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## Definition

An **exception** is an **event** that occurs **during program execution** when an error happens, and **normal code execution is interrupted**.

In other words, an **exception is a "signal" about an error condition**, which the JavaScript engine "throws" and which can be "caught".

## The difference between error and exception

- **Error** is an **object** that contains information about what went wrong.
- **Exception** is the **moment** when that error object **was thrown** during execution.

> In other words:
> * *Error* is **data**,
> * *Exception* is the **event of throwing that data**.

## Example: how an exception occurs

```javascript
function divide(a, b) {
  if (b === 0) {
    throw new Error("Cannot divide by zero!");
  }
  return a / b;
}

try {
  divide(5, 0); // An exception occurs here
} catch (err) {
  console.log("Caught exception:", err.message);
}
```

What happens:

1. The `divide` function runs.
2. The condition `b === 0` is true, and the engine **"throws" the error object**.
3. This event is the **exception**.
4. JavaScript starts looking for the nearest `try...catch`.
5. The `catch` block **catches** the exception, and the program keeps running.

## The exception mechanism (try / throw / catch / finally)

| Keyword | What it does |
|---|---|
| `try` | The block where an error might occur |
| `throw` | Throws an exception |
| `catch` | Catches the exception and handles it |
| `finally` | Always runs - even if there was an error |

Example:

```javascript
try {
  throw new Error("Network error");
} catch (error) {
  console.log("Caught exception:", error.message);
} finally {
  console.log("This code always runs");
}
```

## There are two kinds of exceptions

1. **Automatic (unhandled)**
Occur when the engine itself throws an error:

```javascript
  console.log(a); // ReferenceError → exception thrown automatically
  ```
2. **User-defined (explicit)**
When the programmer throws an exception themselves:

```javascript
  throw new Error("Something went wrong!");
  ```

## What happens if an exception isn't caught

If an exception is **not caught** (no `catch`), it "bubbles up" the call stack.
If, in the end, nobody catches it, the program **terminates with an error**:

```javascript
function a() { b(); }
function b() { throw new Error("Error!"); }

a(); // The error terminates execution
```

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