JavaScript data types and data structures

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)

• Boolean type
• Null type
• Undefined type
• Number type
• BigInt type
• String type
• Symbol type

Objects (collections of properties)

Primitive values

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

Boolean type

This data type represents logical entities. Boolean represents one of two values: true or false.

const dataChecked = true;
const valueCounted = false;

Null type

In JavaScript, null is a special value that represents empty or unknown value.

const number = null;

Undefined type

The undefined data type represents value that is not assigned. If a variable is declared but the value is not assigned, then the value of that variable will be undefined.

let name;
console.log(name); // undefined

let name = undefined;
console.log(name); // undefined

Numeric types

Number represents integer and floating numbers (decimals and exponentials). A number type can also be +Infinity, -Infinity, and NaN (not a number).

const number1 = 3;
const number2 = 3.433;
const number3 = 3e5; // 3 * 10^5

const number4 = 3 / 0;
console.log(number4); // Infinity

const number5 = -3 / 0;
console.log(number5); // -Infinity

// strings can't be divided by numbers
const number6 = "abc" / 3;
console.log(number6); // NaN

NaN 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.

BigInt type

In JavaScript, Number type can only represent numbers less than (253 - 1) and more than -(253 - 1). However, if you need to use a larger number than that, you can use the BigInt data type.

A BigInt number is created by appending n to the end of an integer.

// BigInt value
const num1 = 900719925124740998n;
const num2 = 900719925124740998n;
const num3 = 10;


// Adding two big integers
const result1 = num1 + num2;
console.log(result1); // "1801439850249481996n"


// Error! BitInt and number cannot be added
const result2 = num1 + num2 + num3; 
console.log(result2);  // Uncaught TypeError: Cannot mix BigInt and other types

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.

JavaScript strings are immutable. This means that once a string is created, it is not possible to modify it.

String is used to store text. In JavaScript, strings are surrounded by quotes:

Single quotes: 'Hello'
Double quotes: "Hello"
Backticks: `Hello`

Example::

// Strings
const firstName = "Shivam";
const lastName = "Shukla";
const result = `Name: ${firstName} ${lastName}`;

console.log(result); // Name: Shivam Shukla

Symbol type

A value having the data type Symbol can be referred to as a symbol value. Symbol is an immutable primitive value that is unique.

// Two symbols with the same description

const value1 = Symbol('hello');
const value2 = Symbol('hello');

let result = (value1 === value2) ? true : false;  // false;

// Note: Though value1 and value2 both contain 'hello', they are different as they are of the Symbol type.

Objects

An object is a complex data type that allows us to store collections of data.

const employee = {
    firstName: 'Shivam',
    lastName: 'Shukla',
    email: 'shivam@gmail.com'
};

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 value or a Symbol value.

Data property

Data properties associate a key with a value. It can be described by the following attributes:

value

The value retrieved by a get access of the property. Can be any JavaScript value.

writable

A boolean value indicating if the property can be changed with an assignment.

enumerable

A boolean value indicating if the property can be enumerated by a for...in loop. See also Enumerability and ownership of properties for how enumerability interacts with other functions and syntaxes.

configurable

A boolean value indicating if the property can be deleted, can be changed to an accessor property, and can have its attributes changed.

"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.

Keyed collections: Maps, Sets, WeakMaps, WeakSets

These data structures 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.

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.

Determining types using the typeof operator

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