozove tu require()funkciju s lokalnom stazom datoteke kao jedini argument funkcije, Node prolazi kroz sljedeći slijed koraka:

  • Rješavanje : pronaći apsolutni put datoteke.
  • Učitavanje : za određivanje vrste sadržaja datoteke.
  • Omotavanje : Da bi datoteka dobila svoj privatni opseg. To je ono što objekte requirei moduleobjekte čini lokalnim za svaku datoteku koju trebamo.
  • Procjena : To je ono što VM na kraju radi s učitanim kodom.
  • Predmemoriranje : Tako da kad ponovno zatražimo ovu datoteku, ne prelazimo sve korake drugi put.

U ovom ću članku pokušati objasniti na primjerima ove različite faze i kako oni utječu na način na koji pišemo module u Nodeu.

Prvo da kreiram direktorij za hostiranje svih primjera pomoću mog terminala:

mkdir ~/learn-node && cd ~/learn-node

Sve naredbe u ostatku ovog članka pokrenut će se iznutra ~/learn-node.

Rješavanje lokalnog puta

Dopustite mi da vas upoznam s moduleobjektom. Možete ga provjeriti u jednostavnoj REPL sesiji:

~/learn-node $ node > module Module { id: '', exports: {}, parent: undefined, filename: null, loaded: false, children: [], paths: [ ... ] }

Svaki objekt modula dobiva idsvojstvo da ga identificira. To idje obično puni put do datoteke, ali u REPL sesiji to je jednostavno.

Čvorni moduli imaju jedan-na-jedan odnos s datotekama u datotečnom sustavu. Trebamo modul učitavanjem sadržaja datoteke u memoriju.

Međutim, budući da Node dopušta mnogo načina za zahtijevanje datoteke (na primjer, relativnom stazom ili unaprijed konfiguriranom stazom), prije nego što sadržaj datoteke možemo učitati u memoriju, trebamo pronaći apsolutno mjesto te datoteke.

Kada trebamo 'find-me'modul, bez navođenja puta:

require('find-me');

Čvor će tražiti find-me.jsna svim stazama navedenim module.paths- redom.

~/learn-node $ node > module.paths [ '/Users/samer/learn-node/repl/node_modules', '/Users/samer/learn-node/node_modules', '/Users/samer/node_modules', '/Users/node_modules', '/node_modules', '/Users/samer/.node_modules', '/Users/samer/.node_libraries', '/usr/local/Cellar/node/7.7.1/lib/node' ]

Popis staza je u osnovi popis direktorija node_modules ispod svakog direktorija od trenutnog direktorija do korijenskog direktorija. Također uključuje nekoliko naslijeđenih direktorija čija se uporaba ne preporučuje.

Ako Node ne može pronaći ni find-me.jsna jednom od ovih putova, izbacit će poruku "ne može pronaći pogrešku modula."

~/learn-node $ node > require('find-me') Error: Cannot find module 'find-me' at Function.Module._resolveFilename (module.js:470:15) at Function.Module._load (module.js:418:25) at Module.require (module.js:498:17) at require (internal/module.js:20:19) at repl:1:1 at ContextifyScript.Script.runInThisContext (vm.js:23:33) at REPLServer.defaultEval (repl.js:336:29) at bound (domain.js:280:14) at REPLServer.runBound [as eval] (domain.js:293:12) at REPLServer.onLine (repl.js:533:10)

Ako sada napravite lokalni node_modulesdirektorij i tamo stavite znak find-me.js, require('find-me')redak će ga pronaći.

~/learn-node $ mkdir node_modules ~/learn-node $ echo "console.log('I am not lost');" > node_modules/find-me.js ~/learn-node $ node > require('find-me'); I am not lost {} >

Ako je find-me.js, na primjer, postojala druga datoteka u bilo kojem od drugih putova, ako imamo node_modulesdirektorij ispod početnog direktorija i tamo imamo drugu find-me.jsdatoteku:

$ mkdir ~/node_modules $ echo "console.log('I am the root of all problems');" > ~/node_modules/find-me.js

Kada uđemo require('find-me')iz learn-nodedirektorija - koji ima svoj node_modules/find-me.js, find-me.jsdatoteka pod matičnim direktorijem uopće se neće učitati:

~/learn-node $ node > require('find-me') I am not lost {} >

Ako uklonimo lokalni node_modulesdirektorij iz ~/learn-nodei pokušamo zatražiti find-mejoš jedanput, node_moduleskoristila bi se datoteka ispod direktorija doma :

