Node.js - Intro

tags: nodejs, backend

Node.js

https://nodejs.org/en/

• allows us to run Javascript outside the browser
• Node.js is created using the V8 engine to read and run JS outside the browser
• popular for building servers

Install Node.js

Install

https://nodejs.org/en/download/

• automatically installs NPM for you

Check node version

$ node -v

Run node in the terminal

$ node

Update latest version

• Download the latest version on the website
• Only update NPM:
  ​​​​$npm install npm@latest -g
  

Ref. https://docs.npmjs.com/try-the-latest-stable-version-of-npm

global vs. window

• window object is provided by the browser
• Node uses global instead
• Node comes with its own properties and methods; won't have browser-specific things e.g. document, fetch)

全域變數 var： window 上的全域變數會覆寫到 window，global 則不會（只會在該 js 檔案裡）

• In browser: window.prop

  ​​​​//script1.js
  ​​​​var a = 1
  ​​​​console.log(window.a); // 1
  

• In global: global.prop

  ​​​​var a = 1
  ​​​​console.log(global.a); // undefined
  
  ​​​​global.a = 1
  

process

One of the global object has is process

process

means the process what is running now in the terminal; what the computer is doing

exit the process

process.exit()

Exit the process and back to the terminal

globalThis (ES2020)

globalThis === window // true

• globalThis works outside the browser as well
• exists as both part of the window and Node, so that you can have unified variables across different platforms
• able to have unified code base

Running script.js in Node

1. create a file

$ touch script.js

2. run script.js file

$ node script.js

• after the process is done, runs process.exit to end the process
• within node we can run what global objects we have.
  e.g. Use __dirname to get directory
  ​​​​// script.js
  ​​​​console.log(__dirname);
  

Modules in Node

ES6 import/export

ES6 imports only works in Node version 12.2.0 or higher

Export: export default var1

//script2.js
const largeNumber = 356;
export default largerNumber;

//simplified way:
export const largeNumber = 356;

Import: import var1 from './filePath'

//script.js
import largeNumber from 'script2.js'
//or import doing destructuring
import {largeNumber} from './script2.js'

const a = largeNumber;
const b = 5;
console.log(a+b); // SyntaxError: Unexpected token import

CommonJS modules (in the old version of Node)

If ran in older version which Node has not yet been introduced with the ES6 import/export feature: SyntaxError: Unexpected token import

Export: module.exports = { prop1: value1 }

//script2.js
const largeNumber = 356;
module.exports = largeNumber

// or export an object
module.exports = {
    largerNumber: largeNumber
};

• module is a global object we have access to

Import: require('./filePath')

//script.js
const script2 = require ('./script2.js')

const a = script2.largeNumber; // get access to the property of script2
const b = 5;
console.log(a+b); //361

ES6 Modules in Node

Export: export const variableName = value;

// script2.js
export const largeNumber = 356;

Import: import { variableName } from './filePath'

//script.js
import {largeNumber} from './script2.js'

• {} - destructuring
• './script2' - it's okay not to indicate file type

Run node:

$ node script.js

Got an error:

Warning: To load an ES module, set "type": "module" in the package.json or use the .mjs extension.

Enabling ES6 Modules in Node

Because Node is created with CommonJS import/export in mind, we needs to specify how we want Node to read the file.

Ref. https://nodejs.org/api/esm.html#enabling

Node.js has two module systems: CommonJS modules and ECMAScript modules. Authors can tell Node.js to use the ECMAScript modules loader via the .mjs file extension, the package.json "type" field, or the --input-type flag.

Outside of those cases, Node.js will use the CommonJS module loader.

Two methods to load ES module in Node:

• .mjs file method:

  1. Rename .js to .mjs files
  2. Make sure the syntax we're importing is .mjs
     • file path:import {largeNumber} from './script2.mjs'
     • run: $ node script.mjs

• package.json method:

  1. create a package.json file

     ​​​​​​​​$ npm init -y
     

     • -y to say yes to all the options

  2. in package.json file, add "type": "module"

Types of Modules in Node

Built-in modules: pre-installed with Node

• e.g. fs module - file system module
• e.g. http module - used to build a server
• these modules are without the path

Package (e.g. NPM)
nodemon

1. create a package.json file

$ npm init

2. install nodemon in dev

$ npm install nodemon --save-dev

• you'll see nodemon in package.json's devDependencies
  
  
• devDependencies: dependencies we only use when we're developing; won't be included when we actually deployed the app
• Beacause it's is in thenode_modules/.bin file, we can use the command $ nodemon.

3. Set scripts in package.json file:

"scripts":{
    "start": "nodemon script.js"
},

4. Run $npm start to start nodeman. It will constantly watch the changes and keeps running the $ node script.js command.
   
   

Node Module - Path

• __dirname - 目錄路徑
• __filename - 檔名

var path = require('path'); // 取用 path 模組

// 取得目錄路徑
console.log(**path.dirname**('xx/yy/zz.js')); // Return: **/xx/yy** --> 回傳「資料夾名稱」
 
// 合併路徑
// Syntax: **.join(目前路徑, 欲合併的路徑)**
console.log(**path.join**(__dirname, '/xx')); // Return: /Users/userName/Desktop/project/xx

// 取得檔名
console.log(**path.basename**('xx/yy/zz.js')); // zz.js

// 取得副檔名
console.log(**path.extname**('xx/yy/zz.js')); // .js

//分析路徑
console.log(**path.parse**('xx/yy/zz.js'))  // {root:'/', dir:'/xx/yy', base:'zz.js', ext:'.js', name:'zz'}

當你載入 var path = require('path'); ，便可用下述語法取得檔案與目錄路徑

• 抓目錄路徑： path.**dirname**('**/xx/yy**/zz.js')：回傳 /xx/yy
• 路徑合併：path.**join**(__dirname,'/xx') 回傳 前後路徑合併
• 抓檔名： path.**basename**('/xx/yy/**zz.js**') -  回傳 zz.js
• 抓副檔名： path.**extname**('/xx/yy/zz.**js**') -  回傳 js
• 分析路徑： path.**parse**('/xx/yy/zz.js') - 回傳 上述綜合物件

Ref. Node.js PATH API文件