Requerir módulos en Node.js: todo lo que necesita saber

Actualización: este artículo ahora es parte de mi libro "Node.js más allá de lo básico".

Lea la versión actualizada de este contenido y más sobre Node en jscomplete.com/node-beyond-basics .

Node utiliza dos módulos principales para administrar las dependencias de los módulos:

  • El requiremódulo, que parece estar disponible en el ámbito global, no es necesario require('require').
  • El modulemódulo, que también parece estar disponible en el ámbito global, no es necesario require('module').

Puede pensar en el requiremódulo como el comando y el modulemódulo como el organizador de todos los módulos necesarios.

Requerir un módulo en Node no es un concepto tan complicado.

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

El objeto principal exportado por el requiremódulo es una función (como se usa en el ejemplo anterior). Cuando Node invoca esa require()función con una ruta de archivo local como único argumento de la función, Node sigue la siguiente secuencia de pasos:

  • Resolución : Para encontrar la ruta absoluta del archivo.
  • Cargando : Para determinar el tipo de contenido del archivo.
  • Envoltura : Para dar al archivo su ámbito privado. Esto es lo que hace que los objetos requirey modulesean locales para cada archivo que necesitamos.
  • Evaluación : esto es lo que hace la VM con el código cargado.
  • Almacenamiento en caché : para que cuando volvamos a requerir este archivo, no repasamos todos los pasos en otro momento.

En este artículo, intentaré explicar con ejemplos estas diferentes etapas y cómo afectan la forma en que escribimos módulos en Node.

Permítanme primero crear un directorio para alojar todos los ejemplos usando mi terminal:

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

Todos los comandos del resto de este artículo se ejecutarán desde dentro ~/learn-node.

Resolviendo un camino local

Déjame presentarte el moduleobjeto. Puede comprobarlo en una simple sesión de REPL:

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

Cada objeto de módulo obtiene una idpropiedad para identificarlo. Esta idsuele ser la ruta completa al archivo, pero en una sesión REPL es simplemente.

Los módulos de nodo tienen una relación de uno a uno con los archivos del sistema de archivos. Requerimos un módulo cargando el contenido de un archivo en la memoria.

Sin embargo, dado que Node permite muchas formas de requerir un archivo (por ejemplo, con una ruta relativa o una ruta preconfigurada), antes de que podamos cargar el contenido de un archivo en la memoria, necesitamos encontrar la ubicación absoluta de ese archivo.

Cuando requerimos un 'find-me'módulo, sin especificar una ruta:

require('find-me');

El nodo buscará find-me.jsen todas las rutas especificadas por module.paths- en orden.

~/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' ]

La lista de rutas es básicamente una lista de directorios node_modules debajo de cada directorio desde el directorio actual hasta el directorio raíz. También incluye algunos directorios heredados cuyo uso no se recomienda.

Si Node no puede encontrar find-me.jsen ninguna de estas rutas, arrojará un "error de módulo no se puede encontrar".

~/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)

Si ahora crea un node_modulesdirectorio local y coloca find-me.jsallí, la require('find-me')línea lo encontrará.

~/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 {} >

Si find-me.jsexistiera otro archivo en cualquiera de las otras rutas, por ejemplo, si tenemos un node_modulesdirectorio en el directorio de inicio y tenemos un find-me.jsarchivo diferente allí:

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

Cuando estamos require('find-me')dentro del learn-nodedirectorio, que tiene el suyo node_modules/find-me.js, el find-me.jsarchivo del directorio de inicio no se cargará en absoluto:

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

Si eliminamos el node_modulesdirectorio local debajo ~/learn-nodey tratamos de find-mesolicitarlo una vez más, node_modulesse usaría el archivo debajo del directorio de inicio :

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

Requerir una carpeta

Los módulos no tienen por qué ser archivos. También podemos crear una find-mecarpeta debajo node_modulesy colocar un index.jsarchivo allí. La misma require('find-me')línea usará el index.jsarchivo de esa carpeta :

~/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. {} >

Tenga en cuenta que volvió a ignorar la node_modulesruta del directorio de inicio, ya que ahora tenemos una local.