~/learn-node $ rm -r node_modules/ ~/learn-node $ node > require('find-me') I am the root of all problems {} >

Zahtijeva mapu

Moduli ne moraju biti datoteke. Također možemo stvoriti find-memapu ispod node_modulesi tamo smjestiti index.jsdatoteku. Isti require('find-me')će redak koristiti index.jsdatoteku te mape :

~/learn-node $ mkdir -p node_modules/find-me ~/learn-node $ echo "console.log('Found again.');" > node_modules/find-me/index.js ~/learn-node $ node > require('find-me'); Found again. {} >

Primijetite kako je ponovno ignorirao put kućnog direktorija node_modulesjer sada imamo lokalni.

index.jsDatoteka će se prema zadanim postavkama kada tražimo mapu, ali možemo kontrolirati ono što naziv datoteke za početak u mapu pomoću mainnekretninu u package.json. Na primjer, da bi se require('find-me')linija razlučila u drugu datoteku u find-memapi, sve što trebamo učiniti je dodati package.jsondatoteku tamo i odrediti koju datoteku treba koristiti za rješavanje ove mape:

~/learn-node $ echo "console.log('I rule');" > node_modules/find-me/start.js ~/learn-node $ echo '{ "name": "find-me-folder", "main": "start.js" }' > node_modules/find-me/package.json ~/learn-node $ node > require('find-me'); I rule {} >

zahtijevati.razriješiti

Ako želite samo riješiti modul, a ne izvršiti ga, možete koristiti require.resolvefunkciju. Ovo se ponaša potpuno isto kao glavna requirefunkcija, ali ne učitava datoteku. I dalje će izbaciti pogrešku ako datoteka ne postoji i vratit će puni put do datoteke kad je pronađena.

> require.resolve('find-me'); '/Users/samer/learn-node/node_modules/find-me/start.js' > require.resolve('not-there'); Error: Cannot find module 'not-there' at Function.Module._resolveFilename (module.js:470:15) at Function.resolve (internal/module.js:27:19) at repl:1:9 at ContextifyScript.Script.runInThisContext (vm.js:23:33) at REPLServer.defaultEval (repl.js:336:29) at bound (domain.js:280:14) at REPLServer.runBound [as eval] (domain.js:293:12) at REPLServer.onLine (repl.js:533:10) at emitOne (events.js:101:20) at REPLServer.emit (events.js:191:7) >

To se može koristiti, na primjer, za provjeru je li instaliran dodatni paket ili ne i koristiti ga samo kad je dostupan.

Relativni i apsolutni putovi

Osim rješavanja modula iz node_modulesdirektorija, modul također možemo smjestiti gdje god želimo i zahtijevamo ga ili relativnim stazama ( ./i ../) ili apsolutnim stazama koje počinju sa /.

Ako se, primjerice, find-me.jsdatoteka nalazila u libmapi umjesto u node_modulesmapi, možemo je zatražiti s:

require('./lib/find-me');

Odnos roditelja i djeteta između datoteka

Stvorite lib/util.jsdatoteku i dodajte console.logliniju tamo da je prepoznate. Također, console.logsam moduleobjekt:

~/learn-node $ mkdir lib ~/learn-node $ echo "console.log('In util', module);" > lib/util.js

Učinite isto za index.jsdatoteku, što ćemo izvršiti pomoću naredbe node. Neka ova index.jsdatoteka zahtijeva lib/util.js:

~/learn-node $ echo "console.log('In index', module); require('./lib/util');" > index.js

Sada izvršite index.jsdatoteku s čvorom:

~/learn-node $ node index.js In index Module { id: '.', exports: {}, parent: null, filename: '/Users/samer/learn-node/index.js', loaded: false, children: [], paths: [ ... ] } In util Module { id: '/Users/samer/learn-node/lib/util.js', exports: {}, parent: Module { id: '.', exports: {}, parent: null, filename: '/Users/samer/learn-node/index.js', loaded: false, children: [ [Circular] ], paths: [...] }, filename: '/Users/samer/learn-node/lib/util.js', loaded: false, children: [], paths: [...] }

Imajte na umu kako je glavni indexmodul (id: '.')sada naveden kao nadređeni lib/utilmodulu. Međutim, lib/utilmodul nije naveden kao podređeno mjesto indexmodula. Umjesto toga, tamo imamo [Circular]vrijednost jer je ovo kružna referenca. Ako Node ispiše lib/utilobjekt modula, ući će u beskonačnu petlju. Zato lib/utilreferencu jednostavno zamjenjuje s [Circular].

