Type	Example
string	"Hello"
number	100
boolean	true / false
array	number[]
object	{name:string}
any	let x:any
unknown	let y:unknown
null	null
undefined	undefined

Common Basic Explicit Types in TypeScript

Explicit types mean we manually define the datatype while creating variables.

---

1. string

Used to store text values.

Example

let companyName: string = "Cybotrix";
let city: string = 'Bangalore';

Valid Values

"Hello"
'World'
`Template String`

Invalid

let age: string = 25; // ❌ Error

Common String Methods

companyName.length
companyName.toUpperCase()
companyName.toLowerCase()

---

2. number

Used for numeric values.

Example

let salary: number = 50000;
let percentage: number = 99.5;

Supports

• Integer
• Decimal
• Negative numbers

let x: number = -10;
let y: number = 10.55;

Invalid

let amount: number = "500"; // ❌ Error

Mathematical Operations

let total: number = 10 + 20;

---

3. boolean

Stores only:

• true
• false

Example

let isLoggedIn: boolean = true;
let isAdmin: boolean = false;

Mostly Used For

• Conditions
• Authentication
• Flags

Example

if (isLoggedIn) {
    console.log("Welcome");
}

Invalid

let status: boolean = "true"; // ❌ Error

---

4. array

Used to store multiple values.

---

Array of Numbers

let marks: number[] = [10, 20, 30];

---

Array of Strings

let cities: string[] = ["Bangalore", "Delhi"];

---

Mixed Array (not recommended)

let data: any[] = [10, "Raj", true];

---

Array Generic Syntax

let users: Array<string> = ["Raj", "Amit"];

---

Common Methods

marks.push(40);
marks.pop();
marks.map();
marks.filter();

---

5. object

Used to store key-value pairs.

Example

let employee: {name: string, age: number} = {
    name: "Raj",
    age: 25
};

---

Access Object Values

console.log(employee.name);

---

Invalid

let employee: {name: string} = {
    name: 100 // ❌ Error
};

---

Optional Property

let user: {name: string, phone?: number} = {
    name: "Amit"
};

? means optional.

---

6. any

any disables type checking.

TypeScript behaves like normal JavaScript.

Example

let data: any = "Hello";

data = 100;
data = true;
data = [];

Everything is allowed.

---

Why Avoid any?

Because it removes TypeScript safety.

let value: any = "Raj";

value.toFixed(); // ❌ Runtime error possible

---

Use Cases

• Third-party libraries
• Dynamic APIs
• Temporary development

---

7. unknown

Safer version of any.

You must check type before using it.

Example

let value: unknown = "Hello";

---

Invalid Direct Usage

value.toUpperCase(); // ❌ Error

---

Correct Way

if (typeof value === "string") {
    console.log(value.toUpperCase());
}

---

Difference Between any vs unknown

any	unknown
No type safety	Type safe
Can access anything	Need type checking
Dangerous	Safer

---

8. null

Represents intentionally empty value.

Example

let userData: null = null;

---

Real Example

let selectedUser: string | null = null;

Means:

• either string
• or null

---

Common Use

• Empty database response
• No selection
• Reset values

---

9. undefined

Means variable declared but value not assigned.

Example

let username: undefined = undefined;

---

JavaScript Example

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

---

Difference Between null vs undefined

null	undefined
Intentionally empty	Value not assigned
Assigned manually	Automatically assigned
Object type historically	Separate primitive

---

Combined Example

let name: string = "Raj";
let age: number = 25;
let isActive: boolean = true;

let skills: string[] = ["React", "Node"];

let employee: {name:string, city:string} = {
    name: "Raj",
    city: "Bangalore"
};

let data: any = 100;

let apiResponse: unknown = "Success";

let emptyValue: null = null;

let notAssigned: undefined = undefined;

---

Interview Summary

Type	Purpose
string	Text
number	Numeric values
boolean	True/False
array	Multiple values
object	Key-value data
any	Disable type checking
unknown	Safe dynamic type
null	Intentional empty
undefined	Unassigned value