JavaScript data types and data structures - JavaScript 编辑

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

Data and Structure types

The latest ECMAScript standard defines nine types:

• Six Data Types that are primitives, checked by typeof operator:
  • undefined : typeof instance === "undefined"
  • Boolean : typeof instance === "boolean"
  • Number : typeof instance === "number"
  • String : typeof instance === "string"
  • BigInt : typeof instance === "bigint"
  • Symbol : typeof instance === "symbol"
• Structural Types:
  • Object : typeof instance === "object". Special non-data but Structural type for any constructed object instance also used as data structures: new Object, new Array, new Map, new Set, new WeakMap, new WeakSet, new Date and almost everything made with new keyword;
  • Function : a non-data structure, though it also answers for typeof operator: typeof instance === "function". This is merely a special shorthand for Functions, though every Function constructor is derived from Object constructor.
• Structural Root Primitive:
  • null : typeof instance === "object". Special primitive type having additional usage for its value: if object is not inherited, then null is shown;

Keep in mind the only valuable purpose of typeof operator usage is checking the Data Type. If we wish to check any Structural Type derived from Object it is pointless to use typeof for that, as we will always receive "object". The indeed proper way to check what sort of Object we are using is instanceof keyword. But even in that case there might be misconceptions.

As we can see the meaning of every primitive type is obvious except of undefined and null which are almost the same. This happens as the concept of Time is strictly connected with the purpose of algorithms. We can purport something that does not yet exist or does not exist anymore: undefined. But when we wish to be able to represent something that exists being empty, we have to invent another keyword. And that is what null stands for: the beginning of structural meaning.

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 Boolean for more details.

Null type

The Null type has exactly one value: null. See null and Null for more details.

Undefined type

A variable that has not been assigned a value has the value undefined. See undefined and Undefined for more details.

Number type

ECMAScript has two built-in numeric types: Number and BigInt (see below).

The Number type is a double-precision 64-bit binary format IEEE 754 value (numbers between -(2⁵³ − 1) and 2⁵³ − 1). In addition to representing 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 Number.MAX_VALUE or Number.MIN_VALUE.

Starting with ECMAScript 2015, 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 with two representations: 0 is represented as both -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 binary (bitwise) operators.

Caution: 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 safest value that can be incremented with Numbers by using the constant Number.MAX_SAFE_INTEGER. With the introduction of BigInts, you can operate with numbers beyond the Number.MAX_SAFE_INTEGER.

This example demonstrates, where incrementing the Number.MAX_SAFE_INTEGER returns the expected result:

> const x = 2n ** 53n;
9007199254740992n
> const y = x + 1n;
9007199254740993n

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 TypeError will be thrown.

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 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.

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 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 String or a Symbol value.

There are two types of object properties which have certain attributes: 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 Object.defineProperty() to learn more.

Data property

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

Accessor property

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

"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), 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:

Keyed collections: Maps, Sets, WeakMaps, WeakSets

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

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.

Specifications

See also

• Nicholas Zakas collection of common data structure and common algorithms in JavaScript.
• Search Tre(i)es implemented in JavaScript