02 - Values and Types

The kinds of values that can be represented and manipulated in a programming language are known as types, and one of the most fundamental characteristics of a programming language is the set of types it supports.

Values are chunks of information. Each value has a specific type that defines how it behaves and interacts with other values.

There are eight basic types in JavaScript:

number
bigint
string
boolean
null
undefined
object
symbol

Types of Values

Primitive Types

Primitive types can hold only one value at a time, and they are immutable (i.e., they can’t be changed). These types include:

String: Represents a sequence of characters.
let name = "Eag";

Number: Represents both integer and floating-point numbers. JavaScript doesn’t distinguish between them — all numbers are of the same type.
let age = 30;

Boolean: Represents a true or false value.
let isApproved = false;

Undefined: Indicates that a variable has been declared but not assigned a value.
let firstName = undefined; // Value not initialized

Null: Represents an intentionally empty or non-existent value.
let lastName = null; // Value intentionally left blank

BigInt: Used to represent very large integers that are beyond the limit of the standard Number type.
let largeNumber = 1234567890123456789012345678901234567890n; // BigInt

Symbol: Represents a unique and immutable value, often used to create unique identifiers.
let sym = Symbol("id");

Reference Types

Reference types can store collections of data and more complex entities. These include:

• Object: Is a member of type object, an unordered collection of named values (properties) where each property has a name and a value (either primitive or object).
  let user = { name: "John", age: 30 };

• Array: A special type of object for storing ordered collections of numbered values, which is different from ordinary objects.
  let numbers = [1, 2, 3];

• Function: A special type of object used to define callable blocks of code.

function greet() {
  console.log("Hello!");
}

• Symbol: Used to create unique identifiers, ensuring no collisions in object properties.

’typeof’ Operator

The typeof operator is used to determine the type of a value or variable. It returns a string indicating the type of the operand.
The type can change based on the value it holds.

typeof 0              // "number"
typeof 10n            // "bigint"
typeof 'foo'          // "string"
typeof Symbol("id")   // "symbol"

typeof Math           // "object"  (Math is an object)
typeof null           // "object"  (This is a known quirk in JavaScript)
typeof alert          // "function" (Functions are objects)

Note: The behavior of typeof with null is incorrect — it returns "object", which is a known issue in JavaScript, but it’s kept for compatibility reasons. null is not an object; it is its own unique type.

#17sep24