Module Caching | Engineering Interview Kit

TL;DR

When a module is loaded via require(), Node.js caches it in memory. Subsequent calls to require() for the exact same file will return the cached object, rather than executing the file’s code again. This improves performance and allows modules to act as singletons to share state.

Mental Model

How It Works

Node.js stores cached modules in the require.cache object. The keys in this object are the absolute file paths of the modules.

Because of this caching:

Code inside a module is only executed once, no matter how many times it is required across your application.
If you export an object or instance, it behaves like a Singleton. All files requiring it will share the exact same reference.

Example

// counter.js
let count = 0;
module.exports = {
    increment: () => count++,
    getCount: () => count
};

// fileA.js
const counter = require('./counter');
counter.increment();

// fileB.js
const counter = require('./counter');
console.log(counter.getCount()); // Output: 1!
// Because of module caching, fileB gets the exact same object modified by fileA.

Common Interview Questions

Can you invalidate or clear the module cache?

Yes. You can delete the specific entry from the require.cache object using its absolute path.

delete require.cache[require.resolve('./my-module')];
// Next require('./my-module') will execute the file again.

Is module caching based on the file name or the requested path?

It is based on the resolved absolute filename. Even if you require a module using different relative paths from different files (e.g., require('../util') vs require('./util')), they resolve to the same absolute path, so they hit the same cache.