I have a confusion with module.exports. As far as I understood module.exports is an object that is exposed to other modules,
exports=module.exports={}
But in the index.js of morgan package in node.js I found this.
module.exports = morgan
module.exports.compile = compile
module.exports.format = format
module.exports.token = token
morgan, compile, format and token are all functions.
Can you explain whats happening here? How is a function(morgan) assigned to module.exports ? After the first line is executed, is module.exports a function instead of an JSON object?
A Node module basically works like this:
var module = {
exports: {}
};
(function (exports, require, module, __filename, __dirname) {
// your module code here
})(module.exports, require, module, __filename, __dirname);
var exported = module.exports;
By default, exports, and module.exports both point to the same object. You can add properties to the object as normal. However, what if you want to return a function or some other object instead of just the default standard object?
In that case, you can set module.exports to something else, and that will be the new exported object.
module.exports = function() {};
And of course, that function can have properties too, so your code is kind-of like this:
module.exports = function(){};
module.exports.compile = function() {};
module.exports.format = function() {};
module.exports.token = function() {};
Which would be equivalent to:
var morgan = function() {};
var compile = function() {};
var format = function() {};
var token = function() {};
morgan.compile = compile;
morgan.format = format;
morgan.token = token;
module.exports = morgan;
How is a function(morgan) assigned to module.exports ? After the first line is executed, is module.exports a function instead of an JSON object?
Yes, module.exports will be a function, in place of the default object (however there is no JSON here, JSON is not a JavaScript object, but an encoding format).
You might want to read this answer for a more in-depth explanation:
What is the purpose of node js module exports and how do you use it?
Morgan is simply adding parameters to the module being exported (in this case compile, format, and token. When you require the module in your application using something like morgan = require('morgan'), you can then call morgan.format to return the format that was appended to the object.
Hope that clears things up a bit!
module.exports is an object that is exposed to other modules and files. This allows a convenient way to export variables, functions and more javascript features.
As you can see here , when you initialize any variable to {} you are initializing an object.
An object can contain variables and functions, this is one of the things that makes Javascript really cool. You can easily pass complex objects with functions as parameters and do real nice clean code.
So, short story, think about module.exports as an object that is exposed to other modules and files with variables and functions. Just like passing an Object Oriented object as a parameter in Java or Ruby.
Here is easily explained: Read More
Can you explain whats happening here?
An assignment statement is happening.
// path-to-my-module.js
module.exports = morgan
module.exports.compile = compile
module.exports.format = format
module.exports.token = token
For each of these 4 statements, there is an assignment statement. Looking at the first line, module.exports = morgan, that would mean the module.exports object is going to have a property called morgan with its value being the value of the identifier morgan
So when you use it like this:
var m = require('./path-to-my-module.js');
console.log(m.morgan);
console.log(m);
Related
On this page (http://docs.nodejitsu.com/articles/getting-started/what-is-require), it states that "If you want to set the exports object to a function or a new object, you have to use the module.exports object."
My question is why.
// right
module.exports = function () {
console.log("hello world")
}
// wrong
exports = function () {
console.log("hello world")
}
I console.logged the result (result=require(example.js)) and the first one is [Function] the second one is {}.
Could you please explain the reason behind it? I read the post here: module.exports vs exports in Node.js . It is helpful, but does not explain the reason why it is designed in that way. Will there be a problem if the reference of exports be returned directly?
module is a plain JavaScript object with an exports property. exports is a plain JavaScript variable that happens to be set to module.exports.
At the end of your file, node.js will basically 'return' module.exports to the require function. A simplified way to view a JS file in Node could be this:
var module = { exports: {} };
var exports = module.exports;
// your code
return module.exports;
If you set a property on exports, like exports.a = 9;, that will set module.exports.a as well because objects are passed around as references in JavaScript, which means that if you set multiple variables to the same object, they are all the same object; so then exports and module.exports are the same object.
But if you set exports to something new, it will no longer be set to module.exports, so exports and module.exports are no longer the same object.
Renee's answer is well explained. Addition to the answer with an example:
Node does a lot of things to your file and one of the important is WRAPPING your file. Inside nodejs source code "module.exports" is returned. Lets take a step back and understand the wrapper. Suppose you have
greet.js
var greet = function () {
console.log('Hello World');
};
module.exports = greet;
the above code is wrapped as IIFE(Immediately Invoked Function Expression) inside nodejs source code as follows:
(function (exports, require, module, __filename, __dirname) { //add by node
var greet = function () {
console.log('Hello World');
};
module.exports = greet;
}).apply(); //add by node
return module.exports; //add by node
and the above function is invoked (.apply()) and returned module.exports.
