Most Node.js developers can explain the purpose of the require() function, but few truly understand its inner workings. Despite its frequent use for loading libraries and modules, its underlying behavior often remains a mystery.
Intrigued by this, I delved into the Node core to uncover the mechanics behind require(). Instead of encountering a simple function, I stumbled upon the core of Node’s module system: module.js. This file houses a powerful yet largely unnoticed core module that governs the loading, compiling, and caching of every utilized file. Surprisingly, require() merely scratches the surface of this intricate system.
module.js:
function Module(id, parent) {
this.id = id;
this.exports = {};
this.parent = parent;
// …
}The Module type within module.js serves two primary purposes in Node.js. Firstly, it provides a foundation for all Node.js modules, ensuring consistency and enabling the attachment of properties to module.exports. Secondly, it manages Node’s module loading mechanism.
The standalone require() function, commonly used, is actually an abstraction over module.require(), which, in turn, wraps around Module._load(). This load() function handles the actual loading of each file, marking the starting point of our exploration.
Module._load:
Module._load = function(request, parent, isMain) {
// 1. Check Module._cache for the cached module.
// 2. Create a new Module instance if cache is empty.
// 3. Save it to the cache.
// 4. Call module.load() with the given filename.
// This invokes module.compile() after reading the file contents.
// 5. If there's an error loading/parsing the file,
// remove the faulty module from the cache.
// 6. Return module.exports.
};Module._load is responsible for loading new modules and managing the module cache, optimizing performance by reducing redundant file reads and facilitating the sharing of module instances.
If a module isn’t cached, Module._load creates a new base module for that file, instructing it to read the file’s contents before passing them to module._compile().
The magic intensifies with module._compile(), where a special standalone require() function is generated for the module, encapsulating the loaded source code within a new function. This function creates a distinct functional scope for the module, preventing environmental pollution.
In conclusion, our journey through the require() code path has unveiled the intricate mechanisms that power it. Additionally, the revelation that require('module') is possible epitomizes the versatility of Node’s module system.
For those eager to delve deeper, exploring the source code of module.js offers further enlightenment. And for the intrepid few who unravel the mystery of ‘NODE_MODULE_CONTEXTS,’ accolades await.