Sada je još važnije, što se događa ako lib/utilmodul zahtijeva glavni indexmodul? Tu ulazimo u ono što je poznato kao kružna modularna ovisnost, koja je dopuštena u Nodeu.

Da bismo ga bolje razumjeli, prvo shvatimo nekoliko drugih koncepata na objektu modula.

izvozi, module.exports i sinkrono učitavanje modula

U bilo kojem modulu izvoz je poseban objekt. Ako ste primijetili gore, svaki put kad smo ispisali objekt modula, on je imao svojstvo export, koje je do sada bilo prazan objekt. Ovom posebnom izvozu možemo dodati bilo koji atribut. Na primjer, izvezimo atribut id za index.jsi lib/util.js:

// Add the following line at the top of lib/util.js exports.id = 'lib/util'; // Add the following line at the top of index.js exports.id = 'index';

Kada sada izvršimo index.js, vidjet ćemo ove atribute kako se upravljaju na objektu svake datoteke module:

~/learn-node $ node index.js In index Module { id: '.', exports: { id: 'index' }, loaded: false, ... } In util Module { id: '/Users/samer/learn-node/lib/util.js', exports: { id: 'lib/util' }, parent: Module { id: '.', exports: { id: 'index' }, loaded: false, ... }, loaded: false, ... }

Uklonio sam neke atribute u gornjem izlazu kako bi bio kratak, ali imajte na umu kako exportsobjekt sada ima atribute koje smo definirali u svakom modulu. Na taj objekt izvoza možete staviti onoliko atributa koliko želite, a zapravo možete cijeli objekt promijeniti u nešto drugo. Na primjer, da bismo promijenili objekt izvoza u funkciju umjesto u objekt, radimo sljedeće:

// Add the following line in index.js before the console.log module.exports = function() {};

Kad index.jssada pokrenete , vidjet ćete kako je exportsobjekt funkcija:

~/learn-node $ node index.js In index Module { id: '.', exports: [Function], loaded: false, ... }

Imajte na umu kako nismo učinili exports = function() {}da exportsobjekt pretvorimo u funkciju. To zapravo ne možemo učiniti, jer je exportsvarijabla unutar svakog modula samo referenca na module.exportskoju se upravlja izvoženim svojstvima. Kad dodijelimo exportsvarijablu, ta se referenca gubi i umjesto promjene module.exportsobjekta uveli bismo novu varijablu .

module.exportsPredmet u svakom modulu je ono što je requirefunkcija vraća kada tražimo taj modul. Na primjer, promijenite require('./lib/util')redak u index.js:

const UTIL = require('./lib/util'); console.log('UTIL:', UTIL);

Gore će uhvatiti svojstva izvozi u lib/utilu UTILkonstantu. Kad index.jssada pokrenemo , izvest će se posljednji redak:

UTIL: { id: 'lib/util' }

Let’s also talk about the loaded attribute on every module. So far, every time we printed a module object, we saw a loaded attribute on that object with a value of false.

The module module uses the loaded attribute to track which modules have been loaded (true value) and which modules are still being loaded (false value). We can, for example, see the index.js module fully loaded if we print its module object on the next cycle of the event loop using a setImmediate call:

// In index.js setImmediate(() => { console.log('The index.js module object is now loaded!', module) });

The output of that would be:

The index.js module object is now loaded! Module { id: '.', exports: [Function], parent: null, filename: '/Users/samer/learn-node/index.js', loaded: true, children: [ Module { id: '/Users/samer/learn-node/lib/util.js', exports: [Object], parent: [Circular], filename: '/Users/samer/learn-node/lib/util.js', loaded: true, children: [], paths: [Object] } ], paths: [ '/Users/samer/learn-node/node_modules', '/Users/samer/node_modules', '/Users/node_modules', '/node_modules' ] }

Note how in this delayed console.log output both lib/util.js and index.js are fully loaded.

The exports object becomes complete when Node finishes loading the module (and labels it so). The whole process of requiring/loading a module is synchronous. That’s why we were able to see the modules fully loaded after one cycle of the event loop.

