Recitation #3

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What is Node.js

• Open-source, cross-platform JavaScript runtime environment.
• Event-driven architecture capable of asynchronous (non-blocking) I/O.
• Suitable for fast, powerful, and scalable web applications.
• JavaScript and Node.js are the most popular technologies for web development.

Npm (Node Package Manager)

• Largest ecosystem of open-source libraries in the world.
• npm registry - an online database of public and paid-for private packages.

How to find reliable, secure packages:

1. Run npm audit after installing to check for vulnerabilities.
2. Look for an active GitHub repository (stars, forks, and issues).
3. Check package.json dependencies:

   "dependencies": {
     "axios": "^1.7.9",
     "cra-template": "1.2.0",
     "react": "^18.0.0",
     "react-dom": "^18.0.0",
     "react-scripts": "5.0.1",
     "web-vitals": "^4.2.4"
   }
   

Simple Web Architecture

• Frontend: React / Vue + RESTful APIs
• Backend: Node.js
• Database: PostgreSQL / MySQL / MongoDB

Node.js + MongoDB

What is MongoDB?

• NoSQL document-based database.
• Stores data in JSON-like BSON format.
• Scalable, flexible, and schema-less.

Schema Example:

{
  "name": "Harry Potter",
  "author": {
    "name": "J.K. Rowling",
    "followers": 789
  },
  "views": 123456
}

Node.js Asynchronous Execution

function function1() {
    setTimeout(() => {
        console.log('function1');
    }, 1000); // Delays execution by 1 second
}
function function2() {
    console.log('function2');
}
function main() {
    function1();
    function2();
}

Output

function2
function1

Callback Functions

1. Functions passed to another function
2. Executes the callback after function1 completes

Example

function function1(callback) {
    setTimeout(() => {
        console.log('function1');
        callback();
    }, 1000);
}
function function2() {
    console.log('function2');
}
function main() {
    function1(function2);
}

Output

function1
function2

Promises and Promise Chains

1. Structured callbacks that avoid "callback hell".
2. Resolves the promise after function1 completes.

Example

function function1() {
    return new Promise((resolve) => {
        setTimeout(() => {
            console.log('function1');
            resolve();
        }, 1000);
    });
}
function function2() {
    console.log('function2');
}
function main() {
    function1().then(function2); // Waits for function1 before running function2
}

Output

function1
function2

Generators

1. Functions that can be exited/paused and later renamed

Example

function* main() {
    yield new Promise((resolve) => {
        setTimeout(() => {
            console.log('function1');
            resolve();
        }, 1000);
    });

    console.log('function2');
}
const generator = main();
generator.next().value.then(() => generator.next());

Output

function1
function2

Async/Await

1. Combines generators and promises.
2. Waits for function1 before running function2.
3. Functions that use await must be declared as async.

function function1() {
    return new Promise((resolve) => {
        setTimeout(() => {
            console.log('function1');
            resolve();
        }, 1000);
    });
}
function function2() {
    console.log('function2');
}
async function main() {
    await function1();
    function2();
}

function1
function2