# Asynchronous Programming

Asynchronous programming allows a program to start a potentially long-running task (like fetching data from a server or reading a file) and continue executing other code while waiting for that task to finish, rather than blocking execution.

## 1. The Problem: Blocking vs Non-Blocking

*   **Synchronous (Blocking):** Code executes line-by-line. If one line takes 5 seconds (e.g., a network request), the entire program freezes for 5 seconds.
*   **Asynchronous (Non-Blocking):** The program initiates a task and moves on. When the task finishes, the program is notified.

## 2. Callbacks

The earliest way to handle async operations. You pass a function as an argument to another function, and that function is called ("called back") when the task is done.

```javascript
// JavaScript Example
function fetchData(callback) {
    setTimeout(() => {
        callback("Data received");
    }, 1000);
}

fetchData((data) => {
    console.log(data);
});
```

**Drawback:** "Callback Hell" - deeply nested callbacks make code hard to read and maintain.

## 3. Promises

A Promise is an object representing the eventual completion (or failure) of an asynchronous operation. It allows you to chain operations linearly.

*   **States:** Pending, Fulfilled (Resolved), Rejected.

```javascript
// JavaScript Example
fetch("https://api.example.com/data")
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(error => console.error(error));
```

## 4. Async / Await

Syntactic sugar built on top of Promises. It makes asynchronous code look and behave like synchronous code, making it much easier to read and debug.

*   **`async`**: Declares a function that returns a Promise.
*   **`await`**: Pauses the execution of the function until the Promise is resolved.

```javascript
// JavaScript Example
async function getData() {
    try {
        const response = await fetch("https://api.example.com/data");
        const data = await response.json();
        console.log(data);
    } catch (error) {
        console.error(error);
    }
}
```

[[programming/control-flow]]
[[programming/functions-and-scope]]