Node.js Handbook - How `require()` Actually Works
Update Dec. 18, 2014: An updated version to this article has been posted to The Node Way, a new series on Node.js design patterns and best practices needed to build beautiful applications. Check it out at thenodeway.io.
Almost any Node.js developer can tell you what the
require() function does, but how many of us actually know how it works? We use it every day to load libraries and modules, but its behavior otherwise is a mystery.
Curious, I dug into Node core to find out what was happening under the hood. But instead of finding a single function, I ended up at the heart of Node’s module system:
module.js. The file contains a surprisingly powerful yet relatively unknown core module that controls the loading, compiling, and caching of every file used.
require(), it turned out, was just the tip of the iceberg.
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The Module type found in
module.js has two main roles inside of Node.js. First, it provides a foundation for all Node.js modules to build off of. Each file is given a new instance of this base module on load, which persists even after the file has run. This is why we are able attach properties to
module.exports and return them later as needed.
The module’s second big job is to handle Node’s module loading mechanism. The stand-alone
require function that we use is actually an abstraction over
module.require, which is itself just a simple wrapper around
Module._load. This load method handles the actual loading of each file, and is where we’ll begin our journey.
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Module._load is responsible for loading new modules and managing the module cache. Caching each module on load reduces the number of redundant file reads and can speed up your application significantly. In addition, sharing module instances allows for singleton-like modules that can keep state across a project.
If a module doesn’t exist in the cache,
Module._load will create a new base module for that file. It will then tell the module to read in the new file’s contents before sending them to
If you notice step #6 above, you’ll see that
module.exports is returned to the user. This is why you use
module.exports when defining your public interface, since that’s exactly what
Module._load and then
require will return. I was surprised that there wasn’t more magic going on here, but if anything that’s for the better.
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This is where the real magic happens. First, a special standalone
require function is created for that module. THIS is the require function that we are all familiar with. While the function itself is just a wrapper around
Module.require, it also contains some lesser-known helper properties and methods for us to use:
require(): Loads an external module
require.resolve(): Resolves a module name to its absolute path
require.main: The main module
require.cache: All cached modules
require.extensions: Available compilation methods for each valid file type, based on its extension
require is ready, the entire loaded source code is wrapped in a new function, which takes in
exports, and all other exposed variables as arguments. This creates a new functional scope just for that module so that there is no pollution of the rest of the Node.js environment.
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Finally, the function wrapping the module is run. The entire
Module._compile method is executed synchronously, so the original call to
Module._load just waits for this code to run before finishing up and returning
module.exports back to the user.
And so we’ve reached the end of the require code path, and in doing so have come full circle by creating the very
require function that we had begun investigating in the first place.
If you’ve made it all this way, then you’re ready for the final secret:
require('module'). That’s right, the module system itself can be loaded VIA the module system. INCEPTION. This may sound strange, but it lets userland modules interact with the loading system without digging into Node.js core. Popular modules like mockery and rewire are built off of this.
If you want to learn more, check out the module.js source code for yourself. There is plenty more there to keep you busy and blow your mind. Bonus points for the first person who can tell me what ‘NODE_MODULE_CONTEXTS’ is and why it was added.
 However, both of these modules are built on private Module methods, like
Module._findPath. You could argue that this isn’t much better…