eolas/Programming_Languages/NodeJS/Modules.md

---
tags:
  - Programming_Languages
  - backend
  - node-js
  - node-modules
---


 > 
 > Modules are small files where we define our variables and functions. Values defined in modules are scoped to that specific module, constituting a unique name space. This avoids name clashes in large programs.

* Every file in a Node application is considered a module.

* The variables and methods in a module are equivalent to `private` properties and methods in object-oriented programming. 

* If you wish to use a function or variable defined in a module outside of its modular container you need to explicitly export it and make it public. 

## Structure of a module

Node keeps an internal record of the properties of a module. To see this we can log the property `module` to the console.

````js
// index.js
console.log(module)

````

This gives us:

````json
Module {  
 id: '.',  
 path: '/home/thomas/repos/node-learning',  
 exports: {},  
 filename: '/home/thomas/repos/node-learning/index.js',  
 loaded: false,  
 children: [],  
 paths: [  
   '/home/thomas/repos/node-learning/node_modules',  
   '/home/thomas/repos/node_modules',  
   '/home/thomas/node_modules',  
   '/home/node_modules',  
   '/node_modules'  
 ]  
}
````

## Exports

* Whenever we export a property or method from a module we are directly targeting the `exports` property of the module object. 
* Once we add exports to a file they will be displayed under that property of the module object. 
* We can export the entire module itself as the export (typically used when the module is a single function or class) or individual properties. 

### Exporting a whole module

*The example below is a module file that consists in a single function*

````js

module.exports = function (...params) {
	// function body
}

````

Note the module is unnamed. We would name it when we import:

````js

const myFunction = require('./filenme')

````

### Exporting sub-components from a module

In the example below we export a variable and function from the same module. Note only those values prefixed with `exports` are exported. 

````js

exports.myFunc = (...params) => {
	// function bod[]()y
}

exports.aVar = 321.3

var nonExportedVar = true

````

This time the exports are already name so we would import with the following:

````js

const { myFunc, aVar } = require("./filename");

````

We can also do the exporting at the bottom when the individual components are named: 

````js

const myNamedFunc = (val) => {
  return val + 1;
};

function anotherNamedFunc(val) {
  return val * 2;
}

// This time we export at the bottom
exports.myNamedFunc = myNamedFunc;
exports.differentName = anotherNamedFunc; // We can use different names

// Or we could export them together
module.exports = { myNamedFunc, anotherNamedFunc };

````

The import is the same:

````js
const { myNamedFunc, anotherNamedFunc } = require("./modules/multiExports");
````

## Structuring modules

The techniques above are useful to know but generally you would want to enforce a stricter structure than a mix of exported and private values in the one file. The best way to do this is with a single default export. 

Here the thing exported could be a composite function or an object that basically acts like a class with methods and properties.

*Export a composite single function*

````js

module.exports = () => { 
	foo() {...} 
	bar() {...} 
}
								
````

*Export an object*

````js

module.exports = { 
	foo : () => {...},
    bar: () => {...} 
}


````

**Both of these structures would be referred to in the same way when importing and using them**

Or you could export an actual class as the default. This is practically the same as the two above other than that you would have to use `new` to initiate an instance of the class.

````js
export default class {
	foo() {}
	bar() {}
} 
````

## Built-in modules

Node has numerous built-in methods that provide helpful utility methods:

* [File system module](File%20system%20module.md)
* [Events module](Events%20module.md)

## Structure of Node module methods

Every method belonging to the in-built modules of Node has a callback structure: a callback function of which the first argument is an error-handler and the second is the (typically asynchronous) returned value. 

For example 

````js

fs.readdir('./', function(err, files) {
	if (err) {
		console.error(err)
	} else {
		console.log(files)	
	}
})

// ['files', 'that', 'were', 'returned']

````