en.javascript.info/1-js/4-data-structures/7-array/article.md

# Arrays 

Arrays is the built-in subtype of objects, suited to store ordered collections.

In this chapter we'll study them in-detail and learn the methods to manipulate them.

[cut]

## Declaration

There are two syntaxes for creating an empty array:

```js
let arr = new Array();
let arr = [];
```

Almost all the time, the second syntax is used. We can supply the initial elements in the brackets:

```js
let fruits = ["Apple", "Orange", "Plum"];
```

Array elements are numbered, starting with zero.

We can get an element by its number in square brackets:

```js run
let fruits = ["Apple", "Orange", "Plum"];

alert( fruits[0] ); // Apple
alert( fruits[1] ); // Orange
alert( fruits[2] ); // Plum
```

We can replace an element:

```js
fruits[2] = 'Pear'; // now ["Apple", "Orange", "Pear"]
```

...Or add to the array:

```js
fruits[3] = 'Lemon'; // now ["Apple", "Orange", "Plum", "Lemon"]
```

The total count of the elements in the array is its `length`:

```js run
let fruits = ["Apple", "Orange", "Plum"];

alert( fruits.length ); // 3
```

We can also use `alert` to show the whole array.

```js run
let fruits = ["Apple", "Orange", "Plum"];

alert( fruits ); // Apple,Orange,Plum
```

An array can store elements of any type.

For instance:

```js run no-beautify
// mix of values
let arr = [ 'Apple', { name: 'John' }, true, function() { alert('hello'); } ];

// get the object at index 1 and then show its name
alert( arr[1].name ); // John

// get the function at index 3 and run it
arr[3](); // hello
```


````smart header="Trailing comma"
An array may end with a comma:
```js 
let fruits = [
  "Apple", 
  "Orange", 
  "Plum"*!*,*/!*
];
```

The "trailing comma" style makes it easier to insert/remove items, because all lines become alike.
````


## Methods pop/push, shift/unshift

A [queue](https://en.wikipedia.org/wiki/Queue_(abstract_data_type)) is one of most common uses of an array. In computer science, this means an ordered collection of elements which supports two operations:

- `push` appends an element to the end.
- `shift` get an element from the beginning, advancing the queue, so that the 2nd element becomes the 1st.

![](queue.png)

In practice we meet it very often. For example, a queue of messages that need to be shown on-screen.

There's another closely related data structure named [stack](https://en.wikipedia.org/wiki/Stack_(abstract_data_type)). 

It supports two operations:

- `push` adds an element to the end.
- `pop` takes an element to the end.

So new elements are added or taken always from the "end".

A stack is usually illustrated as a pack of cards: new cards are added to the top or taken from the top:

![](stack.png)

Stacks are useful when we need to postpone a thing. Like, if we are busy doing something and get a new task to do, we can put (push) it onto the stack, and do so with all new tasks while we are busy. Later when we're done, we can take (pop) the latest task from the stack, and repeat it every time when we get freed. So, no task gets forgotten and the latest task is always the top to execute.

Arrays in Javascript can work both as a queue and as a stack. They allow to add/remove elements both to/from the beginning or the end. 

In computer science such data structure is called [deque](https://en.wikipedia.org/wiki/Double-ended_queue).

**Methods that work with the end of the array:**

`pop`
: Extracts the last element of the array and returns it:

    ```js run
    let fruits = ["Apple", "Orange", "Pear"];

    alert( fruits.pop() ); // remove "Pear" and alert it

    alert( fruits ); // Apple, Orange
    ```

`push`
: Append the element to the end of the array:

    ```js run
    let fruits = ["Apple", "Orange"];

    fruits.push("Pear");

    alert( fruits ); // Apple, Orange, Pear
    ```

    The call `fruits.push(...)` is equal to `fruits[fruits.length] = ...`.

**Methods that work with the beginning of the array:**

`shift`
: Extracts the first element of the array and returns it:

    ```js
    let fruits = ["Apple", "Orange", "Pear"];

    alert( fruits.shift() ); // remove Apple and alert it

    alert( fruits ); // Orange, Pear
    ```

`unshift`
: Add the element to the beginning of the array:

    ```js
    let fruits = ["Orange", "Pear"];

    fruits.unshift('Apple');

    alert( fruits ); // Apple, Orange, Pear
    ```

Methods `push` and `unshift` can add multiple elements at once:

```js run
let fruits = ["Apple"];

fruits.push("Orange", "Peach");
fruits.unshift("Pineapple", "Lemon");

// ["Pineapple", "Lemon", "Apple", "Orange", "Peach"]
alert( fruits );
```

## Internals

An array is a special kind of object. The square brackets used to access a property `arr[0]` actually come from the object syntax. Numbers are used as keys. 

The "extras" are special methods to work with ordered collections of data and also the `length` property but at the core it's still an object.

Remember, there are only 7 basic types in JavaScript. Array is an object and thus behaves like an object. 

For instance, it is passed by reference, and modifications are visible from everywhere:

```js run
let fruits = ["Banana"]

let copy = fruits;

alert( copy === fruits ); // the same object
 
copy.push("Pear"); // add to copy?

alert( fruits ); // Banana, Pear - no, we modified fruits (the same object)
```

But what really makes arrays special is their internal representation. The engine tries to store it's elements in the contiguous memory area, one after another, just as painted on the illustrations in this chapter. There are other optimizations as well, to make it work really fast.

But they all break if we quit working with an array as with an "ordered collection" and start working with it as if it were a regular object.

For instance, technically we can do like that:

```js
let fruits = []; // make an array

fruits[99999] = 5; // assign a property with the index far greater than its length

fruits.age = 25; // create a property with an arbitrary name
```

That's possible, because arrays are objects at base. We can add any properties to them.

