module.exports vs exports in Node.js


Question

I've found the following contract in a Node.js module:

module.exports = exports = nano = function database_module(cfg) {...}

I wonder whats the different between module.exports and exports and why both are used here.

1
680
12/29/2014 10:34:25 AM

Accepted Answer

Setting module.exports allows the database_module function to be called like a function when required. Simply setting exports wouldn't allow the function to be exported because node exports the object module.exports references. The following code wouldn't allow the user to call the function.

module.js

The following won't work.

exports = nano = function database_module(cfg) {return;}

The following will work if module.exports is set.

module.exports = exports = nano = function database_module(cfg) {return;}

console

var func = require('./module.js');
// the following line will **work** with module.exports
func();

Basically node.js doesn't export the object that exports currently references, but exports the properties of what exports originally references. Although Node.js does export the object module.exports references, allowing you to call it like a function.


2nd least important reason

They set both module.exports and exports to ensure exports isn't referencing the prior exported object. By setting both you use exports as a shorthand and avoid potential bugs later on down the road.

Using exports.prop = true instead of module.exports.prop = true saves characters and avoids confusion.

405
8/8/2017 2:09:56 PM

Even though question has been answered and accepted long ago, i just want to share my 2 cents:

You can imagine that at the very beginning of your file there is something like (just for explanation):

var module = new Module(...);
var exports = module.exports;

enter image description here

So whatever you do just keep in mind that module.exports and NOT exports will be returned from your module when you're requiring that module from somewhere else.

So when you do something like:

exports.a = function() {
    console.log("a");
}
exports.b = function() {
    console.log("b");
}

You are adding 2 function 'a' and 'b' to the object on which module.exports points too, so the typeof the returning result will be an object : { a: [Function], b: [Function] }

Of course this is the same result you will get if you are using module.exports in this example instead of exports.

This is the case where you want your module.exports to behave like a container of exported values. Whereas, if you only want to export a constructor function then there is something you should know about using module.exports or exports;(Remember again that module.exports will be returned when you require something, not export).

module.exports = function Something() {
    console.log('bla bla');
}

Now typeof returning result is 'function' and you can require it and immediately invoke like:
var x = require('./file1.js')(); because you overwrite the returning result to be a function.

However, using exports you can't use something like:

exports = function Something() {
    console.log('bla bla');
}
var x = require('./file1.js')(); //Error: require is not a function

Because with exports, the reference doesn't 'point' anymore to the object where module.exports points, so there is not a relationship between exports and module.exports anymore. In this case module.exports still points to the empty object {} which will be returned.

Accepted answer from another topic should also help: Does Javascript pass by reference?


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