Synchronous v/s Asynchronous Code

🔹 1. Synchronous Code

• Code runs line by line, blocking until a task finishes.
• If one operation takes time(eg. reading a large files, querying DB), the whole program waits.
• Eg:

    const fs = require('fs');
    console.log('Start')

    // Synchronous file read (blocks until file is read)
    const data = fs.readFileSync('file.txt', 'utf-8');
    console.log(data)

    console.log('End')

• Output:

    Start
    File content: <file contents>
    End

• 👉 Notice: End waits until the file is fully read.

🔹 2. Asynchronous Code

• Code runs non-blocking.
• Tasks that takes time(I/O, network, DB queries) are delegated, Node.js uses the event loop to continue running other code.
• When the task finishes, a callback, promises, async/await handles the result.
• Eg:

    const fs = require("fs");

    console.log("Start");

    // Asynchronous file read (non-blocking)
    fs.readFile("file.txt", "utf-8", (err, data) => {
        if (err) throw err;
        console.log("File content:", data);
    });

    console.log("End");

• Output:

    Start
    End
    File content: <file contents>

• 👉 Notice: End prints before file content, because reading is done asynchronously.

🔹 3. Comparison Table

Feature	Synchronous	Asynchronous
Execution	Blocking (one at a time)	Non-blocking (can do other work while waiting)
Performance	Slower for I/O-heavy apps	Faster & efficient for I/O
Use case	Scripts, small tasks	APIs, servers, DB queries
Example	fs.readFileSync	fs.readFile / async/await

🔹 4. Real-world Analogy

• Synchronous = At a restaurant, you order food and wait at the counter until it’s ready. Nobody else can order until you get your food.
• Asynchronous = You order food, get a token, and sit down. The restaurant continues serving other customers. When your food is ready, they call your token.

🔹 5. Best Practice in Node.js

• Node.js is single-threaded → long synchronous tasks block everything.
• Always prefer asynchronous code for I/O (file, DB, HTTP).
• Use synchronous code only in quick operations (config loading, small calculations).