Variables and Type Annotations in TypeScript¶

Declaring Variables¶

TypeScript supports the same variable declarations as JavaScript.

let age = 18
const name = "Aman"
var score = 90

`let`¶

Used for variables whose value may change.

let count = 0
count = 5

`const`¶

Used for variables whose value will not change.

const pi = 3.14159

`var` (not recommended)¶

var has function scope instead of block scope and can cause unexpected bugs.

var x = 10

Modern TypeScript code should prefer let and const.

Type Annotations¶

Type annotations explicitly define the type of a variable.

let age: number = 16
let username: string = "Aman"
let isLoggedIn: boolean = true

Syntax:

let variableName: type = value

Example:

let score: number = 95

Type Inference¶

TypeScript automatically detects types when possible.

let message = "Hello"

TypeScript infers:

string

So this is equivalent to:

let message: string = "Hello"

When to Use Type Annotations¶

Use explicit annotations when:

1. Declaring variables without initial value¶

let age: number
age = 16

Without annotation TypeScript would treat it as any.

2. Function parameters¶

function greet(name: string) {
  console.log("Hello " + name)
}

3. Complex data structures¶

let user: { name: string; age: number } = {
  name: "Aman",
  age: 16
}

Type Annotation with Arrays¶

let numbers: number[] = [1, 2, 3, 4]
let names: string[] = ["Aman", "Rahul", "Aryan"]

Alternative syntax:

let numbers: Array<number> = [1, 2, 3]

Type Annotation with Objects¶

let user: {
  name: string
  age: number
} = {
  name: "Aman",
  age: 16
}

Reassigning Variables¶

TypeScript enforces type safety.

let age: number = 16

age = 18      // valid
age = "18"    // error

Error:

Type 'string' is not assignable to type 'number'

Best Practices¶

Prefer `const`¶

const API_URL: string = "https://api.example.com"

Avoid unnecessary annotations¶

Bad:

let name: string = "Aman"

Better:

let name = "Aman"

Use annotations when TypeScript cannot infer correctly.

Quick Example¶

const username: string = "Aman"
let age: number = 16
let isStudent: boolean = true

let hobbies: string[] = ["coding", "music"]

let user: { name: string; age: number } = {
  name: "Aman",
  age: 16
}