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An **error** in JavaScript is an object that represents a problem that occurred while the program was running. When JavaScript hits a situation it cannot handle (for example, dividing by zero, referencing a nonexistent variable, invalid syntax, and so on), it creates an error object and "throws" it. If the error is not "caught", code execution stops.

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### The basic idea

Errors exist to **control unforeseen situations** - so the program does not "crash" unexpectedly, but can handle the problem and keep running correctly.

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### Example of an error occurring

```javascript
console.log(a); // ReferenceError: a is not defined
```

Here the variable `a` is not declared, so the interpreter creates and throws a `ReferenceError`.

---

### Handling errors (try...catch)

So the program does not stop, errors can be **handled** with a `try...catch` construct:

```javascript
try {
  let result = JSON.parse("invalid JSON");
} catch (error) {
  console.log("An error occurred:", error.message);
}
```

In this example the program does not crash, it simply prints a message:

```javascript
An error occurred: Unexpected token i in JSON at position 0
```

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### Main error types

| Error type | When it occurs |
|---|---|
| **ReferenceError** | Referencing a nonexistent variable |
| **TypeError** | Wrong data type (for example, calling something that is not a function) |
| **SyntaxError** | An error in the code's syntax |
| **RangeError** | A value out of its allowed range |
| **EvalError** | Incorrect use of `eval()` |
| **URIError** | An error while working with URI-encoding functions (`decodeURI`, `encodeURI`) |

---

### Creating your own errors

You can create and throw an error yourself with `throw`:

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

try {
  divide(5, 0);
} catch (err) {
  console.log(err.message); // Cannot divide by zero!
}
```

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### Key methods and properties of the Error object

| Property | Description |
|---|---|
| `name` | The error type's name (`TypeError`, `ReferenceError`, etc.) |
| `message` | The error description |
| `stack` | The stack trace (shows where the error occurred) |

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