At this time module.exports and exports pointing to the same reference.
Now, imagine you re-write
greet.js as
exports = function () {
console.log('Hello World');
};
console.log(exports);
console.log(module.exports);
the output will be
[Function]
{}
the reason is : module.exports is an empty object. We did not set anything to module.exports rather we set exports = function()..... in new greet.js. So, module.exports is empty.
Technically exports and module.exports should point to same reference(thats correct!!). But we use "=" when assigning function().... to exports, which creates another object in the memory. So, module.exports and exports produce different results. When it comes to exports we can't override it.
Now, imagine you re-write (this is called Mutation)
greet.js (referring to Renee answer) as
exports.a = function() {
console.log("Hello");
}
console.log(exports);
console.log(module.exports);
the output will be
{ a: [Function] }
{ a: [Function] }
As you can see module.exports and exports are pointing to same reference which is a function. If you set a property on exports then it will be set on module.exports because in JS, objects are pass by reference.
Conclusion is always use module.exports to avoid confusion.
Hope this helps. Happy coding :)
Also, one things that may help to understand:
math.js
this.add = function (a, b) {
return a + b;
};
client.js
var math = require('./math');
console.log(math.add(2,2); // 4;
Great, in this case:
console.log(this === module.exports); // true
console.log(this === exports); // true
console.log(module.exports === exports); // true
Thus, by default, "this" is actually equals to module.exports.
However, if you change your implementation to:
math.js
var add = function (a, b) {
return a + b;
};
module.exports = {
add: add
};
In this case, it will work fine, however, "this" is not equal to module.exports anymore, because a new object was created.
console.log(this === module.exports); // false
console.log(this === exports); // true
console.log(module.exports === exports); // false
And now, what will be returned by the require is what was defined inside the module.exports, not this or exports, anymore.
Another way to do it would be:
math.js
module.exports.add = function (a, b) {
return a + b;
};
Or:
math.js
exports.add = function (a, b) {
return a + b;
};
Rene's answer about the relationship between exports and module.exports is quite clear, it's all about javascript references. Just want to add that:
We see this in many node modules:
var app = exports = module.exports = {};
This will make sure that even if we changed module.exports, we can still use exports by making those two variables point to the same object.
node does something like this:
module.exports = exports = {}
module.exports and exports refer to same object.
This is done just for convenience.
so instead of writing something like this
module.exports.PI = 3.14
we can write
exports.PI = 3.14
so it is ok to add a property to exports but assigning it to a different object is not ok
exports.add = function(){
.
.
}
↑ this is OK and same as module.exports.add = function(){...}
exports = function(){
.
.
}
↑ this is not ok and and empty object will be returned as module.exports still refers to {} and exports refer to different object.
There are two difference between module.exports and exports
When export a single class, variable or function from one module to another module, we use the module.exports. But export to multiple variables or functions from one module to another, we use exports.
module.exports is the object reference that gets returned from the require() calls. But exports is not returned by require().
see more details with examples >> link
As all answers posted above are well explained, I want to add something which I faced today.
When you export something using exports then you have to use it with variable. Like,
File1.js
exports.a = 5;
In another file
File2.js
const A = require("./File1.js");
console.log(A.a);
and using module.exports
File1.js
module.exports.a = 5;
In File2.js
const A = require("./File1.js");
console.log(A.a);
and default module.exports
File1.js
module.exports = 5;
in File2.js
const A = require("./File2.js");
console.log(A);
myTest.js
module.exports.get = function () {};
exports.put = function () {};
console.log(module.exports)
// output: { get: [Function], put: [Function] }
exports and module.exports are the same and a reference to the same object. You can add properties by both ways as per your convenience.
You can think of exports as a shortcut to module.exports within a
given module. In fact, exports is just a variable that gets
initialized to the value of module.exports before the module is
evaluated. That value is a reference to an object (empty object in
this case). This means that exports holds a reference to the same
object referenced by module.exports. It also means that by assigning
another value to exports it's no longer bound to module.exports.
This explanation from MDN is the most clear to me.
Basically, there is one object in memory which is referenced by 2 variables - exports and module.exports.
exports.a = 23
equals
module.exports = {a:23}
But,
exports = {a:23}
does not equal
module.exports = {a:23}
When you assign a new object to exports variable directly, then that variable does not refer to module.exports anymore.
On this page (http://docs.nodejitsu.com/articles/getting-started/what-is-require), it states that "If you want to set the exports object to a function or a new object, you have to use the module.exports object."