This also means that we cannot change the exports object asynchronously. We can’t, for example, do the following in any module:

fs.readFile('/etc/passwd', (err, data) => { if (err) throw err; exports.data = data; // Will not work. });

Circular module dependency

Let’s now try to answer the important question about circular dependency in Node: What happens when module 1 requires module 2, and module 2 requires module 1?

To find out, let’s create the following two files under lib/, module1.js and module2.js and have them require each other:

// lib/module1.js exports.a = 1; require('./module2'); exports.b = 2; exports.c = 3; // lib/module2.js const Module1 = require('./module1'); console.log('Module1 is partially loaded here', Module1);

When we run module1.js we see the following:

~/learn-node $ node lib/module1.js Module1 is partially loaded here { a: 1 }

We required module2 before module1 was fully loaded, and since module2 required module1 while it wasn’t fully loaded, what we get from the exports object at that point are all the properties exported prior to the circular dependency. Only the a property was reported because both b and c were exported after module2 required and printed module1.

Node keeps this really simple. During the loading of a module, it builds the exports object. You can require the module before it’s done loading and you’ll just get a partial exports object with whatever was defined so far.

JSON and C/C++ addons

We can natively require JSON files and C++ addon files with the require function. You don’t even need to specify a file extension to do so.

If a file extension was not specified, the first thing Node will try to resolve is a .js file. If it can’t find a .js file, it will try a .json file and it will parse the .json file if found as a JSON text file. After that, it will try to find a binary .node file. However, to remove ambiguity, you should probably specify a file extension when requiring anything other than .js files.

Requiring JSON files is useful if, for example, everything you need to manage in that file is some static configuration values, or some values that you periodically read from an external source. For example, if we had the following config.json file:

{ "host": "localhost", "port": 8080 }

We can require it directly like this:

const { host, port } = require('./config'); console.log(`Server will run at //${host}:${port}`);

Running the above code will have this output:

Server will run at //localhost:8080

If Node can’t find a .js or a .json file, it will look for a .node file and it would interpret the file as a compiled addon module.

The Node documentation site has a sample addon file which is written in C++. It’s a simple module that exposes a hello() function and the hello function outputs “world.”

You can use the node-gyp package to compile and build the .cc file into a .node file. You just need to configure a binding.gyp file to tell node-gyp what to do.

Once you have the addon.node file (or whatever name you specify in binding.gyp) then you can natively require it just like any other module:

const addon = require('./addon'); console.log(addon.hello());

We can actually see the support of the three extensions by looking at require.extensions.

Looking at the functions for each extension, you can clearly see what Node will do with each. It uses module._compile for .js files, JSON.parse for .json files, and process.dlopen for .node files.

All code you write in Node will be wrapped in functions

Node’s wrapping of modules is often misunderstood. To understand it, let me remind you about the exports/module.exports relation.

We can use the exports object to export properties, but we cannot replace the exports object directly because it’s just a reference to module.exports

exports.id = 42; // This is ok. exports = { id: 42 }; // This will not work. module.exports = { id: 42 }; // This is ok.

How exactly does this exports object, which appears to be global for every module, get defined as a reference on the module object?

Let me ask one more question before explaining Node’s wrapping process.

In a browser, when we declare a variable in a script like this:

var answer = 42;

That answer variable will be globally available in all scripts after the script that defined it.

This is not the case in Node. When we define a variable in one module, the other modules in the program will not have access to that variable. So how come variables in Node are magically scoped?

The answer is simple. Before compiling a module, Node wraps the module code in a function, which we can inspect using the wrapper property of the module module.

~ $ node > require('module').wrapper [ '(function (exports, require, module, __filename, __dirname) { ', '\n});' ] >

Node does not execute any code you write in a file directly. It executes this wrapper function which will have your code in its body. This is what keeps the top-level variables that are defined in any module scoped to that module.

This wrapper function has 5 arguments: exports, require, module, __filename, and __dirname. This is what makes them appear to look global when in fact they are specific to each module.

All of these arguments get their values when Node executes the wrapper function. exports is defined as a reference to module.exports prior to that. require and module are both specific to the function to be executed, and __filename/__dirname variables will contain the wrapped module’s absolute filename and directory path.

You can see this wrapping in action if you run a script with a problem on its first line:

~/learn-node $ echo "euaohseu" > bad.js ~/learn-node $ node bad.js ~/bad.js:1 (function (exports, require, module, __filename, __dirname) { euaohseu ^ ReferenceError: euaohseu is not defined

Note how the first line of the script as reported above was the wrapper function, not the bad reference.

