function is like a mini-program within a program, designed to be reusable
this is used so that you don't have to repeat code in a repetitive task
<h2 id="heading-syntax">syntax</h2>
<pre><code class="lang-javascript">function greet(name) {
 console.log("Hello, " + name + "!");
}

greet("Alice"); // Output: Hello, Alice!
</code></pre>
<h2 id="heading-function-expression">function expression</h2>
with <code>name</code> as a parameter and <code>Bob</code> as an argument
<pre><code class="lang-javascript">const hello = function (name) {
 console.log("Hello, " + name + "!");
};

hello("Bob"); // Output: Hello, Bob!
</code></pre>
<h2 id="heading-function-with-return-value">function with return value</h2>
<pre><code class="lang-javascript">function multiply(num1, num2) { 
 return num1 * num2;
}

const result = multiply(5, 3); 
console.log(result); // Output: 15
</code></pre>
<h2 id="heading-arrow-function">arrow function</h2>
<pre><code class="lang-javascript">const multiply = (num1, num2) =&gt; {
 return num1 * num2;
}

const result = multiply(5, 5);

console.log(result); // Output: 25
</code></pre>
<h2 id="heading-arrow-function-one-line-code">arrow function one-line code</h2>
because it doesn't have a parameter we can leave <code>()</code> empty
because it's one line it's allowed to not use <code>{}</code> after the <code>=&gt;</code>
<pre><code class="lang-javascript">const world = () =&gt; console.log('hello world!');

world() // Output: hello world!
</code></pre>
<h2 id="heading-arrow-function-one-line-code-with-return">arrow function one-line code with return</h2>
because it has only 1 parameter it's allowed to not use <code>()</code>
because it's one-line it's using <code>()</code> as a <code>return</code>
<pre><code class="lang-javascript">const double = num =&gt; (num * 2)

const result = double(5)

console.log(result); // Output: 10
</code></pre>

function is like a mini-program within a program, designed to be reusable

this is used so that you don't have to repeat code in a repetitive task

## syntax

```javascript
function greet(name) {
  console.log("Hello, " + name + "!");
}

greet("Alice"); // Output: Hello, Alice!
```

## function expression

with `name` as a parameter and `Bob` as an argument

```javascript
const hello = function (name) {
  console.log("Hello, " + name + "!");
};

hello("Bob"); // Output: Hello, Bob!
```

## function with return value

```javascript
function multiply(num1, num2) { 
  return num1 * num2;
}

const result = multiply(5, 3); 
console.log(result); // Output: 15
```

## arrow function

```javascript
const multiply = (num1, num2) => {
  return num1 * num2;
}

const result = multiply(5, 5);

console.log(result); // Output: 25
```

## arrow function one-line code

because it doesn't have a parameter we can leave `()` empty

because it's one line it's allowed to not use `{}` after the `=>`

```javascript
const world = () => console.log('hello world!');

world() // Output: hello world!
```

## arrow function one-line code with return

because it has only 1 parameter it's allowed to not use `()`

because it's one-line it's using `()` as a `return`

```javascript
const double = num => (num * 2)

const result = double(5)

console.log(result); // Output: 10
```

JavaScript Function

syntax

function expression

function with return value

arrow function

arrow function one-line code

arrow function one-line code with return