My question is why.
// right
module.exports = function () {
console.log("hello world")
}
// wrong
exports = function () {
console.log("hello world")
}
I console.logged the result (result=require(example.js)) and the first one is [Function] the second one is {}.
Could you please explain the reason behind it? I read the post here: module.exports vs exports in Node.js . It is helpful, but does not explain the reason why it is designed in that way. Will there be a problem if the reference of exports be returned directly?
module is a plain JavaScript object with an exports property. exports is a plain JavaScript variable that happens to be set to module.exports.
At the end of your file, node.js will basically 'return' module.exports to the require function. A simplified way to view a JS file in Node could be this:
var module = { exports: {} };
var exports = module.exports;
// your code
return module.exports;
If you set a property on exports, like exports.a = 9;, that will set module.exports.a as well because objects are passed around as references in JavaScript, which means that if you set multiple variables to the same object, they are all the same object; so then exports and module.exports are the same object.
But if you set exports to something new, it will no longer be set to module.exports, so exports and module.exports are no longer the same object.
Renee's answer is well explained. Addition to the answer with an example:
Node does a lot of things to your file and one of the important is WRAPPING your file. Inside nodejs source code "module.exports" is returned. Lets take a step back and understand the wrapper. Suppose you have
greet.js
var greet = function () {
console.log('Hello World');
};
module.exports = greet;
the above code is wrapped as IIFE(Immediately Invoked Function Expression) inside nodejs source code as follows:
(function (exports, require, module, __filename, __dirname) { //add by node
var greet = function () {
console.log('Hello World');
};
module.exports = greet;
}).apply(); //add by node
return module.exports; //add by node
and the above function is invoked (.apply()) and returned module.exports.
At this time module.exports and exports pointing to the same reference.
Now, imagine you re-write
greet.js as
exports = function () {
console.log('Hello World');
};
console.log(exports);
console.log(module.exports);
the output will be
[Function]
{}
the reason is : module.exports is an empty object. We did not set anything to module.exports rather we set exports = function()..... in new greet.js. So, module.exports is empty.
Technically exports and module.exports should point to same reference(thats correct!!). But we use "=" when assigning function().... to exports, which creates another object in the memory. So, module.exports and exports produce different results. When it comes to exports we can't override it.
Now, imagine you re-write (this is called Mutation)
greet.js (referring to Renee answer) as
exports.a = function() {
console.log("Hello");
}
console.log(exports);
console.log(module.exports);
the output will be
{ a: [Function] }
{ a: [Function] }
As you can see module.exports and exports are pointing to same reference which is a function. If you set a property on exports then it will be set on module.exports because in JS, objects are pass by reference.
Conclusion is always use module.exports to avoid confusion.
Hope this helps. Happy coding :)
Also, one things that may help to understand:
math.js
this.add = function (a, b) {
return a + b;
};
client.js
var math = require('./math');
console.log(math.add(2,2); // 4;
Great, in this case:
console.log(this === module.exports); // true
console.log(this === exports); // true
console.log(module.exports === exports); // true
Thus, by default, "this" is actually equals to module.exports.
However, if you change your implementation to:
math.js
var add = function (a, b) {
return a + b;
};
module.exports = {
add: add
};
In this case, it will work fine, however, "this" is not equal to module.exports anymore, because a new object was created.
console.log(this === module.exports); // false
console.log(this === exports); // true
console.log(module.exports === exports); // false
And now, what will be returned by the require is what was defined inside the module.exports, not this or exports, anymore.
Another way to do it would be:
math.js
module.exports.add = function (a, b) {
return a + b;
};
Or:
math.js
exports.add = function (a, b) {
return a + b;
};
Rene's answer about the relationship between exports and module.exports is quite clear, it's all about javascript references. Just want to add that:
We see this in many node modules:
var app = exports = module.exports = {};
This will make sure that even if we changed module.exports, we can still use exports by making those two variables point to the same object.
node does something like this:
module.exports = exports = {}
module.exports and exports refer to same object.
This is done just for convenience.
so instead of writing something like this
module.exports.PI = 3.14
we can write
exports.PI = 3.14
so it is ok to add a property to exports but assigning it to a different object is not ok
exports.add = function(){
.
.
}
↑ this is OK and same as module.exports.add = function(){...}
exports = function(){
.
.
}
↑ this is not ok and and empty object will be returned as module.exports still refers to {} and exports refer to different object.
