📘TypeScript: Day 1 – Variables, Types & Functions

1. Variables with Types

In TypeScript, you declare the type after a :.

let username: string = "Sagar";
let age: number = 25;
let isStudent: boolean = true;

❌ If you assign the wrong type:

age = "30"; // Error: Type 'string' is not assignable to type 'number'

2. Arrays

You can say what type the array holds:

let numbers: number[] = [1, 2, 3];
let fruits: string[] = ["pineapple", "kiwi"];

❌ This won’t work:

numbers.push("hello"); // Error

3. Objects

let person: { name: string; age: number } = {
  name: "Sagar",
  age: 30
};

4. Functions with Types

You type both parameters and the return value:

function add(a: number, b: number): number {
  return a + b;
}

console.log(add(2, 3));   // ✅ 5
console.log(add(2, "3")); // ❌ Error

Optional parameters:

function greet(name: string, age?: number): string {
  return age ? `${name} is ${age}` : `Hello ${name}`;
}

console.log(greet("Sagar"));       // Hello Sagar
console.log(greet("Sagar", 30));   // Sagar is 25

📝 Day 1 Exercises

Try these in the TypeScript Playground:

1. Create a variable city (string), population (number), isCapital (boolean).

2. Make an array of numbers called scores.

3. Define an object car with { brand: string; year: number }.

4. Write a function multiply That takes two numbers and returns a number.

Solution:

✅ Exercise 1: Create a variable city (string), population (number), isCapital (boolean).

const city: string = 'Palghar'
const population: number = 10000000
const isCapital: boolean = true

console.log(city)        // Palghar
console.log(population)  // 10000000
console.log(isCapital)   // true

✅ Exercise 2: Make an array of numbers called scores.

const scores: number[] = [1, 2, 3, 4, 5]

console.log(scores) // [1, 2, 3, 4, 5]

✅ Exercise 3: Define an object car with { brand: string; year: number }.

const car: { brand: string, year: number } = {
  brand: 'Tata',
  year: 2025
}

console.log(car)
// { brand: 'Tata', year: 2025 }

⚠️ Exercise 4: Write a function multiply That takes two numbers and returns a number.

function multiply(a: number, b: number): number {
  return a * b
}

const output = multiply(5, 3)
console.log(output) // 15

Also Learn

• TypeScript: Day 1 – Variables, Types & Functions

• What is TypeScript?

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