How module.exports and require Work in Node.js

How module.exports and require Work in Node.js

module.exports and require are the core of Node.js's CommonJS module system. Here is how they work.

What module.exports Does

module.exports is an object that Node.js gives every file. Whatever you assign to it becomes what require returns when another file requires this one. You export functions, objects, or values by assigning them to module.exports.

What require Does

require takes a file path, runs that file, and returns its module.exports. The file's code runs once and is cached, so requiring the same file twice returns the same object.

Exporting a Single Value

Assign directly to module.exports: module.exports = function add(a, b) { return a + b }. The requiring file gets the function.

Exporting Multiple Values

Attach to the exports object: exports.add = function() {}; exports.subtract = function() {}. The requiring file gets an object with both. Do not assign a new object to module.exports if you use exports.add, since they are separate references.

The Common Mistake

Mixing exports.add and module.exports = in the same file. exports is a reference to module.exports initially, but reassigning module.exports breaks the link. Pick one approach per file.

Caching

Modules are cached after their first require. This means a module's code runs once. If you need fresh state, export a factory function that creates new state, not a shared object.

The Takeaway

module.exports holds what a file exports, and require returns it. Export a single value by assigning to module.exports, or multiple values with exports.add. Avoid mixing the two, and remember modules are cached after first require.