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Learn how to store data in JavaScript with arrays.

Arrays

Each element in an array has a numbered position known as its _index_. We can access individual items using their index, which is similar to referencing an item in a list based on the item's position. 

Arrays in JavaScript are _zero-indexed_, meaning the positions start counting from `0` rather than `1`. Therefore, the first item in an array will be at position `0`. Let's see how we could access an element in an array: 

<img src="https://content.codecademy.com/courses/learn-javascript-arrays/array%20indices.svg" alt="Diagram outlining how to access the property of an array using the index of the element">

In the code snippet above: 
* `cities` is an array that has three elements.
* We're using bracket notation, `[]` with the index after the name of the array to access the element. 
* `cities[0]` will access the element at index `0` in the array `cities`. You can think of `cities[0]` as accessing the space in memory that holds the string `'New York'`.

You can also access individual characters in a string using bracket notation and the index. For instance, you can write:

```javascript
const hello = 'Hello World';
console.log(hello[6]);
// Output: W
```
The console will display `W` since it is the character that is at index `6`.


Accessing Elements

One way we can create an array is to use an _array literal_. An array literal creates an array by wrapping items in square brackets `[]`.  Remember from the previous exercise, arrays can store any data type &mdash; we can have an array that holds all the same data types or an array that holds different data types. 

<img src="https://content.codecademy.com/courses/learn-javascript-arrays/array%20literal.svg" alt="Diagram outlining an array literal that has 3 separate elements, a comma separates each element (a string, a number, and a boolean) and the elements are wrapped with square brackets">

Let's take a closer look at the syntax in the array example:
* The array is represented by the square brackets `[]` and the content inside.
* Each content item inside an array is called an _element_. 
* There are three different elements inside the array.
* Each element inside the array is a different data type. 

We can also save an array to a variable. You may have noticed we did this in the previous exercise:

```js
let newYearsResolutions = ['Keep a journal', 'Take a falconry class', 'Learn to juggle'];
```
Let's practice by making an array of our own.


Create an Array

Throughout the lesson we went over arrays being mutable, or changeable. Well what happens if we try to change an array inside a function? Does the array keep the change after the function call or is it scoped to inside the function?

Take a look at the following example where we call `.push()` on an array inside a function. Recall, the `.push()` method mutates, or changes, an array:
```js
const flowers = ['peony', 'daffodil', 'marigold'];

function addFlower(arr) {
  arr.push('lily');
}

addFlower(flowers);

console.log(flowers); // Output: ['peony', 'daffodil', 'marigold', 'lily']
```
Let's go over what happened in the example:
* The `flowers` array that has 3 elements. 
* The function `addFlower()` has a parameter of `arr` uses `.push()` to add a `'lily'` element into `arr`.
* We call `addFlower()` with an argument of `flowers` which will execute the code inside `addFlower`.
* We check the value of `flowers` and it now includes the `'lily'` element! The array was mutated! 

So when you pass an array into a function, if the array is mutated inside the function, that change will be maintained outside the function as well. You might also see this concept explained as _pass-by-reference_ since what we're actually passing to the function is a reference to where the variable memory is stored and changing the memory. 


Arrays and Functions

In the previous exercise, you learned how to access elements inside an array or a string by using an index.  Once you have access to an element in an array, you can update its value.

```js
let seasons = ['Winter', 'Spring', 'Summer', 'Fall'];

seasons[3] = 'Autumn';
console.log(seasons); 
//Output: ['Winter', 'Spring', 'Summer', 'Autumn']
```
In the example above, the `seasons` array contained the names of the four seasons.  

However, we decided that we preferred to say `'Autumn'` instead of `'Fall'`.  

The line, `seasons[3] = 'Autumn';` tells our program to change the item at index 3 of the `seasons` array to be `'Autumn'` instead of what is already there. 


Update Elements 

There are many more array methods than just `.push()` and `.pop()`. You can read about these array methods on the Docs entry for [JavaScript Arrays](https://www.codecademy.com/resources/docs/javascript/arrays). 

`.pop()` and `.push()` mutate the array on which they're called.  However, there are times that we don't want to mutate the original array and we can use non-mutating array methods.  Be sure to check the Docs to understand the behavior of the method you are using. 

Some arrays methods that are available to JavaScript developers include: `.join()`, `.slice()`, `.splice()`, `.shift()`, `.unshift()`, and `.concat()` amongst many others. Using these built-in methods makes it easier to do some common tasks when working with arrays. 

Below, we will explore some methods that we have not learned yet. We will use these methods to edit a grocery list. As you complete the steps, you can consult the [Codecademy Docs](https://www.codecademy.com/resources/docs/javascript) to learn what each method does! 


More Array Methods

Earlier we mentioned that arrays can store other arrays. When an array contains another array it is known as a _nested array_. Examine the example below: 

```js
const nestedArr = [[1], [2, 3]];
```
To access the nested arrays we can use bracket notation with the index value, just like we did to access any other element: 

```js
const nestedArr = [[1], [2, 3]];

console.log(nestedArr[1]); // Output: [2, 3]
```
Notice that `nestedArr[1]` will grab the element in index 1 which is the array `[2, 3]`. 
Then, if we wanted to access the elements within the nested array we can _chain_, or add on, more bracket notation with index values.

```js
const nestedArr = [[1], [2, 3]];

console.log(nestedArr[1]); // Output: [2, 3]
console.log(nestedArr[1][0]); // Output: 2
```
In the second `console.log()` statement, we have two bracket notations chained to `nestedArr`. We know that `nestedArr[1]` is the array `[2, 3]`. Then to grab the first element from that array, we use `nestedArr[1][0]` and we get the value of `2`.


