What is splice() in JavaScript

1. What is splice()?

The splice() method in JavaScript is used to change the contents of an array by:

• Adding elements

• Removing elements

• Replacing elements

It modifies the original array (unlike slice() which returns a new array).

Here’s a visual diagram showing how splice() works step-by-step — starting from the original array, removing selected elements, and then inserting new elements at the chosen position.

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2. Syntax

array.splice(start, deleteCount, item1, item2, ...);

Parameters

1. start → The index where changes begin.

2. deleteCount → Number of elements to remove.

3. item1, item2, ... → (optional) Elements to add at the start index.

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3. How it Works

Think of splice() as:

“Go to the position start, remove deleteCount items, and then insert any new items I give you at that same position.”

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4. Examples

Example 1: Removing Elements

let fruits = ['apple', 'banana', 'orange', 'mango'];

// Remove 2 elements starting from index 1
let removed = fruits.splice(1, 2);

console.log(fruits); // ['apple', 'mango']
console.log(removed); // ['banana', 'orange']

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Example 2: Adding Elements

let fruits = ['apple', 'mango'];

// Start at index 1, remove 0 elements, add 'banana' and 'orange'
fruits.splice(1, 0, 'banana', 'orange');

console.log(fruits); // ['apple', 'banana', 'orange', 'mango']

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Example 3: Replacing Elements

let fruits = ['apple', 'banana', 'orange'];

// Start at index 1, remove 1 element, add 'mango'
fruits.splice(1, 1, 'mango');

console.log(fruits); // ['apple', 'mango', 'orange']

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Example 4: Remove All from a Certain Index

let numbers = [1, 2, 3, 4, 5];

// Start at index 2, remove all remaining
numbers.splice(2);

console.log(numbers); // [1, 2]

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5. Key Points

• Modifies original array (in-place).

• Returns an array of removed elements.

• If deleteCount is 0 → no elements are removed (only adding happens).

• If deleteCount is omitted → removes everything from start to end.

• Works with any data type in an array.

What is reduce() in JavaScript

1. What is reduce()?

The reduce() method in JavaScript is used to “reduce” an array into a single value by running a function on each element of the array.

This single value could be:

• A number (sum, product, average, etc.)

• A string (concatenated sentence, joined names, etc.)

• An object (grouped data, frequency count, etc.)

• Even another array (flattening arrays, etc.)

Think of reduce() as:

"Take my array, go through it one item at a time, and keep combining the result until I have only one thing left."

Here’s a visual diagram showing how reduce() processes each step — starting from the initialValue and updating the accumulator after adding each array element. This makes it easier to see the flow from start to finish.

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2. Syntax

array.reduce(callbackFunction, initialValue);

Parameters

1. callbackFunction → The function that runs on each element.

   • It takes 4 arguments:

     function(accumulator, currentValue, currentIndex, array)
     

     • accumulator → The value that carries over between each loop.

     • currentValue → The current element being processed.

     • currentIndex → The index of the current element (optional).

     • array → The original array (optional).

2. initialValue → (optional but highly recommended) The starting value for the accumulator.

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3. How it works step-by-step

Let’s start with a simple example — summing numbers.

const numbers = [1, 2, 3, 4, 5];

const sum = numbers.reduce(function (accumulator, currentValue) {
  return accumulator + currentValue;
}, 0);

console.log(sum); // 15

Step-by-step execution:

• Step 1: Initial accumulator = 0 (from initialValue)

• Step 2: Add first number → 0 + 1 = 1

• Step 3: Add second number → 1 + 2 = 3

• Step 4: Add third number → 3 + 3 = 6

• Step 5: Add fourth number → 6 + 4 = 10

• Step 6: Add fifth number → 10 + 5 = 15

• Final Result: 15

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4. Using Arrow Function

const sum = numbers.reduce((acc, curr) => acc + curr, 0);
console.log(sum); // 15

Here:

