module.exports vs exports in Node.js

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

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

I wonder what's the difference between module.exports and exports and why both are used here.

Solution 1:

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;

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 functions a and b to the object to which module.exports points, 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.

The accepted answer from another topic should also help: Does JavaScript pass by reference?

Solution 2:

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.

Solution 3:

Basically the answer lies in what really happens when a module is required via require statement. Assuming this is the first time the module is being required.

For example:

var x = require('file1.js');

contents of file1.js:

module.exports = '123';

When the above statement is executed, a Module object is created. Its constructor function is:

function Module(id, parent) {
    this.id = id;
    this.exports = {};
    this.parent = parent;
    if (parent && parent.children) {
        parent.children.push(this);
    }

    this.filename = null;
    this.loaded = false;
    this.children = [];
}

As you see each module object has a property with name exports. This is what is eventually returned as part of require.

Next step of require is to wrap the contents of file1.js into an anonymous function like below:

(function (exports, require, module, __filename, __dirname) { 
    //contents from file1.js
    module.exports = '123;
});

And this anonymous function is invoked the following way, module here refers to the Module Object created earlier.

(function (exports, require, module, __filename, __dirname) { 
    //contents from file1.js
    module.exports = '123;
}) (module.exports,require, module, "path_to_file1.js","directory of the file1.js");

As we can see inside the function, exports formal argument refers to module.exports. In essence it's a convenience provided to the module programmer.

However this convenience need to be exercised with care. In any case if trying to assign a new object to exports ensure we do it this way.

exports = module.exports = {};

If we do it following way wrong way, module.exports will still be pointing to the object created as part of module instance.

exports = {};

As as result adding anything to the above exports object will have no effect to module.exports object and nothing will be exported or returned as part of require.

Solution 4:

Initially,module.exports=exports , and the require function returns the object module.exports refers to.

if we add property to the object, say exports.a=1, then module.exports and exports still refer to the same object. So if we call require and assign the module to a variable, then the variable has a property a and its value is 1;

But if we override one of them, for example, exports=function(){}, then they are different now: exports refers to a new object and module.exports refer to the original object. And if we require the file, it will not return the new object, since module.exports is not refer to the new object.

For me, i will keep adding new property, or override both of them to a new object. Just override one is not right. And keep in mind that module.exports is the real boss.

Solution 5:

exports and module.exports are the same unless you reassign exports within your module.

The easiest way to think about it, is to think that this line is implicitly at the top of every module.

var exports = module.exports = {};

If, within your module, you reassign exports, then you reassign it within your module and it no longer equals module.exports. This is why, if you want to export a function, you must do:

module.exports = function() { ... }

If you simply assigned your function() { ... } to exports, you would be reassigning exports to no longer point to module.exports.

If you don't want to refer to your function by module.exports every time, you can do:

module.exports = exports = function() { ... }

Notice that module.exports is the left most argument.

Attaching properties to exports is not the same since you are not reassigning it. That is why this works

exports.foo = function() { ... }