# JavaScript Functions

A **JavaScript function** is a block of "reusable" code designed to perform a particular task, it is executed when "something" invokes it (calls it).

A JavaScript function is defined with the `function` keyword, followed by the name and parentheses **()**. The parentheses may include *parameter* names separated by commas. The code to be executed, by the function, is placed inside curly brackets **{}**. Variables declared inside a function, become LOCAL to the function and can only be accessed from within the function.

> Accessing a function without "()" will return the function object instead of the function result.

Function *arguments* are the values received by the function when it is invoked. Inside the function, the arguments (the parameters) behave as local variables.

Functions often compute a `return` value which is "returned" back to the "caller". When JavaScript reaches a return statement, the function will stop executing.

Functions can be used the same way as variables, where the returned value is assigned to the variable.

Another way of defining a function expression is an *arrow function*. An **arrow function** expression has a shorter syntax compared to function expressions and does not have its own `this`.

JavaScript has several built-in functions:

| Built-in Functions | Description |
| --- | --- |
| eval() | evaluates JavaScript code represented as a string |
| isNaN() | determines whether a value is *NaN* or not |
| parseInt() | parses a string argument and returns an integer |
| parseFloat() | parses a string argument and returns a floating point number |