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Node.js Imports and Exports

Overview

In this lesson, we're going to talk about how Node.js importing and exporting works. Just like Ruby, Node.js gives you a require function, and Node.js's require has a few tricks up its sleeve.

To understand how require works, you'll also need to understand how exports work in Node.js. By the end of this lesson, you'll be able to

  1. Import modules from the Node.js standard library
  2. Write your own modules
  3. Import your modules for use in an application

This lesson is a code-along — by the end, you'll have a working command-line application that colorizes its output. If you need to check something, a complete solution is in the learn-lib directory.

util.inspect the unexpected

In lib/index.js, enter the following:

const util = require('util');

console.log(util);

BOOM! You just required your first module! Now if you enter node lib, you should see a list of the util module's top-level properties. Let's talk about that call to require.

NOTE: You can also enter node lib/index.js, but Node.js knows to look for an index.js file and execute that if you pass it a directory name.

Before we dive into what require is doing in the above, let's create another file by entering touch lib/interface.js. Add the following few lines to lib/interface.js:

function getUserArguments() {
    return process.argv.slice(2);
}

console.log(getUserArguments());

Head on back to your command line and enter something like

node lib/interface.js the quick brown fox

You should see ['the', 'quick', 'brown', 'fox'] in your terminal if you entered the above. How does that work? process.argv is an array of arguments passed to the node process. process.argv[0] is the absolute path to the node executable, process.argv[1] is the absolute path to the current module (!), and any subsequent indices in process.argv point to arguments that the user has passed in (where each argument is delimited by a space).

Let's change the contents of lib/interface.js so that we can use that function elsewhere.

module.exports = function getUserArguments() {
    return process.argv.slice(2);
};

Whoa whoa whoa, where is this module coming from and what is it exportsing? module comes from here. Don't sweat it if that code seems a little incomprehensible. The important thing to know is that in Node.js, all files are modules (but not all modules are files, as we'll soon see).

Now fire up your REPL (just enter node in your terminal), and enter the following:

const interface = require('./lib/interface');

Notice any differences between the call to require here and the call to require above? For one, we're passing a relative path this time; above, we just passed the name of the module. What's going on?

The Room of requirement

Node.js's require has a few tricks up its sleeve for finding modules. When it's called, it first checks whether its argument is a relative path or not. If it's not a relative path — as in require('util') above — then Node.js looks for a node_modules folder, starting in the current directory and working through parent directories in order. If it find such a folder (we'll talk more about this folder in the next lesson), and if that folder contains a module by the given name, then we're done and we get that module.

If Node.js strikes out on this hunt (it will look all the way up the chain to globally installed modules), then it searches its standard library. In the case of 'util', it will find the module that we're looking for there.

If the argument to require is a relative path (as is the case with ./lib/interface), then Node.js will only check that location. If it finds a module there, great! It evaluates the module's code and returns its exports; if not, it throws an error.

module.exports

So what are these exports, anyway? Every file in Node.js, when it's evaluated, has at its disposal the module global and the exports global (among others). To start, module.exports and exports both refer to the same empty object. When the module is imported, it makes these exports available to the module that required it (or to the REPL). So you can overwrite module.exports in its entirety (like we do above) or you can assign properties to the exports object (more on that later).

So when you const interface = require('./lib/interface'); in the REPL, you're assigning the exports of the ./lib/interface module to the variable interface. If you enter interface in the REPL, you'll see:

[Function: getUserArguments]

So now we have the getUserArguments() function from ./lib/interface available as interface in our REPL! Try calling it; you should see [].

Inspector gadget

One of the handy utilities that util provides is inspection through util.inspect(). Let's use our ./lib/interface module and util.inspect() to build a simple command-line utility that lets us look up a module's properties on the fly. We'll have to change ./lib/index.js:

const interface = require('./interface');
const util = require('util');

const userArgs = interface();
const moduleName = userArgs[0];

if (userArgs.length > 1) {
  console.warn('Warning: you provided more than one argument.');
}

try {
  const moduleToInspect = require(moduleName);

  console.log(util.inspect(moduleToInspect, { colors: true }));
} catch (error) {
  console.error(`Unable to inspect module ${moduleName}.`);
  console.error(`Reason: ${error.message}`);
  process.exitCode = 1;
}

Just like above, we require ./lib/interface using a relative path — we use ./interface because relative requires are relative to the file doing the require. In other words, since index.js and interface.js are in the same directory, we just say, "Look in this directory ./ for this file interface.js."

Next, we call interface and take the first item out of the array that it returns. If the user has passed more than one argument, we issue a warning. (This process is a good candidate for refactoring!) Then, we try to require a module of the name that they've passed in. If we succeed, we print the module's exports to console (and we colorize it!); if we fail, we tell the user what went wrong and exit with an error. Try it!

(Confused by that stuff in backticks? These are called "template literals." They work essentially like string interpolation in Ruby ("See? It's #{interpolated}"), just with a slightly different syntax. They're a new feature in ECMAScript 6 that Node.js just made available by default. No more "concatenating " + variables + " into strings!")

# print the `fs` module's properties!
node lib fs

# fail!
node lib poop

# Note: If, for some reason, you have installed the node module
# `poop` (https://www.npmjs.com/package/poop) globally, this will
# not fail. You'll have to try something else.

# !!!
node lib ./interface.js

We did it! We can call this on pretty much any module to print its properties to stdout.

Refactor

There's a bit of ugliness in our code. Currently, we're discarding all but the first user argument in the main file of our little library; but we should really be doing that where we're handling all of the user arguments, in interface.js. Let's get to work.

// lib/interface.js

module.exports = function getUserArgument() {
  const userArgs = process.argv.slice(2);

  if (userArgs.length > 1) {
    console.warn(`Warning: you provided more than one argument.`);
  }

  return userArgs[0];
};

We've just moved our warning to interface.js and made sure that getUserArguments() only returns a string. Since it just returns one thing, let's go ahead and rename it to getUserArgument().

Now for index.js:

const interface = require('./interface');
const util = require('util');

const moduleName = interface();

try {
  const moduleToInspect = require(moduleName);

  console.log(util.inspect(moduleToInspect, { colors: true }));
} catch (error) {
  console.error(`Unable to inspect module ${moduleName}.`);
  console.error(`Reason: ${error.message}`);
}

We don't have to do any argument trimming here, and even though we renamed the function in interface.js, we don't have to rename anything here since the function is the exports object of the interfaces.js module. Code separation FTW!

Review

Whew. We covered quite a bit in this lesson, but you've now built a nice little inspector and learned a lot about modules and require in Node.js.

Check out the resources below for further reading.

Resources

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