Synchronous and Asynchronous in JavaScript (original) (raw)

Last Updated : 26 May, 2026

JavaScript is a single threaded programming language used for web development. It supports synchronous and asynchronous execution. In synchronous execution, statements run one after another, and each task waits for the previous one to finish. In asynchronous execution, some tasks run in the background without stopping the execution of other code.

Synchronous JavaScript

Synchronous programming executes code in sequence, where each statement waits for the previous one to complete before running. This makes the flow simple and predictable.

• Tasks run one after another in a fixed order.
• Time-consuming operations can block the execution of other code until they finish.

**Example:

javascript `

console.log("Hi"); console.log("Geek"); console.log("How are you?");

`

Output

Hi Geek How are you?

In the above code, “Hi” is logged first, then “Geek”, and finally “How are you”. Each statement executes only after the previous one completes.

**Asynchronous JavaScript

Asynchronous programming, on the other hand, allows multiple tasks to run independently of each other. In asynchronous code, a task can be initiated, and while waiting for it to complete, other tasks can proceed. This non-blocking nature helps improve performance and responsiveness, especially in web applications.

Working of **Asynchronous JavaScript

• **Call Stack: Functions are executed in the order they are called. Each function is added to the stack and completed before the next one runs.
• **Web APIs: Functions like setTimeout, HTTP requests, and event listeners are handled by browser Web APIs without blocking the call stack.
• **Callback Queue: After a Web API completes its task, the callback function is moved to the callback queue.
• **Event Loop: The event loop checks the call stack continuously. If it is empty, it moves functions from the callback queue to the stack for execution.

**Example:

javascript `

console.log("Hi");

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

console.log("End");

`

**Output

Hi
End
Geek

So, what the code does is first it logs in Hi then rather than executing the setTimeout function it logs in End and then it runs the setTimeout function.

At first, as usual, the Hi statement got logged in. As we use browsers to run JavaScript, there are the web APIs that handle these things for users. So, what JavaScript does is, it passes the setTimeout function in such web API and then we keep on running our code as usual. So it does not block the rest of the code from executing and after all the code its execution, it gets pushed to the call stack and then finally gets executed. This is what happens in asynchronous JavaScript.

Difference between Synchronous and Asynchronous JavaScript