Variables and Datatypes in JavaScript

Variables and data types are foundational concepts in programming, serving as the building blocks for storing and manipulating information within a program. In JavaScript, getting a good grasp of these concepts is important for writing code that works well and is easy to understand.

Variables

A variable is like a container that holds data that can be reused or updated later in the program. In JavaScript, variables are declared using the keywords var, let, or const.

1. var Keyword

The var keyword is used to declare a variable. It has a function-scoped or globally-scoped behavior.

var n = 5;
console.log(n);
 
var n = 20; // reassigning is allowed
console.log(n);

Output:

5
20

2. let Keyword

The let keyword is introduced in ES6, has block scope, and cannot be re-declared in the same scope.

let n = 10;
n = 20; // Value can be updated
// let n = 15; // Cannot redeclare
console.log(n);

Output:

20

3. const Keyword

The const keyword declares variables that cannot be reassigned. It’s block-scoped as well.

const n = 100;
// n = 200; This will throw an error
console.log(n);

Output:

100

For more details, read the article – JavaScript Variables.

Data Types

JavaScript supports various data types, which can be broadly categorized into primitive and non-primitive types.

Primitive Datatypes

Primitive datatypes represent single values and are immutable.

1. Number

Represents numeric values (integers and decimals).

let n = 42;
let pi = 3.14;

2. String

Represents text enclosed in single or double quotes.

let s = "Hello, World!";

3. Boolean

Represents a logical value (true or false).

let bool = true;

4. Undefined

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

let notAssigned;
console.log(notAssigned);

Output:

undefined

5. Null

Represents an intentional absence of any value.

let empty = null;

6. Symbol

Represents unique and immutable values, often used as object keys.

let sym = Symbol('unique');

7. BigInt

Represents integers larger than Number.MAX_SAFE_INTEGER.

let bigNumber = 123456789012345678901234567890n;

Non-Primitive Datatypes

Non-primitive types are objects and can store collections of data or more complex entities.

1. Object

Represents key-value pairs.

let obj = {
    name: "Amit",
    age: 25
};

2. Array

Represents an ordered list of values.

let a = ["red", "green", "blue"];

3. Function

Represents reusable blocks of code.

function fun() {
    console.log("Coding75");
}