But the engine will see that we're working with the array as with a regular object. Array-specific optimizations are not suited for such cases and will be turned off, their benefits disappear.

The ways to misuse an array:

- Add a non-numeric property like `arr.test = 5`. 
- Make holes, like: add `arr[0]` and then `arr[1000]` (and nothing between them).
- Fill the array in the reverse order, like `arr[1000]`, `arr[999]` and so on.

Please think of arrays as about special structures to work with the *ordered data*. They provide special methods for that. Arrays are carefully tuned inside Javascript engines to work with contiguous ordered data, please use them this way. And if you need arbitrary keys, chances are high that you actually require a regular object `{}`.

## Performance

Methods `push/pop` run fast, while `shift/unshift` are slow.

![](array-speed.png)

Why is it faster to work with the end of an array than with its beginning? Let's see what happens during the execution:

```js
fruits.shift(); // take 1 element from the start
```

It's not enough to take and remove the element with the number `0`. Other elements need to be renumbered as well.

The `shift` operation must do 3 things:

1. Remove the element with the index `0`.
2. Move all elements to the left, renumber them from the index `1` to `0`, from `2` to `1` and so on.
3. Update the `length` property.

![](array-shift.png)

**The more elements in the array, the more time to move them, more in-memory operations.**

The similar thing happens with `unshift`: to add an element to the beginning of the array, we need first to move existing elements to the right, increasing their indexes.

And what's with `push/pop`? They do not need to move anything. To extract an element from the end, the `pop` method cleans the index and shortens `length`.

The actions for the `pop` operation:

```js
fruits.pop(); // take 1 element from the end
```

![](array-pop.png)

**The `pop` method does not need to move anything, because other elements keep their indexes. That's why it's blazingly fast.**

The similar thing with the `push` method.

## Loops

We've already seen two suitable variants of `for` loop:

```js run
let arr = ["Apple", "Orange", "Pear"];

*!*
for (let i = 0; i < arr.length; i++) {
*/!*
  alert( arr[i] );
}

*!*
for(let item of arr) {
*/!*
  alert( item );
}
```

Technically, because arrays are objects, it is also possible to use `for..in`:

```js run
let arr = ["Apple", "Orange", "Pear"];

*!*
for (let key in arr) {
*/!*
  alert( arr[key] ); // Apple, Orange, Pear
}
```

But that's actually a bad idea. There are potential problems with it:

1. The loop `for..in` iterates over *all properties*, not only the numeric ones.

    There are so-called "array-like" objects in the browser and in other environments, that *look like arrays*. That is, they have `length` and indexes properties, and can be used in `for..of`, but they have *other non-numeric properties and methods*, which we usually don't need in the loop. The `for..in` will list them. If we need to work with array-like objects, then these "extra" properties can become a problem.

2. The `for..in` loop is optimized for generic objects, not arrays, and thus is 10-100 times slower.

So we should never use `for..in` for arrays.

## A word about "length"

The `length` property automatically updates when we modify the array. It is actually not the *count* of values in the array, but the greatest numeric index plus one.

For instance, a single element with a large index gives a big length:

```js run
let fruits = [];
fruits[123] = "Apple";

alert( fruits.length ); // 124
```

Note that we usually don't use arrays like that. Want to stick a value in the middle of nowhere? Use an object, unless really know what you're doing.

Another interesting thing about the `length` property is that it's actually writable.

If we increase it manually, nothing interesting happens. But if we decrease it, the array is truncated. The process is irreversable, here's the example:

```js run
let arr = [1, 2, 3, 4, 5];

arr.length = 2; // truncate to 2 elements
alert( arr ); // [1, 2]

arr.length = 5; // return length back
alert( arr[3] ); // undefined: the values do not return
```

So, the simplest way to clear the array is: `arr.length=0`.


## new Array() [#new-array]

There is one more syntax to create an array:

```js
let arr = *!*new Array*/!*("Apple", "Pear", "etc");
```

It's rarely used, because square brackets `[]` are shorter. Also there's a tricky feature with it.

If `new Array` is called with a single argument which is a number, then it creates an array *without items, but with the given length*.

Let's see how one can shoot himself in the foot:

```js run
let arr = new Array(2); // will it create an array of [2] ?

alert( arr[0] ); // undefined! no elements.

alert( arr.length ); // length 2
```

In the code above, `new Array(number)` has all elements `undefined`.

To evade such surprises, we usually use square brackets, unless we really know what we're doing.

## Multidimentional arrays

Arrays can have items that are also arrays. We can use it for multidimentional arrays, to store matrices:

```js run
let matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
];

alert( matrix[1][1] ); // the central element
```


## Summary

Array is a special kind of objects, suited to store and manage ordered data items.

- The declaration:

    ```js
    // square brackets (usual)
    let arr = [item1, item2...];

    // new Array (exceptionally rare)
    let arr = new Array(item1, item2...);
    ```

    The call to `new Array(number)` creates an array with the given length, but without elements.

- The `length` property is the array length or, to be precise, its last numeric index plus one. It is auto-adjusted by array methods. 
- If we shorten `length` manually, the array is truncated.

We can use an array as a deque with the following operations:

- `push(...items)` adds `items` to the end.
- `pop()` removes the element from the end and returns it.
- `shift()` removes the element from the beginning and returns it.
- `unshift(...items)` adds items to the beginning.

To loop over the elements of the array:
  - `for(let item of arr)` -- the modern syntax,
  - `for(let i=0; i<arr.length; i++)` -- works fastest, old-browser-compatible.
  - `for(let i in arr)` -- never use.

That were the "extended basics". There are more methods. In the next chapter we'll study them in detail.