• acc = accumulator

• curr = current value

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5. More Examples

Example 1: Find Maximum Value

const numbers = [10, 25, 30, 5, 40];

const max = numbers.reduce((acc, curr) => {
  return curr > acc ? curr : acc;
}, numbers[0]);

console.log(max); // 40

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Example 2: Count Occurrences

const fruits = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple'];

const count = fruits.reduce((acc, fruit) => {
  acc[fruit] = (acc[fruit] || 0) + 1;
  return acc;
}, {});

console.log(count);
// { apple: 3, banana: 2, orange: 1 }

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Example 3: Flatten an Array

const nested = [[1, 2], [3, 4], [5, 6]];

const flat = nested.reduce((acc, curr) => acc.concat(curr), []);

console.log(flat); // [1, 2, 3, 4, 5, 6]

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Example 4: Sum of Object Values

const items = [
  { name: 'Book', price: 200 },
  { name: 'Pen', price: 50 },
  { name: 'Bag', price: 500 }
];

const total = items.reduce((acc, item) => acc + item.price, 0);

console.log(total); // 750

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6. Common Mistakes Beginners Make

❌ Forgetting to set the initialValue (can cause unexpected results if the array is empty).
❌ Confusing accumulator with currentValue.
❌ Thinking reduce() only works for numbers (it works for any data type!).

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7. Why Use reduce()?

• It’s powerful: Can replace many for or forEach loops.

• It’s flexible: Works with numbers, strings, objects, arrays.

• It’s cleaner: Keeps logic in one place.

Mail Proxy

1. Simple Meaning

A Mail Proxy in Nginx is like a middleman for email traffic — similar to how a reverse proxy works for websites, but here it’s for email protocols like:

• SMTP (sending mail)

• IMAP (reading mail)

• POP3 (downloading mail)

Instead of connecting directly to the mail server, your email client connects to Nginx Mail Proxy, which then forwards the connection to the correct mail server.

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2. Real-Life Example

Imagine:

• You have two mail servers — one for Gmail, one for Outlook.

• You want to provide one single address to users: mail.example.com.

• Users connect here, and Nginx decides which actual server to send them to.

Like a post office front desk:

• Customer hands you a letter (email connection).

• You check where it needs to go.

• You forward it to the right delivery office (mail server).

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3. Why Use Mail Proxy?

• Single Entry Point → Users don’t need to remember different server addresses.

• Security → Hide actual mail server IPs from public.

• SSL/TLS Offloading → Nginx handles encryption before passing to backend.

• Load Balancing for Mail → Distribute connections among multiple mail servers.

• Protocol Handling → Can support IMAP, POP3, SMTP in one place.

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4. How it Works in Nginx

Nginx listens for email client connections on ports like:

• 25, 465, 587 → SMTP

• 110, 995 → POP3

• 143, 993 → IMAP

When an email client connects:

1. Nginx authenticates the user (via a backend or script).

2. Based on authentication, Nginx selects the right mail server.

3. Nginx forwards traffic between the client and the mail server.

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5. Example Nginx Mail Proxy Config

mail {
    # Enable mail proxy for POP3, IMAP, SMTP
    server {
        listen 143;        # IMAP
        protocol imap;
        proxy_pass_error_message on;
        proxy on;
        starttls on;
        ssl_certificate /etc/ssl/cert.pem;
        ssl_certificate_key /etc/ssl/key.pem;
        auth_http 127.0.0.1:9000/auth;
    }

    server {
        listen 25;         # SMTP
        protocol smtp;
        proxy on;
        starttls on;
        ssl_certificate /etc/ssl/cert.pem;
        ssl_certificate_key /etc/ssl/key.pem;
        auth_http 127.0.0.1:9000/auth;
    }
}

Key parts:

• protocol imap/smtp/pop3 → Defines which protocol the block handles.

• starttls on; → Allows upgrading from plain to encrypted connection.

• ssl_certificate → Handles SSL encryption.

• auth_http → Calls an HTTP backend to authenticate users and tell Nginx which server to use.

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6. Where It’s Used in Real Life

• Large companies with multiple mail clusters behind one public address.

• ISPs offering email hosting for many domains.

• Mail services that hide backend changes (you can move mail servers without changing client settings).

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7. Advantages

• Centralized security & SSL

• Easier scaling

• Easier migration between mail servers

• Unified configuration for multiple domains

HTTP Cache

1. Simple Meaning

HTTP Cache means storing a copy of the server’s response so that the next time someone requests the same thing, it can be served faster without asking the backend again.

Think of it like:

• You go to a shop and ask for a Coke.