Moreover, since every module gets wrapped in a function, we can actually access that function’s arguments with the arguments keyword:

~/learn-node $ echo "console.log(arguments)" > index.js ~/learn-node $ node index.js { '0': {}, '1': { [Function: require] resolve: [Function: resolve], main: Module { id: '.', exports: {}, parent: null, filename: '/Users/samer/index.js', loaded: false, children: [], paths: [Object] }, extensions: { ... }, cache: { '/Users/samer/index.js': [Object] } }, '2': Module { id: '.', exports: {}, parent: null, filename: '/Users/samer/index.js', loaded: false, children: [], paths: [ ... ] }, '3': '/Users/samer/index.js', '4': '/Users/samer' }

The first argument is the exports object, which starts empty. Then we have the require/module objects, both of which are instances that are associated with the index.js file that we’re executing. They are not global variables. The last 2 arguments are the file’s path and its directory path.

The wrapping function’s return value is module.exports. Inside the wrapped function, we can use the exports object to change the properties of module.exports, but we can’t reassign exports itself because it’s just a reference.

What happens is roughly equivalent to:

function (require, module, __filename, __dirname) { let exports = module.exports; // Your Code... return module.exports; }

If we change the whole exports object, it would no longer be a reference to module.exports. This is the way JavaScript reference objects work everywhere, not just in this context.

The require object

There is nothing special about require. It’s an object that acts mainly as a function that takes a module name or path and returns the module.exports object. We can simply override the require object with our own logic if we want to.

For example, maybe for testing purposes, we want every require call to be mocked by default and just return a fake object instead of the required module exports object. This simple reassignment of require will do the trick:

require = function() { return { mocked: true }; }

After doing the above reassignment of require, every require('something') call in the script will just return the mocked object.

The require object also has properties of its own. We’ve seen the resolve property, which is a function that performs only the resolving step of the require process. We’ve also seen require.extensions above.

There is also require.main which can be helpful to determine if the script is being required or run directly.

Say, for example, that we have this simple printInFrame function in print-in-frame.js:

// In print-in-frame.js const printInFrame = (size, header) => { console.log('*'.repeat(size)); console.log(header); console.log('*'.repeat(size)); };

The function takes a numeric argument size and a string argument header and it prints that header in a frame of stars controlled by the size we specify.

We want to use this file in two ways:

  1. From the command line directly like this:
~/learn-node $ node print-in-frame 8 Hello

Passing 8 and Hello as command line arguments to print “Hello” in a frame of 8 stars.

2. With require. Assuming the required module will export the printInFrame function and we can just call it:

const print = require('./print-in-frame'); print(5, 'Hey');

To print the header “Hey” in a frame of 5 stars.

Those are two different usages. We need a way to determine if the file is being run as a stand-alone script or if it is being required by other scripts.

This is where we can use this simple if statement:

if (require.main === module) { // The file is being executed directly (not with require) }

So we can use this condition to satisfy the usage requirements above by invoking the printInFrame function differently:

// In print-in-frame.js const printInFrame = (size, header) => { console.log('*'.repeat(size)); console.log(header); console.log('*'.repeat(size)); }; if (require.main === module) { printInFrame(process.argv[2], process.argv[3]); } else { module.exports = printInFrame; }

When the file is not being required, we just call the printInFrame function with process.argv elements. Otherwise, we just change the module.exports object to be the printInFrame function itself.

All modules will be cached

Caching is important to understand. Let me use a simple example to demonstrate it.

Say that you have the following ascii-art.js file that prints a cool looking header:

We want to display this header every time we require the file. So when we require the file twice, we want the header to show up twice.

require('./ascii-art') // will show the header. require('./ascii-art') // will not show the header.

The second require will not show the header because of modules’ caching. Node caches the first call and does not load the file on the second call.

We can see this cache by printing require.cache after the first require. The cache registry is simply an object that has a property for every required module. Those properties values are the module objects used for each module. We can simply delete a property from that require.cache object to invalidate that cache. If we do that, Node will re-load the module to re-cache it.

However, this is not the most efficient solution for this case. The simple solution is to wrap the log line in ascii-art.js with a function and export that function. This way, when we require the ascii-art.js file, we get a function that we can execute to invoke the log line every time:

require('./ascii-art')() // will show the header. require('./ascii-art')() // will also show the header.

That’s all I have for this topic. Thanks for reading. Until next time!

Learning React or Node? Checkout my books:

  • Learn React.js by Building Games
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