There are two difference between module.exports and exports
When export a single class, variable or function from one module to another module, we use the module.exports. But export to multiple variables or functions from one module to another, we use exports.
module.exports is the object reference that gets returned from the require() calls. But exports is not returned by require().
see more details with examples >> link
As all answers posted above are well explained, I want to add something which I faced today.
When you export something using exports then you have to use it with variable. Like,
File1.js
exports.a = 5;
In another file
File2.js
const A = require("./File1.js");
console.log(A.a);
and using module.exports
File1.js
module.exports.a = 5;
In File2.js
const A = require("./File1.js");
console.log(A.a);
and default module.exports
File1.js
module.exports = 5;
in File2.js
const A = require("./File2.js");
console.log(A);
myTest.js
module.exports.get = function () {};
exports.put = function () {};
console.log(module.exports)
// output: { get: [Function], put: [Function] }
exports and module.exports are the same and a reference to the same object. You can add properties by both ways as per your convenience.
You can think of exports as a shortcut to module.exports within a
given module. In fact, exports is just a variable that gets
initialized to the value of module.exports before the module is
evaluated. That value is a reference to an object (empty object in
this case). This means that exports holds a reference to the same
object referenced by module.exports. It also means that by assigning
another value to exports it's no longer bound to module.exports.
This explanation from MDN is the most clear to me.
Basically, there is one object in memory which is referenced by 2 variables - exports and module.exports.
exports.a = 23
equals
module.exports = {a:23}
But,
exports = {a:23}
does not equal
module.exports = {a:23}
When you assign a new object to exports variable directly, then that variable does not refer to module.exports anymore.
I have been creating software in NodeJS for years, but I have rarely ever looked into the module side of it, since I've never needed to do that stuff myself.
I have created a test.js file, which is my module, all it contains is this (i've tried this.host, self.host, var host ... nothing is working)
var tests = function () {
this.host = "http://127.0.0.1/";
};
exports = tests;
In my server.js I try and use it and I can never get host to actually output
var tests = require('./test.js');
console.log(tests);
console.log(tests.host);
I always get this output, saying that tests variable has no properties ... which I set in the module.
sudo node server.js
{}
undefined
The host variable as you defined it in the tests function is not accessible through tests's prototype.
This means that in order to access it, you should be creating a new instance of tests, using the new operator :
var Tests = require('./tests');
var instance = new Tests();
// Now you can access `instance.host`
Also, as David said, use module.exports to export your function.
Don't do exports = tests. Either do exports.tests = tests or module.exports = tests.
Basically, you have to first decide if you want your module to just have properties that can be directly accessed or if you want it to have a constructor function that creates an object when it is called that then has properties or it could even just be a regular function that you call that returns a value. You have mixed and matched the first two schemes (pieces of each) and thus it does not work. I will show you both schemes:
Here's the scheme where your module exports a constructor function from which you can create an object (when you new it):
// test.js module
var tests = function () {
this.host = "http://127.0.0.1/";
};
module.exports = tests;
// main module server.js
var Tests = require('./test.js');
var t = new Tests();
console.log(t.host);
And, here's the scheme where you just directly export properties:
// test.js module
module.exports = {
host: "http://127.0.0.1/"
};
// main module server.js
var tests = require('./test.js');
console.log(tests);
console.log(tests.host);
Keep in mind that whatever you assign to module.exports is what require() will return after it loads your module. So, in your first case, you're assigning a function that is intended to be a constructor function so you have to use it as a constructor function in order for it to work properly.
In my second example, I assign an object to module.exports so you can then treat it just like an object after loading the module with require(). That means you can then just directly access its properties as you would for an object.
console.log(tests()); will work if the you add return statement inside the function.
I am taking first steps with node.js and obviously one of the first things i tried to do was exporting some data from a module, so i tried this simple case:
dummy.js:
var user = "rally";
module.exports = {
user:user
};
and than required it from a different file like this:
var dummy = require('./dummy.js');
console.log(dummy.user); // rally
So far so good, every thing works, but now i dived into code where there is this definition in the beginning of the module:
module.exports = function(passport,config, mongoose) {}
and i don't understand whats the meaning of it and how can i work with it.
just for trying to understand i defined some variables inside this abstract function but couldn't get their value from any other file.
any idea how can i export variables from module defined like this..? so for example i could require this module and get the "Dummy" variable and use it in a different file
module.exports = function(passport,config, mongoose) {
var dummy = "Dummy";
}
It works exactly the same as the first one does, only that it exports a function instead of an object.
The module that imports the module can then call that function:
var dummy = require('./dummy.js');
dummy();
any idea how can i export variables from module defined like this..?