• The shopkeeper gets it from the warehouse (backend server) — takes 5 minutes.

• Next time, he already has Coke in the fridge (cache) — gives it in 5 seconds.

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2. Why Caching is Important

Without caching:

• Every request hits your backend.

• Backend works harder, even for the same repeated request.

• Slow responses under high traffic.

With caching:

• Common requests are served from stored copies.

• Backend is less busy.

• Responses are much faster.

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3. Types of Caching in HTTP

1. Browser Cache → Stored in the user’s browser.

2. Proxy Cache → Stored in a middle server like Nginx.

3. CDN Cache → Stored in geographically distributed servers.

We’re focusing on Proxy Cache in Nginx.

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4. How Nginx HTTP Cache Works

1. First request → Nginx checks if response is cached.

2. If not cached → Nginx asks backend → stores response in cache → sends to client.

3. If cached → Nginx sends stored copy directly to client.

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5. Nginx HTTP Cache Basic Setup

Let’s say you want to cache image files for 1 hour.

proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=my_cache:10m inactive=60m;

server {
    listen 80;
    server_name example.com;

    location /images/ {
        proxy_cache my_cache;
        proxy_cache_valid 200 1h;
        proxy_pass http://localhost:3000;
    }
}

Explanation:

• /var/cache/nginx → Folder where cached files are stored.

• keys_zone=my_cache:10m → Name + memory used for cache metadata.

• inactive=60m → If not used for 60 minutes, remove from cache.

• proxy_cache_valid 200 1h → Cache only successful responses (200 OK) for 1 hour.

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6. Example Use Case

Without Cache:

• 1000 users request the same /images/logo.png

• Backend gets 1000 hits.

With Cache:

• First request → Backend

• Next 999 requests → Served from Nginx cache (0 backend hits).

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7. Advanced Cache Controls

• Cache different URLs differently:

location /api/ {
    proxy_cache my_cache;
    proxy_cache_valid 200 10m;
}
location /images/ {
    proxy_cache my_cache;
    proxy_cache_valid 200 1h;
}

• Bypass cache for logged-in users:

location / {
    if ($http_cookie ~* "session_id") {
        proxy_no_cache 1;
        proxy_cache_bypass 1;
    }
}

• Clear cache manually (delete files in /var/cache/nginx).

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8. Benefits of HTTP Cache

• Faster load time

• Less load on backend

• Better scalability

• Lower bandwidth cost

Load Balancer

1. Simple Meaning

A Load Balancer is like a traffic police 🚦 for your servers.
It stands in front of multiple backend servers and splits incoming requests among them so that no single server gets overloaded.

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2. Real-Life Example

Imagine you have a restaurant chain with 3 kitchens 🍽️:

• Kitchen 1

• Kitchen 2

• Kitchen 3

If all customers go to Kitchen 1, it will be crowded and slow.
Instead, a receptionist (load balancer) sends:

• First customer → Kitchen 1

• Second customer → Kitchen 2

• Third customer → Kitchen 3

• Fourth customer → Kitchen 1 again (and so on…)

This way:

• All kitchens work equally

• Customers get food faster

• No single kitchen is overloaded

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3. Why Load Balancing is Needed

Without load balancing:

• One server can crash from too much traffic.

• Other servers remain idle.

• Users get slow responses or timeouts.

With load balancing:

• Traffic is distributed evenly.

• If one server fails, others take over.

• Faster response time.

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4. Nginx as a Load Balancer

Nginx can sit in front of multiple servers and send requests to them based on different strategies.

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4.1 Basic Load Balancing Example

You have 3 Node.js servers:

• server1.example.com

• server2.example.com

• server3.example.com

Nginx config:

# Define backend servers
upstream backend_servers {
    server server1.example.com;
    server server2.example.com;
    server server3.example.com;
}

# Use them in a reverse proxy
server {
    listen 80;
    server_name example.com;

    location / {
        proxy_pass http://backend_servers;
    }
}

Here, Nginx sends requests in Round Robin style:
1st request → server1
2nd request → server2
3rd request → server3
4th request → server1 again…

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5. Load Balancing Methods in Nginx

Nginx supports multiple strategies:

1. Round Robin (Default) → Sends requests one by one to each server.

2. Least Connections → Sends request to the server with the fewest active connections.

   upstream backend_servers {
       least_conn;
       server server1.example.com;
       server server2.example.com;
   }
   

3. IP Hash → Same client always goes to the same server.

   upstream backend_servers {
       ip_hash;
       server server1.example.com;
       server server2.example.com;
   }
   

4. Weighted Round Robin → Give some servers more traffic if they are more powerful.

   upstream backend_servers {
       server server1.example.com weight=3;
       server server2.example.com weight=1;
   }
   

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6. Extra Benefits of Load Balancing

• High Availability → If one server is down, traffic is sent to others.

