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JavaScript

Programming languages all have built-in data structures, but these often differ from one language to another. This article attempts to list the built-in data structures available in JavaScript and what properties they have. These can be used to build other data structures. Wherever possible, comparisons with other languages are drawn.

Dynamic typing

JavaScript is a loosely typed and dynamic language. Variables in JavaScript are not directly associated with any particular value type, and any variable can be assigned (and re-assigned) values of all types:

let foo = 42;    // foo is now a number
foo     = 'bar'; // foo is now a string
foo     = true;  // foo is now a boolean

JavaScript types

The set of types in the JavaScript language consists of primitive values and objects.

Primitive values (immutable datum represented directly at the lowest level of the language)
Objects (collections of properties)

Primitive values

All types except objects define immutable values (that is, values which can't be changed). For example (and unlike in C), Strings are immutable. We refer to values of these types as "primitive values".

Boolean type

Boolean represents a logical entity and can have two values: true and false. See Boolean and {{jsxref("Boolean")}} for more details.

Null type

The Null type has exactly one value: null. See {{jsxref("null")}} and Null for more details.

Undefined type

A variable that has not been assigned a value has the value undefined. See {{jsxref("undefined")}} and Undefined for more details.

Numeric types

ECMAScript has two built-in numeric types: Number and BigInt — along with the related value NaN.

Number type

The Number type is a double-precision 64-bit binary format IEEE 754 value. It is capable of storing floating-point numbers between 2^-1074 and 2^1024, but can only safely store integers in the range -(2^53 − 1) to 2^53 − 1. Values outside of the range from {{jsxref("Number.MIN_VALUE")}} to {{jsxref("Number.MAX_VALUE")}} are automatically converted to either +Infinity or -Infinity, which behave similarly to mathematical infinity, but with some slight differences; see {{jsxref("Number.POSITIVE_INFINITY")}} for details.

Note: You can check if a number is in the double-precision floating-point number range using {{jsxref("Number.isSafeInteger()")}} Outside the range from {{jsxref("Number.MIN_SAFE_INTEGER")}} to {{jsxref("Number.MAX_SAFE_INTEGER")}}, JavaScript can no longer safely represent integers; they will instead be represented by a double-precision floating point approximation.

The number type has only one integer with multiple representations: 0 is represented as both -0 and +0 (where 0 is an alias for +0). In practice, there is almost no difference between the different representations; 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 {{jsxref("Operators", "binary (bitwise) operators")}}.

Note: Although bitwise operators can be used to represent several Boolean values within a single number using bit masking, this is usually considered a bad practice. 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 limitations of local storage, or in extreme cases (such as 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.

BigInt type

The BigInt type is a numeric primitive in JavaScript that can represent integers with arbitrary precision. With BigInts, you can safely store and operate on large integers even beyond the safe integer limit for Numbers.

A BigInt is created by appending n to the end of an integer or by calling the constructor.

You can obtain the largest safe value that can be incremented with Numbers by using the constant {{jsxref("Number.MAX_SAFE_INTEGER")}}. With the introduction of BigInts, you can operate with numbers beyond the {{jsxref("Number.MAX_SAFE_INTEGER")}}.

This example demonstrates, where incrementing the {{jsxref("Number.MAX_SAFE_INTEGER")}} returns the expected result:

// BigInt
> const x = BigInt(Number.MAX_SAFE_INTEGER);
9007199254740991n
> x + 1n === x + 2n; // 9007199254740992n === 9007199254740993n
false

// Number
> Number.MAX_SAFE_INTEGER + 1 === Number.MAX_SAFE_INTEGER + 2; // 9007199254740992 === 9007199254740992
true

You can use the operators +, *, -, **, and % with BigInts—just like with Numbers. A BigInt is not strictly equal to a Number, but it is loosely so.

A BigInt behaves like a Number in cases where it is converted to boolean: if, ||, &&, Boolean, !.

BigInts cannot be operated on interchangeably with Numbers. Instead a {{jsxref("TypeError")}} will be thrown.

NaN

{{jsxref("NaN")}} ("Not a Number") is typically encountered when the result of an arithmetic operation cannot be expressed as a number. It is also the only value in JavaScript that is not equal to itself.

String type

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 some programming languages (such as 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 {{jsxref("String.substr()")}}.
A concatenation of two strings using the concatenation operator (+) or {{jsxref("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.

Symbol type

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 {{jsxref("Symbol")}} object wrapper in JavaScript.

Objects

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 {{Glossary("String", "String value")}} or a {{Glossary("Symbol", "Symbol value")}}.

There are two types of object properties: The data property and the accessor property.

Note: Each property has corresponding attributes. Attributes are used internally by the JavaScript engine, so you cannot directly access them. That's why attributes are listed in double square brackets, rather than single.

See {{jsxref("Object.defineProperty()")}} to learn more.

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]] cannot 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 cannot be deleted, cannot be changed to an accessor property, and attributes other than [[Value]] and [[Writable]] cannot 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.

Accessor property

Associates a key with one of two accessor functions (get and set) to retrieve or store a value.

Note: It's important to recognize it's accessor property — not accessor method. We can give a JavaScript object class-like accessors by using a function as a value — but that doesn't make the object a class.

An accessor property 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`

"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-keyed 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), and so on. 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 determine the equivalent C data types:

Type	Value Range	Size in bytes	Description	Web IDL type	Equivalent C type
{{jsxref("Int8Array")}}	`-128` to `127`	1	8-bit two's complement signed integer	`byte`	`int8_t`
{{jsxref("Uint8Array")}}	`0` to `255`	1	8-bit unsigned integer	`octet`	`uint8_t`
{{jsxref("Uint8ClampedArray")}}	`0` to `255`	1	8-bit unsigned integer (clamped)	`octet`	`uint8_t`
{{jsxref("Int16Array")}}	`-32768` to `32767`	2	16-bit two's complement signed integer	`short`	`int16_t`
{{jsxref("Uint16Array")}}	`0` to `65535`	2	16-bit unsigned integer	`unsigned short`	`uint16_t`
{{jsxref("Int32Array")}}	`-2147483648` to `2147483647`	4	32-bit two's complement signed integer	`long`	`int32_t`
{{jsxref("Uint32Array")}}	`0` to `4294967295`	4	32-bit unsigned integer	`unsigned long`	`uint32_t`
{{jsxref("Float32Array")}}	`1.2E-38` to `3.4E38`	4	32-bit IEEE floating point number (7 significant digits e.g., `1.1234567`)	`unrestricted float`	`float`
{{jsxref("Float64Array")}}	`5E-324` to `1.8E308`	8	64-bit IEEE floating point number (16 significant digits e.g., `1.123...15`)	`unrestricted double`	`double`
{{jsxref("BigInt64Array")}}	`-2^63` to `2^63 - 1`	8	64-bit two's complement signed integer	`bigint`	`int64_t (signed long long)`
{{jsxref("BigUint64Array")}}	`0` to `2^64 - 1`	8	64-bit unsigned integer	`bigint`	`uint64_t (unsigned long long)`

Keyed collections: Maps, Sets, WeakMaps, WeakSets

These data structures, introduced in ECMAScript Edition 6, take object references as keys. {{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.

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 {{jsxref("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.

Determining types using the `typeof` operator

The typeof operator can help you to find the type of your variable.

Please read the reference page for more details and edge cases.