> For the complete documentation index, see [llms.txt](https://js201.gitbook.io/js-101/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://js201.gitbook.io/js-101/functions.md).

# Functions

Functions are one of the most powerful and essential notions in programming. Functions like mathematical functions perform transformations, they take input values called **arguments** and **return** an output value.  

Functions can be created in two ways: using `function declaration` or `function expression` . The *function name* can be omitted in f`unction expression` making it an `anonymous function`.  Functions, like variables, must be declared. Let's declare a function `double` that accepts an *argument* called `x` and **returns** the double of x :

```javascript
// an example of a function declaration
function double(x) {
  return 2 * x;
}
```

> *Note:* the function above **may** be referenced before it has been defined.

Functions are also values in JavaScript; they can be stored in variables (just like numbers, strings, etc ...) and given to other functions as arguments :

```javascript
// an example of a function expression
let double = function (x) {
  return 2 * x;
};
```

> *Note:* the function above **may not** be referenced before it is defined, just like any other variable.

{% hint style="info" %}
 A callback is a function passed as an argument to another function.
{% endhint %}

An arrow function is a compact alternative to traditional functions which has some semantic differences with some limitations. These function doesn't have their own bindings to `this`, `arguments` and `super`, and cannot be used as constructors. An example of an arrow function.

```javascript
const double =  (x) =>  2 * x;
```

{% hint style="warning" %}
The `this` keyword in the arrow function represents the object that defined the arrow function. 
{% endhint %}