Synchronous vs Asynchronous (Simple Thumb Rule)

1. Synchronous Code (Blocking)

Thumb Rule:
Synchronous code waits until the current task finishes before moving to the next line.

Example:

const fs = require("fs");

let contents = fs.readFileSync("a.txt", "utf-8");
console.log(contents);

What happens here?

• readFileSync() reads the file.

• JavaScript waits until the file is completely read.

• Only after that, it executes console.log().

If reading takes 5 seconds, the program will pause for 5 seconds.

This is called blocking code.

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2. Asynchronous Code (Non-Blocking)

Thumb Rule:
Asynchronous code does not wait. It moves to the next line immediately and runs a callback later.

Example:

const fs = require("fs");

fs.readFile("a.txt", "utf-8", (err, contents) => {
  console.log(contents);
});

What happens here?

• readFile() starts reading the file.

• JavaScript does not wait.

• When the file reading finishes, the callback function runs.

This is called non-blocking code.

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3. Asynchronous using Promises

Instead of callbacks, we can use Promises.

Example:

function setTimeoutPromisified(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

function callback() {
  console.log("3 seconds have passed");
}

setTimeoutPromisified(3000).then(callback);

What happens here?

• setTimeoutPromisified returns a Promise.

• After 3 seconds, the promise resolves.

• .then(callback) runs the function.

✔ Cleaner than callbacks
✔ Avoids callback nesting
✔ Easier to chain