Programlama dilleri dil ile birlikte gelen sabit veri yapılarına sahiptir, fakat bu veri yapıları dilden dile değişiklik gösterir. Bu makale JavaScript'teki sabit veri yapılarını ve bu veri yapılarının özelliklerini listeler; böylece bu veri yapıları kullanılarak farklı veri yapıları yaratılabilir. Makale boyunca karşılaştırmanın mümkün olduğu bölümlerde, farklı dillerle kıyaslama yapılmıştır.
JavaScript esnek (ya da dinamik) veri tipine sahip bir dildir. JavaScript dilinde değişkenler belirli bir veri tipi ile direkt ve sabit olarak ilişkilendirilmez ve herhangi bir değişkene herhangi bir veri tipine sahip bir değer atanabilir, bu değer daha sonra farklı bir veri tipinin değeri ile değiştirilebilir.
var foo = 42; // foo değişkeni şuan bir sayıdır foo = 'bar'; // foo değişkeni şuan karakter dizisidir foo = true; // foo değişkeni şuan boolean veri türüdür
Son ECMAScript standartları 7 veri tipi tanımlar:
Nesne dışındaki bütün veri tipleri sonradan değiştirilemez değerler tanımlar. Örnek olarak C dilinin tersine karakter dizileri (String) değiştirilemez. Bu tipteki değerler ilkel değerler (primitive values) olarak tanımlanır.
Boolean veri tipi doğru ( true ) ve yanlış ( false ) olmak üzere iki mantıksal değerden birine sahip olan veri tiplerini temsil eder.
Boş olan değişkenlerin alabiileceği tek değer vardır: null. Daha fazla detay için buraya bakınız {{jsxref("null")}} and {{Glossary("Null")}}.
Herhangi bir değer atanmamış değişkenlerin değeri undefined (tanımlanmamış)dır. Daha fazla detay için buraya {{jsxref("undefined")}} ve buraya {{Glossary("Undefined")}} bakınız.
According to the ECMAScript standard, there is only one number type: the double-precision 64-bit binary format IEEE 754 value (numbers between -(253 -1) and 253 -1). There is no specific type for integers. In addition to being able to represent floating-point numbers, the number type has three symbolic values: +Infinity, -Infinity, and NaN (not-a-number).
To check for the largest available value or smallest available value within +/-Infinity, you can use the constants {{jsxref("Number.MAX_VALUE")}} or {{jsxref("Number.MIN_VALUE")}} and starting with ECMAScript 2015, you are also able to check if a number is in the double-precision floating-point number range using {{jsxref("Number.isSafeInteger()")}} as well as {{jsxref("Number.MAX_SAFE_INTEGER")}} and {{jsxref("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 the 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.
JavaScript's {{jsxref("Global_Objects/String", "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:
+) or {{jsxref("String.concat()")}}.It can be tempting to use strings to represent complex data. Doing this comes with short-term benefits:
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.
Symbols are new to JavaScript in ECMAScript 2015. 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 {{Glossary("Symbol")}} and the {{jsxref("Symbol")}} object wrapper in JavaScript.
In computer science, an object is a value in memory which is possibly referenced by an {{Glossary("Identifier", "identifier")}}.
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.
Associates a key with a value and has the following attributes:
| 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, can't be changed to an accessor property and attributes other than [[Value]] and [[Writable]] can't be changed. |
false |
| 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. |
Associates a key with one or two accessor functions (get and set) to retrieve or store a value and has the following attributes:
| 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.
A JavaScript object is a mapping between keys and values. Keys are strings (or {{jsxref("Symbol")}}s) and values can be anything. This makes objects a natural fit for hashmaps.
Functions are regular objects with the additional capability of being callable.
When representing dates, the best choice is to use the built-in Date utility in JavaScript.
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 2015 and present an array-like view of an underlying binary data buffer. The following table helps you to find the equivalent C data types:
{{page("/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray", "TypedArray_objects", "", 0, 3)}}
These data structures take object references as keys and are introduced in ECMAScript Edition 6. {{jsxref("Set")}} and {{jsxref("WeakSet")}} represent a set of objects, while {{jsxref("Map")}} and {{jsxref("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.
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 {{Glossary("JSON")}} and {{jsxref("JSON")}} for more details.
JavaScript has a standard library of built-in objects. Please have a look at the reference to find out about more objects.
typeof operatorThe typeof operator can help you to find the type of your variable. Please read the reference page for more details and edge cases.
| Specification | Status | Comment |
|---|---|---|
| {{SpecName('ES1')}} | {{Spec2('ES1')}} | Initial definition. |
| {{SpecName('ES5.1', '#sec-8', 'Types')}} | {{Spec2('ES5.1')}} | |
| {{SpecName('ES2015', '#sec-ecmascript-data-types-and-values', 'ECMAScript Data Types and Values')}} | {{Spec2('ES2015')}} | Added Symbol. |
| {{SpecName('ESDraft', '#sec-ecmascript-data-types-and-values', 'ECMAScript Data Types and Values')}} | {{Spec2('ESDraft')}} |