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Усі мови програмування мають вбудовані структури даних, які, втім, у різних мовах дещо відрізняються. У цій статті ми розглянемо наявні у JavaScript структури даних та їх властивості, а також створення на їх ґрунті інших структур. За можливості наведемо порівняння з іншими мовами.

Динамічна типізація

JavaScript є мовою з нестрогою типізацією або ж динамічною мовою. Оголошення змінної в JavaScript не визначає типу даних, а отже всяка змінна з отриманням нового значення отримує і новий тип. Звідси випливає, що тип з'ясовується перед виконанням певної операції, позаяк від нього залежить і результат:

// Одразу по оголошенню змінна foo разом з початковим значенням отримує тип Number
var foo = 42;

console.log(foo + 1);  // виводить 43

// Змінна отримує нове значення й тепер має тип String
foo = 'bar';

console.log(foo + 1);  // виводить "bar1"

// Разом із значенням true тип змінної foo обертається на Boolean
foo = true;

Типи даних

Згідно з останнім стандартом ECMAScript у мові є сім типів даних:

Прості величини

Усі типи даних, окрім об'єкта, передають незмінювані величини (можна встановити нове значення змінної, але неможливо змінити того значення, яке вона вже має). На відміну від, наприклад, мови C, рядки є незмінюваними. Такі величини називають простими або «примітивними».

Тип boolean

Тип Boolean передає два логічних значення: true та false.

Тип null

Тип Null, власне, передає лише одне-єдине значення: null. Див. також null та Null.

Тип undefined

Змінна, допоки не отримає нового (початкового), має значення undefined. Див. також undefined та Undefined.

Тип number

Відповідно до стандарту ECMAScript, існує лише один тип чисел: 64-розрядне число подвійної точності з рухомою комою в форматі IEEE 754 (приймає значення від -(253 -1) до 253 -1). Для цілих чисел немає окремого типу даних. Окрім чисел з рухомою комою цей формат підтримує три особливих значення: +Infinity, -Infinity та NaN (так зване «не число»).

To check for the largest available value or smallest available value within +/-Infinity, you can use the constants Number.MAX_VALUE or Number.MIN_VALUE and starting with ECMAScript 6, you are also able to check if a number is in the double-precision floating-point number range using Number.isSafeInteger() as well as Number.MAX_SAFE_INTEGER and Number.MIN_SAFE_INTEGER. Beyond this range, integers in JavaScript are not safe anymore and will be a double-precision floating point approximation of the value.

The number type has only one integer that has two representations: 0 is represented as -0 and +0. ("0" is an alias for +0). In the praxis, this has almost no impact. For example +0 === -0 is true. However, you are able to notice this when you divide by zero:

> 42 / +0
Infinity
> 42 / -0
-Infinity

Although a number often represents only its value, JavaScript provides some binary operators. These can be used to represent several Boolean values within a single number using bit masking. However, this is usually considered a bad practice, since JavaScript offers other means to represent a set of Booleans (like an array of Booleans or an object with Boolean values assigned to named properties). Bit masking also tends to make code more difficult to read, understand, and maintain. It may be necessary to use such techniques in very constrained environments, like when trying to cope with the storage limitation of local storage or in extreme cases when each bit over the network counts. This technique should only be considered when it is the last measure that can be taken to optimize size.

Тип string

JavaScript's String type is used to represent textual data. It is a set of "elements" of 16-bit unsigned integer values. Each element in the String occupies a position in the String. The first element is at index 0, the next at index 1, and so on. The length of a String is the number of elements in it.

Unlike in languages like C, JavaScript strings are immutable. This means that once a string is created, it is not possible to modify it. However, it is still possible to create another string based on an operation on the original string. For example:

  • A substring of the original by picking individual letters or using String.substr().
  • A concatenation of two strings using the concatenation operator (+) or String.concat().

Beware of "stringly-typing" your code!

It can be tempting to use strings to represent complex data. Doing this comes with short-term benefits:

  • It is easy to build complex strings with concatenation.
  • Strings are easy to debug (what you see printed is always what is in the string).
  • Strings are the common denominator of a lot of APIs (input fields, local storage values, XMLHttpRequest responses when using responseText, etc.) and it can be tempting to only work with strings.

With conventions, it is possible to represent any data structure in a string. This does not make it a good idea. For instance, with a separator, one could emulate a list (while a JavaScript array would be more suitable). Unfortunately, when the separator is used in one of the "list" elements, then, the list is broken. An escape character can be chosen, etc. All of this requires conventions and creates an unnecessary maintenance burden.

Use strings for textual data. When representing complex data, parse strings and use the appropriate abstraction.

Тип symbol

Symbols are new to JavaScript in ECMAScript Edition 6. A Symbol is a unique and immutable primitive value and may be used as the key of an Object property (see below). In some programming languages, Symbols are called atoms. For more details see Symbol and the Symbol object wrapper in JavaScript.

Об'єкти

In computer science, an object is a value in memory which is possibly referenced by an identifier.

Properties

In JavaScript, objects can be seen as a collection of properties. With the object literal syntax, a limited set of properties are initialized; then properties can be added and removed. Property values can be values of any type, including other objects, which enables building complex data structures. Properties are identified using key values. A key value is either a String or a Symbol value.

