I have two files, a main and a module file.
//module.js
module.exports.filesystem = "hello!";
//main.js
var filefunctions = require('./module.js');
console.log(filefunctions.filesystem);
//returns "hello!"
This works as expected. I'm not sure why the following, doesn't though:
//module.js
var filefunctions = require('./module.js');
console.log(filefunctions[0]);
//main.js
exports.filesystem = "hello!";
I was under the impression that module.exports would create an array, like
module.exports = {
filesystem: "hello!";
}
Is there something obvious in the syntax that I'm missing? Just trying to wrap my head around these concepts. Any help is appreciated. Thanks!
your assumption of below code is correct,
module.exports = {
filesystem: "hello!";
}
the mistake is, you cannot access object attributes as obj[0], can access them as obj[Object.keys(obj)[0]], try the below code:
console.log(filefunctions[Object.keys(filefunctions)[0]]);
Related
I'm having trouble understanding the difference between exporting modules like:
module.exports.getUserIP = function getUserIP(req) {
var ip = req.headers['x-forwarded-for'];
return ip;
}
Or just declaring it:
function getUserIP(req) {
// retrieve user IP from req object
// Build this function to be more accurate/use more sources.
var ip = req.headers['x-forwarded-for'];
return ip;
}
and exporting at the bottom:
module.exports = { getUserIP }
or even:
module.exports = {getUserIP:getUserIP}
or
module.exports = {'getUserIP':getUserIP}
My problem is: when i call the function getUserIP from another file:
var mainbody = require('./app.js');//getUserIP is in here.
const gl = require('geoip-lite');
var ax = require('axios');
module.exports.getloc = function getloc(req, ip, property) {
//return location from IP.
if (req) {
var ipGuest = mainbody.getUserIP(req); //HERE
} else {
var ipGuest = ip;
}....
I get an error message:
Error Message
However, when I use the FIRST method to export the function:
module.exports.getUserIP = function getUserIP(req) {
var ip = req.headers['x-forwarded-for'];
return ip;
}
Then it works perfectly.
What's the difference?
Better way is to use
module.exports = { getUserIP: getUserIP }
This way you can just look at the export statement at the end of your file and know which functions are being exported from a particular file
The module.exports = {getUserIP}; is nothing but a shorthand of the above syntax(ES6 Magic). What it typically does is allows you to write this way { getUserIP } if the key name to be same as function/variable name like { getUserIP: getUserIP } where getUserIP can be a variable or a function or a ES6 class.
All the examples you show will work properly, but they do have some different affects.
By default module.exports is already initialized to an empty object. So, when you do something like this:
module.exports.getUserIP = function() {...}
You are assigning a new property to the existing object that module.exports already pointed to. One advantage of this scheme is that you can easily add more properties the same way.
module.exports.getUserRegion = function() {}
This will add one more property to that same object without disturbing the first one you already added.
On the other hand, all of these are identical:
module.exports = {getUserIP: getUserIP}
module.exports = {'getUserIP':getUserIP}
module.exports = { getUserIP } // ES6 shorthand for the previous syntax
and, they all end up with the same result as each other, but they all replace module.exports with a new object that has your one new property in it.
If you then tried to add another property:
module.exports = {getUserRegion};
That would again assign a whole new object to module.exports and you would have just wiped out the object that previously had getUserIP on it. When assigning a new object, you would typically assign an object that had all your properties on it:
module.exports = {getUserIP: function() {...}, getUserRegion: function() {...}};
Thus, not wiping out something you had already put there.
All of your schemes should work fine as long as you aren't overwriting module.exports with a new object and thus overwriting the object that already had some of your methods on it.
To understand this exporting modules concept, just think module.export is a simple object. you can bind anything to that object as do with normal javascript objects.
Finally when you require that module by require('path to js') you will get that exported object. If you export number of items in your module you can return them back by giving the names of the tag.
I am pretty certain there is a way to pass a variable using require.
So it would look something like this:
var model = require('model')(mongoose);
With the above line of code, I want to pass my model file my database information (mongoose), so that if I access the same model with a different database, I can pass it different database information.
However, even if the above syntax is correct, I am not sure what my model file itself would have to look like. Can anyone help me out with this?
module.exports = function (mongoose) {
// . . .
return model;
};
You can pass moongoose by argument to that file
var model = require('model')(mongoose);
Your module will look like this, you can make an object in module.exports and can attach multiple properties to that object and in the end return it from the function
module.exports = function (mongoose) {
model ={};
model.properties = {};
model.yourfunction1 = function(){};
return model;
};
I guess I can't assign anything else to module.exports in this case?
