Synchronous, Asynchronous, and Promise in Javascript

1. Synchronous JavaScript
Synchronous means code executes one line at a time, in order. The next line waits until the current line finishes.
Example:

console.log("First");
console.log("Second");
console.log("Third");

Output :

First
Second
Third

Explanation:

• First line executes.
• Then the second line executes.
• Finally, the third line executes. Each statement runs in sequence.

2. Asynchronous JavaScript
Asynchronous means some operations can start and finish later, allowing other code to run while waiting.

console.log("First");

setTimeout(() => {
    console.log("Second");
}, 2000);

console.log("Third");

Output :

First
Third
Second

Explanation:

• "First" prints immediately.
• setTimeout() schedules the callback to run after at least 2 seconds.
• "Third" prints without waiting for the timer.
• "Second" prints when the timer callback run

3. What is a Promise?
A Promise is a JavaScript object that represents the eventual success or failure of an asynchronous operation.

A Promise has three states:

• Pending: The operation has not finished.
• Fulfilled: The operation succeeded.
• Rejected: The operation failed.

Example:

let myPromise = new Promise((resolve, reject) => {
    resolve("Task completed!");
});

myPromise.then((result) => {
    console.log(result);
});

Output :

Task completed!

Explanation:

• new Promise() creates a Promise.
• resolve() indicates success.
• .then() handles the successful result.
• reject() is used to indicate failure.

Here's an example of a rejected Promise:

let myPromise = new Promise((resolve, reject) => {
    reject("Something went wrong");
});

myPromise.catch((error) => {
    console.log(error);
});

Output :

Something went wrong