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

Arrays basics

As we've seen before, objects in Javascript store arbitrary keyed values. Any string can be a key.

But quite often we find that we need an ordered collection, where we have a 1st, a 2nd, a 3rd element and so on. For example, we need that to store a list of something: users, goods, HTML elements etc.

It's difficult to use an object here, because it provides no methods to manage the order of elements. We can't easily access the n-th element. Also we can't insert a new property "before" the existing ones, and so on. It's just not meant for such use.

For this purpose, there is a special type of objects in JavaScript, named "an array".

[cut]

Declaration

There are two syntaxes for creating an empty array:

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

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

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

Array elements are numbered, starting with zero.

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

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

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

We can replace an element:

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

...Or add to the array:

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

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

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

alert( fruits.length ); // 3

We can also use alert to show the whole array.

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

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

An array can store elements of any type.

For instance:

// 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

Methods pop/push, shift/unshift

A queue 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.

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. 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:

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 a data structure is called deque.

The methods that work with the end of the array:

pop

Extracts the last element of the array and returns it:

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:

let fruits = ["Apple", "Orange"];

fruits.push("Pear");

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

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

The methods that work with the beginning of the array:

shift

Extracts the first element of the array and returns it:

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:

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

fruits.unshift('Apple');

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

Methods push and unshift can add multiple elements at once:

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. Numbers are used as keys. And there are special methods and optimizations to work with ordered collections of data, 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:

let fruits = ["Banana"]

let arr = fruits;

alert( arr === fruits ); // the same object
 
arr.push("Pear"); // modify it?

alert( fruits ); // Banana, Pear
alert( arr === fruits ); // still the same single 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.

But things 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 go like that:

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 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 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].
• Fill the array in 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. And there's the length property for that too, which auto-increases and decreases when we add/remove the data.

Arrays are specially suited and carefully tuned inside Javascript engines to work with 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.

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

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.

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:

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

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.

The for..of and other loops

To process all elements, we can use the regular for loop:

let arr = ["Apple", "Orange", "Pear"];

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

That's the most optimized and fastest way to loop over the array.

There's an alternative kind of loops: for..of.

The syntax:

for(let item of arr) {
  // item is an element of arr
}

Reminds of for..in, right? But a totally different beast.

The for..of loop works with iterable objects. An iterable is an object that has a special method named object[Symbol.iterator]. We don't need to go any deeper now, because this topic deserves a special chapter and it's going to get it.

For now it's enough to know that arrays and many other data structures in modern browsers are iterable. That is, they have proper built-in methods to work with for..of.

So we can loop over an array like this:

let arr = ["Apple", "Orange", "Pear"];

*!*
for (let fruit of arr) {
*/!*
  alert( fruit ); // Apple, Orange, Pear
}

Looks clean and nice, right?

````warn header="Don't use for..in for arrays" Technically, because arrays are objects, we could use for..in:

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.

   In the browser as well as in other environments, there are many collections of elements that look like arrays. That is, they have length and indexes properties, but they have other non-numeric properties too, which we usually don't need. The for..in loop will list them. If we need to work with arrays and those array-like structures, then these "extra" properties can become a problem.

2. The for (let i=0; i<arr.length; i++) loop in modern engines runs very fast, 10-100 times faster than for..in, because it is specially optimized for arrays.

So, as a generic recipe, please use:
  - `for(let item of arr)` -- as a nice-looking variant, probably most of time,
  - `for(let i=0; i<arr.length; i++)` -- as a fastest, old-browser-compatible version,
  - `for(let i in arr)` -- never.




## 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, 3);
alert( arr[0] ); // 2, created an array of [2, 3], all fine

*!*
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
```


## Object.keys(obj)

In the section about objects we talked about iterating over properties in a `for..in` loop.

The method [Object.keys(obj)](mdn:js/Object/keys) returns an array of properties in exactly the same order as `for..in`. 

It should be called exactly as given, not `obj.keys()`, but `Object.keys(obj)`.

For instance:

```js run
let phoneCodeByCountry = {
  44: "London",
  7: "Russia",
  1: "Usa"
};

let codes = Object.keys(phoneCodeByCountry);

alert( codes ); // 1, 7, 44
alert( typeof codes[0] ); // string
```


Note that object keys are always converted to strings, the last `typeof` highlights that.



## Summary

An 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)` -- looks nice,
  - `for(let i=0; i<arr.length; i++)` -- works fastest, old-browser-compatible.
  - `for(let i in arr)` -- never use.

To get an array of object properties:
  - `Object.keys(obj)`

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