# Understanding Asynchronous JavaScript: Why Your Array Is Empty?

```javascript
const worker = (delay) =>
  new Promise((resolve) =>
    setTimeout(() => resolve(`Finished ${delay}`), delay)
  );

async function run() {
  let results = [];
  let items = [2, 10, 2];

  for (let i = 0; i < items.length; i++) {
    const data = await worker(items[i]);
    results.push(data);
  }

  console.log("Final results:", results);
}

run();
```

## The Confusion

*   Inside the callback → `results` contains data
    
*   Outside the callback → `results` is still `[]`
    

* * *

## The Real Reason: Asynchronous Execution

### The key lies in this line:

```plaintext
setTimeout(...)
```

`setTimeout` is **asynchronous**.

That means:

*   The `for` loop runs immediately.
    
*   The callback runs later.
    
*   JavaScript does NOT wait for it.
    

* * *

## How JavaScript Actually Works

JavaScript:

*   Is single-threaded
    
*   Uses the Event Loop
    
*   Sends `setTimeout` to Web APIs
    
*   Pushes callback to Callback Queue
    
*   Executes it when Call Stack is empty
    

So your loop finishes before any timeout completes.

* * *

## Final Takeaway

If you remember just one thing from this blog:

> The loop runs synchronously.  
> setTimeout runs asynchronously.  
> JavaScript does NOT wait unless you tell it to.