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In Node js, module.exportmodules are exposed on the server side and provided in CommonJS format.

Node js modules can be called as pre-built code packages that contain javascript objects and functions and can be used by external applications.

Using modules in Node JS

Modules are an essential part of writing programs because they make our lives as developers easy by allowing us to 重用代码,,, and .调试易于理解管理复杂的应用程序

There are three types of modules in Node js; those modules which essentially come with the installation of Node js are called Coremodules. These modules are easy to use in our programs as we only need to access their functionality.

A typical example is HTTPthe module.

const http = require('http');

Using third-party modules in Node JS

We also have 第三方modules that we need to install before using them in our program. These modules are usually installed using npm packages.

A typical example of such modules is the express package. Once installed, these packages are added as dependencies package.jsonunder the .

We can then require 关键字import them into our program using .

npm install express

const express = require('express');

Using local modules in Node JS

Finally, we have localmodules. As the name suggests, these are modules that we create ourselves to perform various operations.

Unlike the previous two types of modules, these modules require us to first export them to make them available, and then import them into the program where we intend to implement them.

There are two widely used export and import modules on the server side, especially in Node js. These are CommonJS and ESM.

The CommonJS format allows us to create modules in unique contexts in different Node js files.

Modules that conform to this format can be called CommonJsmodules. In this format, we use module.exportsobjects to expose a module and use the default requirekeyword to import them into the program.

On the other hand, ESMthe __register__ format uses exportthe __name__ keyword to identify exportmodules and imports them importaccording to the __register__ ES6syntax using the __register__ keyword.

Use in Node JSmodule.experts

Using module.exports, we can export literals, , functions, classesand objects. keywordmodule stands for currentmodule, while the keyword export exposes any object as a module.

module.exportsWe can export them as a module by simply assigning them to . This is because an export is itself an object, so any literal assigned to it will be exported as a module.

In the following example, we create a Node js application, our main file is index.jsand description.jscontains the text we want to export as a module.

Use the one that contains text description.js.

let tesla = "Largest electric car manufacturer";
 
module.exports = tesla;

Use index.jsas our main file.

var desc_tesla = require('./description.js');

console.log(desc_tesla);

Output:

Largest electric car manufacturer

In the Node application above, we exported a String literaland imported it by specifying description.jsthe path to the root directory where index.js.

We can also export functions as modules, just create a function as we normally do in javascript and assign the name of the function to module.exports.

Now that the module system knows that we want to expose the function as a module, we can index.jsaccess it by simply requiring it in our <module> file as shown below.

Use the one that contains text description.js.

function factorial(n) {
    if ((n == 0) || (n === 1)) {
        return 1;
    } else {
 
        return (n * factorial(n-1));
    }
}

module.exports = factorial;

Use index.jsas our main file.

var factorial = require('./description.js');
 
console.log(factorial(7));

Output:

5040

Similarly, we can export objects as modules. Objects are independent entities with properties and types.

We can assign the object to module.exportsor assign the object name to module.exports. Once we have exposed the object, we can use the following require()method to access it at our desired location.

Use the one that contains text description.js.

const car = {
    make: 'Tesla',
    model: 'S plaid',
    top_speed: '200mph',
    year: 2019
}

module.exports = car;

Use index.jsas our main file.

var car = require('./description.js');
 
console.log(car.make);
console.log(car.model);
console.log(car.top_speed);

Output:

Tesla
S plaid
200mph

Once we have imported the object index.js, we can access the object's properties using dot notation.

In the examples above, we have been using ./description.jsimport-module where ./specifying description.jsis located in the root folder.

However, this may not always be the case; in some cases, the modules may be located in different folders. We need to specify the absolute location of importing such modules, which ./begin with , pointing to the root folder.