Node.js module.exports vs. exports

Always, in CommonJS, we use module.exports or exports to export functions, variables, etc. However, it's generally recommended to use module.exports for consistency and to avoid potential issues.

Here's why:

• Returned Value: When you use require to import a module in another file, only the value assigned to module.exports is actually returned. exports itself is not returned.
• Reference: exports is simply a reference to module.exports. Initially, they both point to the same empty object.

Based on the two rules above, if you export like this way:

exports = {
  add: (a, b) => a + b,
  MAX: 20,
};

and use like this:

const lib = require("./lib");
 
console.log(lib); // {}
console.log(lib.add(1, 2)); // TypeError: lib.add is not a function
console.log(lib.MAX); // undefined

it will be throw errors.

In javascript, we know if we change the attributes of an object, that will be affect all the values referenced to this object:

const a = { v: 1 };
const b = a;
b.v = 2;
console.log(a); // { v: 2 }

but if we re-assign the whole reference object to other value, then any changes to the object will not make any effects to the original one:

const a = { v: 1 };
let b = a;
b.v = 2;
console.log(a); // { v: 2 }
 
b = { v: 3 };
console.log(a); // { v: 2 }

since the module just return module.exports when it required, assigning a new object to exports can break the reference between exports and module.exports.

So, that's the why we should always use module.exports instead of exports in CJS modules.

While it's technically possible to use exports for individual property assignments, it's generally discouraged due to the potential for confusion and the risk of accidentally breaking the reference.

Here's an alternative way to use exports (but consider module.exports for better practice):

exports.add = (a, b) => a + b;
exports.MAX = (a, b) => 20;

or

exports = {
  add: (a, b) => a + b,
  MAX: 20,
};
 
module.exports = exports;

The reason is obvious.

However, to maintain consistency and avoid confusion, it's strongly recommended to use module.exports throughout your code.