Since functions are just objects, you can also assign properties to it:
module.exports = function(passport,config, mongoose) {}
module.exports.user = 'rally';
However I'd argue that this is less expected if a module directly exports a function. You are probably better off exporting the function as its own export:
exports.login = function(passport,config, mongoose) {}
exports.user = 'rally';
WHAT IS A MODULE?
A module encapsulates related code into a single unit of code. When creating a module, this can be interpreted as moving all related functions into a file.
// dummy.js
var exports = module.exports = {};
The utility of dummy.js increases when its encapsulated code can be utilized in other files. This is achieved by using exports.
HOW ARE THEY INVOKED?
You could declare your functions outside of the module.exports block. Functions inside exports can be invoked exactly the same way as variables or any other object.
EXAMPLE
//dummy.js
var myVariable = "foo";
var myFunction = function(){
//some logic
};
module.exports{
myVariable : myVariable,
myFunction : myFunction,
myVariableTypeTwo : "bar",
myFunctionTypeTwo : function () {
//some logic
}
}
We can now access the publicly available methods of dummy.js as a property from any js file.
var dummy = require('./dummy.js');
dummy.myVariable; //foo
dummy.myFunction();
dummy.myVariableTypeTwo; //bar
dummy.myFunctionTypeTwo();
NOTE
In the code above, we could have replaced module.exports with exports and achieved the same result. If this seems confusing, remember that exports and module.exports reference the same object.
In one of my JS files I include another one. How can I set variables in the included module?
I thought doing something like this would work
var mymodule = require('mymodule.js');
mymodule.myvariable = 'test';
And then in mymodule
this.myvariable === 'test';
But this doesn't work, it's undefined. What are the various options for passing a value into a module? I could just add the variable as a parameter to every function I call in mymodule, but that isn't ideal.
Is there a way to do it without globals, so that I can set the variables independently in various required modules, like this?
var mymodule1 = require('mymodule.js');
var mymodule2 = require('mymodule.js');
mymodule1.myvariable = 'test1';
mymodule2.myvariable = 'test2';
The problem with what you were doing is that you set the variable after importing, but this.myvariable === 'test'; was being called when the module was imported, before your variable was set.
You can have your module export a function and then call the function when you import, passing your variable as an argument.
module.exports = function(myVar) {
var myModule = {
// has access to myVar
...
};
return myModule;
};
When you import,
var myModule = require('myModule')(myVar);
If you use this method, keep in mind that you get a different instance of your module wherever you import, which may not be what you want.
If you want to set values of a module from outside the module, a good option is to have your module export an object with a setter method, and use that to set the value of the variable as a property of the object. This makes it more clear that you want this value to be settable, whereas just doing myModule.myVar = can set you up for confusion later.
module.exports = {
myVar: null,
setMyVar: function(myVar) {
this.myVar = myVar;
},
...
};
In this case you're accessing the same instance of the model wherever you import it.
Edit in response to comment
In the first option you show where you get a different instance each
time, how can I export multiple functions that each share the same
myVar? If that module exports 5 functions each that need myVar, can I
set it in one place like at import time rather than passing it into
each function?
Not entirely sure if I understand what you're describing, but you could do something like this:
module.exports = function(myVar) {
var modules = {};
modules.someModule = {...};
modules.anotherModule = {...};
...
return modules;
};
Each of these sub-modules would have access to the same myVar. So you would import as above and the result would be an object containing each of your five modules as properties. I can't say whether this is a good idea, it's getting pretty convoluted, but maybe it makes sense for your situation.
NodeJS require() will always load the module once so you will need to implement scoping into your module where different instances of the module can exist with their own internal state.
You can implement your module as a JS class like:
var MyModule = function(){};
MyModule.prototype.someFunction = function(params){
this.someVar = params;
}
MyModule.prototype.anotherFunction = function(key, value){
this[key] = value;
}
module.exports = MyModule;
Then in your code;
var MyModule = require('MyModule');
someModule = new MyModule();
// treat someModule like an object with its getters/setters/functions
Should work just fine. Here is a working example:
index.js
var module = require('./module.js');
module.myvar = 'Hello world';
module.test();
module.js
module.exports = {
test: function() {
console.log('var is', this.myvar);
}
};
Keep in mind if you use this in a closure that the scope isn't any longer the module itself. So that could be your problem.
Can you show me the part of the module code where you use this?
This is a module named StrUpperCase.js
exports.StrUpperCase = function(str) {
return str.toUpperCase();
}
In app.js:
var str = "Welcome World...";
const SUC = require('./modules/StrUpperCase');
console.log(SUC.StrUpperCase(str));