Answer to your comment is explained below
Choosing between module.exports and exports depends on you
For exports
exports.object1 = {};
exports.object2 = {};
For module.exports
module.exports = function(){
myobj={}
myobj.object1 = {};
myobj.object2 = {};
return myobj
}
Now calling it will be different
For exports it will be directly available on file variable
var file = require('./file');
console.log(file.object1);
For module.exports you will execute it like a function by appending function parenthesis so that object can be returned
var file = require('./file')();
console.log(file.myobj.object1);
I understand that functionality is scoped inside module.exports. But I always see variables declared outside. So shouldn't it be inside ?
var bcrypt = require('bcrypt');
module.exports = {
...
}
The code outside of the module.exports is not visible outside of the file. Typically includes libraries and functions used within the code you are writing. I guess you could think of this as being "private" functions / variables.
I don't know if you are writing code inside of module.exports but I prefer not to. I find this to be more readable:
var doSomethingImpl = function(){
}
module.exports = {
doSomething: doSomethingImpl
}
or
var doSomethingImpl = function(){
}
module.exports.doSomething = doSomethingImpl
or even
module.exports.doSomething = function(){
}
I'm really confused about a variable scope issue with a file required via a path in a config file. Why does my Mongoose schema method "see" the required objects when called from within the model file but not when called from my app.js file? I'm convinced that I must be doing something obviously wrong but I can't see it.
The Node project has the following (simplified) structure:
|models
-index.js
-story.js
-post.js
-app.js
-config.js
This is config.js:
config = {};
config.test = 'test';
config.models = __dirname + '/models';
module.exports = config;
This is story.js:
var config = require('../config.js');
var models = require(config.models);
var foo = {};
foo.bar = 'baz';
var storySchema = mongoose.Schema
({
author: {type: mongoose.Schema.Types.ObjectId},
root: {type: mongoose.Schema.Types.ObjectId, default: null}
});
storySchema.methods.test = function()
{
console.log(foo.bar);
console.log(config.test);
console.log(models);
}
var Story = exports.model = mongoose.model('story', storySchema);
When I create a new Story in app.js and call its test() method, I get this output:
baz (so I know it's seeing objects in the same file)
test (so I know it's seeing variables in the config file)
{} (this "should" log my models object but it logs an empty object, why?)
When I create a new Story object within the story.js file, and run it (node ./models.story.js) the values returned are as expected (the models object is logged rather than an empty object).
Update, here are the index.js and app.js files:
index.js:
module.exports = {
post: require('./post'),
story: require('./story')
};
app.js:
var config = require('./config');
var models = require(config.models);
var story = new models.story.model();
story.test();
I believe the issue is that you've created a circular dependency. Story executes require(config.models) which requires Story again inside index.js.
Rather than storing a string and requireing it everywhere, try storing the models directly in config.models:
config.js
module.exports = {
test: 'test',
models: require(__dirname + '/models')
};
In case anyone runs into this same issue, I wanted to point to a couple resources I came across that helped me resolve the issue. As ssafejava pointed out, the problem does have to do with circular dependency (although ssafejava's solution did not entirely resolve the issue) . What worked for me was designing this dependency out of my application but there are other options if doing so is not possible. See the following issues' comments for a better explanation (in particular, see 'isaacs' comments):
https://github.com/joyent/node/issues/1490
https://github.com/joyent/node/issues/1418
Consider I want to expose a method called Print
Binding method as prototype:
File Saved as Printer.js
var printerObj = function(isPrinted) {
this.printed = isPrinted;
}
printerObj.prototype.printNow = function(printData) {
console.log('= Print Started =');
};
module.exports = printerObj;
Then access printNow() by putting code require('Printer.js').printNow() in any external .js node program file.
Export method itself using module.exports:
File Saved as Printer2.js
var printed = false;
function printNow() {
console.log('= Print Started =');
}
module.exports.printNow = printNow;
Then access printNow() by putting code require('Printer2.js').printNow() in any external .js node program file.
Can anyone tell what is the difference and best way of doing it with respect to Node.js?
Definitely the first way. It is called the substack pattern and you can read about it on Twitter and on Mikeal Rogers' blog. Some code examples can be found at the jade github repo in the parser:
var Parser = exports = module.exports = function Parser(str, filename, options){
this.input = str;
this.lexer = new Lexer(str, options);
...
};
Parser.prototype = {
context: function(parser){
if (parser) {
this.contexts.push(parser);
} else {
return this.contexts.pop();
}
},
advance: function(){
return this.lexer.advance();
}
};
In the first example you are creating a class, ideally you should use it with "new" in your caller program:
var PrinterObj = require('Printer.js').PrinterObj;
var printer = new PrinterObj();
printer.PrintNow();
This is a good read on the subject: http://www.2ality.com/2012/01/js-inheritance-by-example.html
In the second example you are returning a function.
The difference is that you can have multiple instances of the first example (provided you use new as indicated) but only one instance of the second approach.