require() basics in Node.js

The require function is Node.js’s mechanism for breaking up large projects into small, manageable files. The require function lets you includefunctions from external modules and other files in a clean and elegant way. You already used the require function. once before in order to include the MongoDB Node.js driver.

var mongodb = require(‘mongodb’);

In this lesson you’ll learn how to integrate your own code with the require function. In this example you have two files called index.js and myfile.js, and the directory called Test. The index.js file will be the main entry point for your program — that is, you’ll run node index.js. The test directory contains two more files, index.js and myotherfile.js. The top level index.js file will access code from all of these files. Let’s take a look at the top level index.js file.

Now, Node.js uses file level scoping. This means that by default, any variable or function declared in a file is not accessible outside of that file. Node.js has a global object, but using it is almost always a mistake.The require function is the preferred way to share code between files. So in this index.js file, you call require on myfile.js and the test directory. First let’s take a look at what the myfile.js file looks like.

All myfile.js is assign a function to this module.exports property. Now, you can think of module.exports as the return value that somebody gets when they call require on the file. In this case, calling require on myfile.js gives you a function that prints this hello from myfile.js message. Now let’s consider what happens when you call require of test. Recall that test is a directory, so when you call require on a directory, Node.js looks for an index.js file in that directory and uses that file. In other words, require of ./test is equivalent to require on ./test/index.js. Now let’s take a look at test/index.js.

exports.other = require(‘./myotherfile’);

Now, this file introduces two subtleties about require. First, notice that this file uses exports.other instead of module.exports. The exports variable is a convenient shorthand for module.exports. This file would do the same thing if you used module.exports.other, rather than exports.other. The only difference is that you can’t directly assign to the exports variable. For instance, if you were to use exports rather than module.exports in, say, that myfile.js file, Node.js would error out. You can only assign properties on the exports variable. The other subtlety is that this file calls require on myotherfile.js without specifying the test directory. This is because require resolves file names relative to the current files directory. So in other words, the current working directory when you call require on myotherfile within test/index.js is the test directory itself. So you don’t need to specify test here. Finally, let’s take a look at the myotherfile.js file that’s in the test directory.

This file uses the same module.exports pattern that you saw in myfile.js, however it prints a slightly different message through the screen. Now that you’ve taken a look through all the files, let’s run node index.js. So as you can see, this file prints the message from myfile.js, followed by the message from otherfile.js from the test directory.

Hello from myfile.js
Hello from test directory

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