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The `void` operator in JavaScript is a fairly rare but important tool.
It is used to **evaluate an expression without returning a value**,
meaning it **always returns** `undefined`, regardless of what the expression does.

---

## Syntax

```javascript
void expression
```

```javascript
void(expression)
```

---

## Main idea

> The `void` operator **evaluates the expression**,
> but **always returns** `undefined`.

---

### Example

```javascript
void 123;       // undefined
void 'hello';   // undefined
void (5 + 5);   // undefined
```

> It doesn't matter what the expression on the right evaluates to,
> the result of `void` is **always** `undefined`.

---

## Example with a function

```javascript
function sayHi() {
  console.log('Hello!');
  return 'Hi';
}

console.log(sayHi());   // "Hi"
console.log(void sayHi()); // runs, but returns undefined
```

Here, `sayHi()` is called,
but the result (`'Hi'`) is **ignored**, and `undefined` is returned.

---

## Where it's used in practice

### **To prevent navigation via an** `<a>` **link**

Sometimes you need a link click to do nothing:

```javascript
<a href="javascript:void(0)">Click me</a>
```

> Here:
>
> - `void(0)` is executed → `undefined`
> - the browser doesn't navigate anywhere, because `href` doesn't provide a URL.

This is a **legacy practice**;
in modern code it's better to use `href="#"` with `event.preventDefault()`.

---

### **For IIFEs (immediately invoked function expressions)**

`void` used to be used to **guarantee correct execution of an IIFE**:

```javascript
void function() {
  console.log('It worked!');
}();
```

> Without `void`, the parser could confuse the expression with a function declaration.
> This need is rare nowadays,
> but historically `void` was used as a "safety net".

---

### **In arrow functions, to "return nothing"**

```javascript
const log = msg => void console.log(msg);

log('Hello'); // logs to the console, but returns undefined
```

> Here, `void` is used to **perform an action without returning a value**.

---

### **In bookmarklets (mini-scripts in bookmarks)**

An example of a "bookmarklet":

```javascript
javascript:void(alert('Hello'))
```

> Without `void`, the browser would try to navigate to a "page" with the expression's result (e.g., `undefined`),
> and `void` prevents that.

---

## Example: comparison with a regular call

```javascript
function calc() {
  return 42;
}

console.log(calc());      // 42
console.log(void calc()); // undefined
```

> The function runs, but `void` forces the expression to return `undefined`.

---

## Summary

| What it does | Evaluates an expression and returns `undefined` |
|---|---|
| Returns | `undefined` |
| Used for | Ignoring the expression's result |
| Common scenarios | `javascript:void(0)` links, IIFEs, arrow functions without a return |

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