Categories of Variables in JavaScript

JavaScript variables can be categorized in two main ways:

1. Variable Declaration Types

var (Legacy)

Function-scoped and can be redeclared and reassigned. Hoisted to the top.

javascript

var name = "John";
var name = "Jane"; // Can be redeclared
name = "Bob"; // Can be reassigned
console.log(name); // Bob

let (Block-scoped)

Cannot be redeclared in the same scope but can be reassigned. Preferred in modern JavaScript.

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let age = 25;
// let age = 30; // Error: Identifier 'age' has already been declared
age = 30; // Can be reassigned
console.log(age); // 30

const (Block-scoped)

Cannot be redeclared or reassigned. Must be initialized at declaration.

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const country = "USA";
// country = "Canada"; // Error: Assignment to constant variable
console.log(country); // USA

2. Data Types (Values Variables Can Hold)

Primitive Data Types

1. String

javascript

let name = "John";
let char = 'A';
let template = `Hello ${name}`;

2. Number

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let age = 25;
let price = 99.99;
let negNum = -10;

3. Boolean

javascript

let isActive = true;
let isDeleted = false;

4. Undefined

javascript

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

5. Null

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let user = null;
console.log(user); // null

6. Symbol (Unique identifier)

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let id = Symbol("id");
let another = Symbol("id");
console.log(id === another); // false

7. BigInt (Large integers)

javascript

let largeNum = 123456789012345678901234567890n;
let bigNum = BigInt("9007199254740992");

Complex Data Types (Objects)

1. Object

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let person = {
  name: "John",
  age: 30,
  city: "New York"
};

2. Array

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let colors = ["red", "green", "blue"];
let mixed = [1, "hello", true, {name: "John"}];

3. Function

javascript

let greet = function(name) {
  return `Hello ${name}`;
};

4. Date

javascript

let today = new Date();

5. RegExp (Regular Expression)

javascript

let pattern = /[a-z]+/g;

Comparison Table

FeaturevarletconstScopeFunctionBlockBlockRedeclarationYesNoNoReassignmentYesYesNoHoistingYes (undefined)Yes (TDZ)Yes (TDZ)

Best Practices

• Use const by default for variables that won't change
• Use let when you need to reassign values
• Avoid var in modern JavaScript
• Use meaningful variable names