• Scalability → Add more servers as traffic grows.

• Failover → Automatic backup servers.

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7. Example: API with Load Balancer

You have:

• API Server 1 → localhost:3001

• API Server 2 → localhost:3002

Nginx Config:

upstream api_servers {
    server localhost:3001;
    server localhost:3002;
}

server {
    listen 80;
    server_name api.example.com;

    location / {
        proxy_pass http://api_servers;
    }
}

✅ Traffic gets distributed between the two API servers.

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8. Simple Flow

Client → Nginx Load Balancer → Multiple Backend Servers → Response

Reverse Proxy

1. Simple Meaning

A Reverse Proxy is like a middleman between the internet user (client) and your actual application/server.

When someone visits your website:

1. Their request first goes to Nginx (the middleman).

2. Nginx forwards it to the correct backend server (Node.js, PHP, etc.).

3. The backend sends the response to Nginx.

4. Nginx sends it back to the user.

The user never directly talks to your backend server — only to Nginx.

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2. Real-Life Example

Imagine:

• You own a restaurant 🍽️

• There’s a reception counter at the entrance

• Customers never directly enter the kitchen

• They tell the receptionist (reverse proxy) their order

• The receptionist tells the chef (backend server)

• The chef prepares the food and gives it to the receptionist

• The receptionist gives the food to the customer

Here:

• Customer = User’s browser

• Receptionist = Nginx Reverse Proxy

• Chef = Your backend application

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3. Why Use a Reverse Proxy?

• Hide your kitchen → Nobody sees your actual backend server or its IP address.

• Handle multiple chefs → If you have many backend servers, the receptionist can send customers to the less busy one.

• Extra services → Receptionist can give menus (static files) directly without disturbing the chef.

• Security → Receptionist checks if the customer is allowed before passing the order.

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4. Beginner-Friendly Example with Nginx

You have:

• Frontend HTML → /var/www/html

• Backend Node.js → running on port 3000

You want:

• example.com → show frontend

• example.com/api → talk to backend

Nginx config:

server {
    listen 80;
    server_name example.com;

    # Serve website files directly
    location / {
        root /var/www/html;
        index index.html;
    }

    # Reverse proxy for backend
    location /api {
        proxy_pass http://localhost:3000;
    }
}

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5. How it Works (Step-by-Step)

1. User types example.com/api/users in browser.

2. Request goes to Nginx.

3. Nginx sees /api and forwards it to http://localhost:3000.

4. Backend sends JSON data to Nginx.

5. Nginx sends data to user.

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6. Extra Things Nginx Can Do in Reverse Proxy

• HTTPS (secure connection) → So backend only needs HTTP.

• Caching → Store common responses to reply faster.

• Load balancing → Distribute requests between multiple backend servers.

What is Nginx

1. What is Nginx?

Nginx (pronounced “Engine-X”) is a high-performance web server that can also act as:

• Reverse Proxy

• Load Balancer

• HTTP Cache

• Mail Proxy (less common now)

It’s popular because it’s:

• Fast (handles thousands of connections at once)

• Lightweight (low memory usage)

• Stable (used by companies like Netflix, Airbnb, GitHub)

• Flexible (web serving, proxying, streaming, etc.)

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2. Why Nginx Exists?

Imagine you have a website.
When someone types yourwebsite.com, their browser sends a request to your server.
The server’s job is to:

1. Receive the request

2. Find the right files/data

3. Send the response back

Traditional web servers like Apache can handle this, but under heavy traffic, they can slow down.
Nginx was built for speed and concurrency, meaning it can handle many requests at the same time without choking.