Se index.jsutilizará un archivo de forma predeterminada cuando necesitemos una carpeta, pero podemos controlar con qué nombre de archivo comenzar bajo la carpeta usando la mainpropiedad en package.json. Por ejemplo, para hacer que la require('find-me')línea se resuelva en un archivo diferente debajo de la find-mecarpeta, todo lo que tenemos que hacer es agregar un package.jsonarchivo allí y especificar qué archivo debe usarse para resolver esta carpeta:

~/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 {} >

require.resolve

Si solo desea resolver el módulo y no ejecutarlo, puede usar la require.resolvefunción. Esto se comporta exactamente igual que la requirefunción principal , pero no carga el archivo. Todavía arrojará un error si el archivo no existe y devolverá la ruta completa al archivo cuando se encuentre.

> 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) >

Esto se puede usar, por ejemplo, para verificar si un paquete opcional está instalado o no y solo usarlo cuando esté disponible.

Caminos relativos y absolutos

Además de resolver módulos desde dentro de los node_modulesdirectorios, también podemos colocar el módulo en cualquier lugar que queramos y requerirlo con rutas relativas ( ./y ../) o con rutas absolutas que comiencen con /.

Si, por ejemplo, el find-me.jsarchivo estaba en una libcarpeta en lugar de la node_modulescarpeta, podemos solicitarlo con:

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

Relación padre-hijo entre archivos

Cree un lib/util.jsarchivo y agregue una console.loglínea allí para identificarlo. Además, console.logel moduleobjeto en sí:

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

Haga lo mismo con un index.jsarchivo, que es lo que ejecutaremos con el comando de nodo. Hacer que este index.jsarchivo requiera lib/util.js:

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

Ahora ejecute el index.jsarchivo con el nodo:

~/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: [...] }

Tenga en cuenta que el indexmódulo principal (id: '.')ahora aparece como padre del lib/utilmódulo. Sin embargo, el lib/utilmódulo no se incluyó en la lista como hijo del indexmódulo. En cambio, tenemos el [Circular]valor allí porque se trata de una referencia circular. Si Node imprime el lib/utilobjeto del módulo, entrará en un bucle infinito. Es por eso que simplemente reemplaza la lib/utilreferencia con [Circular].

Y lo que es más importante ahora, ¿qué sucede si el lib/utilmódulo requiere el indexmódulo principal ? Aquí es donde entramos en lo que se conoce como dependencia modular circular, que está permitida en Node.

Para comprenderlo mejor, primero comprendamos algunos otros conceptos sobre el objeto del módulo.

exportaciones, exportaciones de módulos y carga síncrona de módulos

In any module, exports is a special object. If you’ve noticed above, every time we’ve printed a module object, it had an exports property which has been an empty object so far. We can add any attribute to this special exports object. For example, let’s export an id attribute for index.js and 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';

When we now execute index.js, we’ll see these attributes as managed on each file’s module object:

~/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, ... }

I’ve removed some attributes in the above output to keep it brief, but note how the exports object now has the attributes we defined in each module. You can put as many attributes as you want on that exports object, and you can actually change the whole object to be something else. For example, to change the exports object to be a function instead of an object, we do the following:

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

When you run index.js now, you’ll see how the exports object is a function:

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

Note how we did not do exports = function() {} to make the exports object into a function. We can’t actually do that because the exports variable inside each module is just a reference to module.exports which manages the exported properties. When we reassign the exports variable, that reference is lost and we would be introducing a new variable instead of changing the module.exports object.

The module.exports object in every module is what the require function returns when we require that module. For example, change the require('./lib/util') line in index.js into:

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

The above will capture the properties exported in lib/util into the UTIL constant. When we run index.js now, the very last line will output:

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
  • Node.js Beyond the Basics