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How to Create a Node.js Module

1. Create a new directory and enter it.

e.g:/d/sayHelloWorld

2. Init your Node.js module.

You will set name, descrption, entry point and etcetera for your Node.js module.
When you are done, there will be a new file in your directory called “package.json”, which contains all of the information you just set.
entry point: your entry file of your Node.js module.
test command: the command that you need to execute your unit test.
npm init

3. Create your entry point.

e.g:/d/sayHelloWorld/index.js

module.exports = {
    say() {
    console.log('Hello World!')
   }
}

How to use your Node.js Module Locally

1. Create a new directory and enter it.

2. Install the Node.js module that you created before.

npm install [Node.js Module Path]   // e.g: npm install ../sayHelloWorld/

3. Enter into the Node shell interface, and test if your Node.js Module is available to use.

node
var test = require([Node.js Module Name]) // e.g: var test = require('sayhelloworld')
test.say() // write out 'Hello World'

Related:
Publish a Node.js Module to the NPM Registry
Install Pakages Using NPM