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3. Basic Nginx Architecture

Think of Nginx as:

• Front desk of a hotel 🏨

  • Guest (Browser) comes to the desk (Nginx)

  • Receptionist (Nginx) either gives information (static files) directly OR calls the right department (backend server) to handle it.

Flow:

Browser → Nginx → (optional) Backend server (Node.js, PHP, etc.) → Response → Browser

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4. Installing Nginx (Example)

Ubuntu/Debian

sudo apt update
sudo apt install nginx

Check if running:

systemctl status nginx

Access in browser:

http://your-server-ip

You should see the Nginx welcome page.

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5. Nginx Configuration Basics

Nginx configs are usually inside:

/etc/nginx/nginx.conf
/etc/nginx/sites-available/
/etc/nginx/sites-enabled/

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5.1 Basic Web Server Config

Example: Serving static HTML files

server {
    listen 80;
    server_name example.com;

    root /var/www/html;
    index index.html;

    location / {
        try_files $uri $uri/ =404;
    }
}

• listen 80; → Listen for HTTP requests on port 80

• server_name → Your domain

• root → Folder where your website files are

• location / → Defines how to handle requests

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5.2 Reverse Proxy Example

You have a Node.js app running on localhost:3000, but you want the world to access it via example.com.

server {
    listen 80;
    server_name example.com;

    location / {
        proxy_pass http://localhost:3000;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
    }
}

Now Nginx forwards requests to your Node.js app.

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6. Key Nginx Concepts

6.1 Static vs Dynamic Content

• Static = HTML, CSS, JS, images → Served directly by Nginx

• Dynamic = Generated by backend (Node.js, PHP) → Nginx passes the request to the backend

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6.2 Reverse Proxy

Nginx sits in front of your backend servers and:

• Hides their real IP

• Balances load

• Caches responses

• Adds security

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6.3 Load Balancing

If you have multiple backend servers, Nginx can distribute traffic.

upstream backend_servers {
    server backend1.example.com;
    server backend2.example.com;
}

server {
    listen 80;
    location / {
        proxy_pass http://backend_servers;
    }
}

Nginx sends requests round-robin (evenly) to each server.

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6.4 Caching

Nginx can store responses temporarily to speed up future requests.

location /images/ {
    proxy_cache my_cache;
    proxy_cache_valid 200 1h;
}

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6.5 Security

• Rate limiting (stop bots & abuse)

limit_req_zone $binary_remote_addr zone=one:10m rate=5r/s;

server {
    location /login {
        limit_req zone=one burst=10;
    }
}

• SSL/TLS (HTTPS)

server {
    listen 443 ssl;
    ssl_certificate /etc/ssl/cert.pem;
    ssl_certificate_key /etc/ssl/key.pem;
}

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7. Nginx Process Model (Why It’s Fast)

• Master process → Reads config, manages workers

• Worker processes → Handle requests

• Event-driven → Workers handle many connections at once without blocking

This is why Nginx can handle tens of thousands of concurrent connections.

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8. Real-World Example Setup

You have:

• React Frontend (static files in /var/www/react)

• Node.js API (running on localhost:5000)

Nginx Config:

server {
    listen 80;
    server_name example.com;

    location / {
        root /var/www/react;
        index index.html;
        try_files $uri /index.html;
    }

    location /api/ {
        proxy_pass http://localhost:5000/;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
    }
}

✅ Static files served directly
✅ API requests sent to backend

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9. Advanced Features

• Gzip compression → Smaller response size

gzip on;
gzip_types text/plain application/json;

• HTTP/2 support

listen 443 ssl http2;

• WebSocket support (real-time apps)

proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";

• Redirect HTTP → HTTPS

server {
    listen 80;
    return 301 https://$host$request_uri;
}

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10. Summary Table

Feature	Purpose
Web Server	Serve static files fast
Reverse Proxy	Forward requests to backend
Load Balancing	Distribute load across servers
Caching	Speed up repeated requests
SSL/TLS	Secure connections
Rate Limiting	Control abusive traffic

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If you want, I can now create a full real-world Nginx config file that:

• Serves React frontend

• Proxies API to Node.js

• Redirects HTTP to HTTPS

• Enables Gzip, caching, and rate limiting
  so you’ll have a production-ready setup.