TypeScript - lesson 5 | HackIndia Skills

TypeScript - lesson 5 | HackIndia Skills | HackIndia

Example

Basic function types

Here is a small college-event helper. It calculates the payable amount after a student discount.

The return type is number. If someone returns a string like "₹120", TypeScript will complain before the build passes.

type TicketType = "student" | "regular";

function calculateTicketPrice(basePrice: number, ticketType: TicketType): number {
  if (ticketType === "student") {
    return basePrice * 0.7;
  }

  return basePrice;
}

const amountToPay = calculateTicketPrice(200, "student");

Example

Objects in functions

Most useful functions take objects. Use the aliases and interfaces you learned earlier instead of repeating shapes everywhere.

This makes the function readable and keeps your app data consistent.

interface Expense {
  title: string;
  amount: number;
  paidBy: string;
}

function formatExpense(expense: Expense): string {
  return `${expense.title}: ₹${expense.amount} paid by ${expense.paidBy}`;
}

const chaiExpense: Expense = {
  title: "Team chai",
  amount: 120,
  paidBy: "Asha"
};

console.log(formatExpense(chaiExpense));

Tip

Optional vs default

Use an optional parameter when the value may be missing. Use a default parameter when you want a fallback.

Default parameters are cleaner for common UI helpers, because the function always has a usable value inside.

function createGreeting(name: string, college = "your college"): string {
  return `Hi ${name} from ${college}`;
}

createGreeting("Ravi");
createGreeting("Ravi", "VIT");

function sendInvite(email: string, phone?: string): string {
  if (phone) {
    return `Invite sent to ${email} and ${phone}`;
  }

  return `Invite sent to ${email}`;
}

Common mistake

Do not hide errors

Do not use any for function parameters just to remove red lines. That turns TypeScript back into JavaScript at the exact place where bugs enter.

If you are unsure, ask AI to suggest a type from sample data. Then check the result yourself: are numbers really numbers, can values be missing, and are there only fixed options?

// Weak: accepts anything
function saveUser(user: any) {
  return user.email.toLowerCase();
}

// Better
type User = {
  name: string;
  email: string;
};

function saveUser(user: User): string {
  return user.email.toLowerCase();
}

Type functions that do work

Main idea

Where to type

Basic function types

Type real work

Objects in functions

Optional vs default

Do not hide errors

Do this now

Quick check