CommonJS Modules

Definition

The original, default module system in Node.js. It uses `require()` to import modules and `module.exports` to export them.

Explain Like I'm New

Imagine every JS file is a locked room. CommonJS is the system for passing notes under the door. You use `module.exports` to shove a note under the door out to the world, and you use `require()` to grab notes from other rooms and pull them inside.

Real World Example

Importing a utility function: `const formatData = require('./utils')`. This is how 90% of legacy Node.js applications and NPM packages are wired together.

Common Use Cases

•Legacy Node.js projects
•Dynamic importing (requiring files inside an `if` statement)

Interactive Example

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Interview Questions

basic

What keywords are used for importing and exporting in CommonJS?

intermediate

Is `require()` synchronous or asynchronous?

advanced

What is the difference between `module.exports` and `exports`?

Flash Cards

Question

Is require() sync or async?

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Answer

It is completely Synchronous! It blocks the thread while it reads the file from the hard drive, compiles it, and caches it. This is why you should only `require()` at the very top of your files, never inside a loop or active request route.

Question

Difference between module.exports and exports?

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Answer

`exports` is just a shortcut reference to `module.exports`. If you assign properties to it (`exports.run = () => {}`), it works. But if you try to overwrite it completely (`exports = myFunction`), the reference breaks, and you export nothing. Always use `module.exports = ...` to be safe.