There are two types of object properties which have certain attributes: The data property and the accessor property.

Data property

Associates a key with a value and has the following attributes:

Attributes of a data property
Attribute Type Description Default value
[[Value]] Any JavaScript type The value retrieved by a get access of the property. undefined
[[Writable]] Boolean If false, the property's [[Value]] can't be changed. false
[[Enumerable]] Boolean If true, the property will be enumerated in for...in loops. See also Enumerability and ownership of properties false
[[Configurable]] Boolean If false, the property can't be deleted and attributes other than [[Value]] and [[Writable]] can't be changed. false
Obsolete attributes (as of ECMAScript 3, renamed in ECMAScript 5)
Attribute Type Description
Read-only Boolean Reversed state of the ES5 [[Writable]] attribute.
DontEnum Boolean Reversed state of the ES5 [[Enumerable]] attribute.
DontDelete Boolean Reversed state of the ES5 [[Configurable]] attribute.

Accessor property

Associates a key with one or two accessor functions (get and set) to retrieve or store a value and has the following attributes:

Attributes of an accessor property
Attribute Type Description Default value
[[Get]] Function object or undefined The function is called with an empty argument list and retrieves the property value whenever a get access to the value is performed. See also get. undefined
[[Set]] Function object or undefined The function is called with an argument that contains the assigned value and is executed whenever a specified property is attempted to be changed. See also set. undefined
[[Enumerable]] Boolean If true, the property will be enumerated in for...in loops. false
[[Configurable]] Boolean If false, the property can't be deleted and can't be changed to a data property. false

Note: Attribute is usually used by JavaScript engine, so you can't directly access it(see more about Object.defineProperty()).That's why the attribute is put in double square brackets instead of single.

"Normal" objects, and functions

A JavaScript object is a mapping between keys and values. Keys are strings (or Symbols) and values can be anything. This makes objects a natural fit for hashmaps.

Functions are regular objects with the additional capability of being callable.

Dates

When representing dates, the best choice is to use the built-in Date utility in JavaScript.

Indexed collections: Arrays and typed Arrays

Arrays are regular objects for which there is a particular relationship between integer-key-ed properties and the 'length' property. Additionally, arrays inherit from Array.prototype which provides to them a handful of convenient methods to manipulate arrays. For example, indexOf (searching a value in the array) or push (adding an element to the array), etc. This makes Arrays a perfect candidate to represent lists or sets.

Typed Arrays are new to JavaScript with ECMAScript Edition 6 and present an array-like view of an underlying binary data buffer. The following table helps you to find the equivalent C data types:

TypedArray objects

Type Value Range Size in bytes Description Web IDL type Equivalent C type
Int8Array -128 to 127 1 8-bit two's complement signed integer byte int8_t
Uint8Array 0 to 255 1 8-bit unsigned integer octet uint8_t
Uint8ClampedArray 0 to 255 1 8-bit unsigned integer (clamped) octet uint8_t
Int16Array -32768 to 32767 2 16-bit two's complement signed integer short int16_t
Uint16Array 0 to 65535 2 16-bit unsigned integer unsigned short uint16_t
Int32Array -2147483648 to 2147483647 4 32-bit two's complement signed integer long int32_t
Uint32Array 0 to 4294967295 4 32-bit unsigned integer unsigned long uint32_t
Float32Array 1.2x10-38 to 3.4x1038 4 32-bit IEEE floating point number ( 7 significant digits e.g. 1.1234567) unrestricted float float
Float64Array 5.0x10-324 to 1.8x10308 8 64-bit IEEE floating point number (16 significant digits e.g. 1.123...15) unrestricted double double

Keyed collections: Maps, Sets, WeakMaps, WeakSets

These data structures take object references as keys and are introduced in ECMAScript Edition 6. Set and WeakSet represent a set of objects, while Map and WeakMap associate a value to an object. The difference between Maps and WeakMaps is that in the former, object keys can be enumerated over. This allows garbage collection optimizations in the latter case.

One could implement Maps and Sets in pure ECMAScript 5. However, since objects cannot be compared (in the sense of "less than" for instance), look-up performance would necessarily be linear. Native implementations of them (including WeakMaps) can have look-up performance that is approximately logarithmic to constant time.

Usually, to bind data to a DOM node, one could set properties directly on the object or use data-* attributes. This has the downside that the data is available to any script running in the same context. Maps and WeakMaps make it easy to privately bind data to an object.

Structured data: JSON

JSON (JavaScript Object Notation) is a lightweight data-interchange format, derived from JavaScript but used by many programming languages. JSON builds universal data structures. See JSON and JSON for more details.

More objects in the standard library

JavaScript has a standard library of built-in objects. Please have a look at the reference to find out about more objects.

З'ясування типу за допомогою оператора typeof

The typeof operator can help you to find the type of your variable. Please read the reference page for more details and edge cases.

Специфікації

Специфікація Статус Коментар
ECMAScript 1st Edition (ECMA-262) Standard Первинне визначення.
ECMAScript 5.1 (ECMA-262)
The definition of 'Types' in that specification.
Standard  
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'ECMAScript Data Types and Values' in that specification.
Standard  
ECMAScript Latest Draft (ECMA-262)
The definition of 'ECMAScript Data Types and Values' in that specification.
Living Standard  

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