Nested Arrays

One of an array's built-in properties is `length` and it returns the number of items in the array. We access the [`.length`](https://www.codecademy.com/resources/docs/javascript/strings/length?page_ref=catalog) property just like we do with strings. Check the example below:

```js
const newYearsResolutions = ['Keep a journal', 'Take a falconry class'];

console.log(newYearsResolutions.length);
// Output: 2
```
In the example above, we log `newYearsResolutions.length` to the console using the following steps: 
* We use _dot notation_, chaining a period with the property name to the array, to access the `length` property of the `newYearsResolutions` array. 
* Then we log the `length` of `newYearsResolution` to the console. 
* Since `newYearsResolution` has two elements, `2` would be logged to the console. 

When we want to know how many elements are in an array, we can access the `.length` property. 

The .length property

Another array method, [`.pop()`](https://www.codecademy.com/resources/docs/javascript/arrays/pop?page_ref=catalog), removes the last item of an array. 

```js
const newItemTracker = ['item 0', 'item 1', 'item 2'];

const removed = newItemTracker.pop();

console.log(newItemTracker); 
// Output: [ 'item 0', 'item 1' ]
console.log(removed);
// Output: item 2
```
* In the example above, calling `.pop()` on the `newItemTracker` array removed `item 2` from the end. 
* `.pop()` does not take any arguments, it simply removes the last element of `newItemTracker`. 
* `.pop()` returns the value of the last element. In the example, we store the returned value in a variable `removed` to be used for later. 
* `.pop()` is a method that mutates the initial array. 

When you need to mutate an array by removing the last element, use `.pop()`. 

The .pop() Method

You may recall that you can declare [variables](https://www.codecademy.com/resources/docs/javascript/variables?page_ref=catalog) with both the `let` and `const` keywords.  Variables declared with `let` can be reassigned.

Variables declared with the `const` keyword cannot be reassigned. However, elements in an array declared with `const` remain mutable. Meaning that we can change the contents of a `const` array, but cannot reassign a new array or a different value.  

The instructions below will illustrate this concept more clearly. Pay close attention to the similarities and differences between the `condiments` array and the `utensils` array as you complete the steps. 


Arrays with let and const

Let's learn about some built-in JavaScript methods that make working with arrays easier. These methods are specifically called on arrays to make common tasks, like adding and removing elements, more straightforward. 

One method, [`.push()`,](https://www.codecademy.com/resources/docs/javascript/arrays/push?page_ref=catalog) allows us to add items to the end of an array. Here is an example of how this is used:

```js
const itemTracker = ['item 0', 'item 1', 'item 2'];

itemTracker.push('item 3', 'item 4');

console.log(itemTracker); 
// Output: ['item 0', 'item 1', 'item 2', 'item 3', 'item 4'];
```
So, how does `.push()` work? 

* We access the `push` method by using dot notation, connecting `push` to `itemTracker` with a period. 
* Then we call it like a function. That's because `.push()` is a function and one that JavaScript allows us to use right on an array. 
* `.push()` can take a single argument or multiple arguments separated by commas. In this case, we're adding two elements: `'item 3'` and `'item 4'` to `itemTracker`.
* Notice that `.push()` changes, or _mutates_, `itemTracker`. You might also see `.push()` referred to as a _destructive_ array method since it changes the initial array.

If you're looking for a method that will mutate an array by adding elements to it, then `.push()` is the method for you! 

The .push() Method

Organizing and storing data is a foundational concept of programming. 

One way we organize data in real life is by making lists. Let's make one here:

```pseudo
New Year's Resolutions:

1. Keep a journal 
2. Take a falconry class
3. Learn to juggle
```
Let's now write this list in JavaScript, as an _array_:

```js
let newYearsResolutions = ['Keep a journal', 'Take a falconry class', 'Learn to juggle'];
```

[Arrays](https://www.codecademy.com/resources/docs/javascript/arrays?page_ref=catalog) are JavaScript's way of making lists. Arrays can store any data types (including strings, numbers, and booleans). Like lists, arrays are ordered, meaning each item has a numbered position. 

Here's an array of the concepts we'll cover: 
```js
let concepts = ['creating arrays', 'array structures', 'array manipulation'];
```

By the end of this lesson you'll have another tool under your belt that helps you manage chunks of data! 


Nice work! In this lesson, we learned these concepts regarding arrays:

* Arrays are lists that store data in JavaScript. 
* Arrays are created with brackets `[]`.
* Each item inside of an array is at a numbered position, or index, starting at `0`. 
* We can access one item in an array using its index, with syntax like: `myArray[0]`.
* We can also change an item in an array using its index, with syntax like `myArray[0] = 'new string'`; 
* Arrays have a `length` property, which allows you to see how many items are in an array.
* Arrays have their own methods, including `.push()` and `.pop()`, which add and remove items from an array, respectively.
* Arrays have many methods that perform different tasks, such as `.slice()` and `.shift()`, you can find documentation at the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array" target="_blank" rel="noopener noreferrer">Mozilla Developer Network</a> website. 
* Some built-in methods are mutating, meaning the method will change the array, while others are not mutating. You can always check the documentation.  
* Variables that contain arrays can be declared with `let` or `const`. Even when declared with `const`, arrays are still mutable. However, a variable declared with `const` cannot be reassigned. 
* Arrays mutated inside of a function will keep that change even outside the function. 
* Arrays can be nested inside other arrays. 
* To access elements in nested arrays chain indices using bracket notation. 

Learning how to work with and manipulate arrays will help you work with chunks of data! 
