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Eric Hu

JavaScript Data Types

JavaScript data types

JavaScript can work with primitive types (number, string, boolean, null, undefined, symbol, bigint) and objects (arrays, functions, plain objects).

Type	Category	Description
`number`	Primitive	Numeric values, including integers and floats.
`string`	Primitive	Textual data wrapped in quotes, backticks, or double quotes.
`boolean`	Primitive	Logical values: `true` or `false`.
`null`	Primitive	Intentional absence of any value.
`undefined`	Primitive	Variable declared but not assigned a value.
`symbol`	Primitive	Unique, immutable identifier.
`bigint`	Primitive	Arbitrary-precision integers.
`object`	Structural	Collections of key-value pairs, arrays, functions, dates, etc.

number

Represents both integers and floating-point values.

const age = 30;
const price = 19.99;
const notANumber = NaN;
const infinity = Infinity;
 
typeof age; // "number"
typeof NaN; // "number"

string

Immutable textual data.

const name = "Ada";
const greeting = 'Hello'; // prettier-ignore
const template = `Hi, ${name}`;
 
typeof name; // "string"

boolean

Logical type with only two values.

const isActive = true;
const isAdmin = false;
 
typeof isActive; // "boolean"

null

Represents an absence of value.

const user = null;
 
typeof user; // "object" (historical quirk)
user === null; // true

typeof null gives "object". This is a long-standing JavaScript quirk.

Instead, use value === null to check for null.

undefined

A variable that has been declared but not assigned a value.

let score;
 
typeof score; // "undefined"
score === undefined; // true

symbol

Unique and immutable value, often used as object keys.

const id = Symbol("id");
const anotherId = Symbol("id");
 
id === anotherId; // false

bigint

For integers larger than Number.MAX_SAFE_INTEGER.

const huge = 9007199254740993n;
const alsoHuge = BigInt(9007199254740993);
 
typeof huge; // "bigint"

object

Collections of properties. Includes plain objects, arrays, functions, dates, and more.

const person = { name: "Ada", age: 30 };
const numbers = [1, 2, 3];
const greet = () => "hello";
 
typeof person; // "object"
typeof numbers; // "object"
typeof greet; // "function"
Array.isArray(numbers); // true

JavaScript data types

JavaScript can work with primitive types (number, string, boolean, null, undefined, symbol, bigint) and objects (arrays, functions, plain objects).

Type

Category

Description

number

Primitive

Numeric values, including integers and floats.

string

Primitive

Textual data wrapped in quotes, backticks, or double quotes.

boolean

Primitive

Logical values: true or false.

null

Primitive

Intentional absence of any value.

undefined

Primitive

Variable declared but not assigned a value.

symbol

Primitive

Unique, immutable identifier.

bigint

Primitive

Arbitrary-precision integers.

object

Structural

Collections of key-value pairs, arrays, functions, dates, etc.

object

Collections of properties. Includes plain objects, arrays, functions, dates, and more.

const person = { name: "Ada", age: 30 };
const numbers = [1, 2, 3];
const greet = () => "hello";
 
typeof person; // "object"
typeof numbers; // "object"
typeof greet; // "function"
Array.isArray(numbers); // true

JavaScript Data Types

JavaScript data types

number

string

boolean

null

undefined

symbol

bigint

object

Full-Stack AI Developer Roadmap

JavaScript Data Types

JavaScript data types

number

string

boolean

null

undefined

symbol

bigint

object

Full-Stack AI Developer Roadmap