TypeScript - lesson 3 | HackIndia Skills

TypeScript - lesson 3 | HackIndia Skills | HackIndia

Example

Simple typed values

This is the kind of code you might have in a hackathon app that tracks orders, events, or project tasks.

let teamName: string = "Code Mandali";
let membersCount: number = 4;
let isSubmitted: boolean = false;

let tags: string[] = ["typescript", "hackathon", "frontend"];
let scores: number[] = [8, 9, 7];

let selectedCoupon: string | null = null;
selectedCoupon = "HACK50";

Example

Union for status

Instead of saving any random string as a payment status, list the only allowed values. This fits Indian app flows like UPI payment, event registration, or food ordering.

let paymentStatus: "pending" | "paid" | "failed" = "pending";

paymentStatus = "paid";
// paymentStatus = "done"; // TypeScript error

function statusLabel(status: "pending" | "paid" | "failed") {
  if (status === "paid") {
    return "Payment received";
  }

  if (status === "failed") {
    return "Payment failed";
  }

  return "Waiting for payment";
}

Common mistake

Do not reach for any

any turns off checking for that value. It is useful only as a short temporary step when converting old JavaScript.

If you use any for form data, API data, or user input, TypeScript cannot help you catch wrong assumptions.

// Weak
let amount: any = "499";
console.log(amount.toFixed(2)); // Breaks at runtime

// Safer
let amountText: string = "499";
let amountInRupees: number = Number(amountText);
console.log(amountInRupees.toFixed(2));

Tip

Narrow before use

When a value has a union type, check what it is before using methods that only work on one type. This is called narrowing.

AI tools often write code that assumes a value exists. Check for null and undefined in AI-generated code before you copy it.

let phoneNumber: string | undefined = undefined;

if (phoneNumber) {
  console.log(phoneNumber.toUpperCase());
} else {
  console.log("No phone number added yet");
}

Use basic and union types

The core idea

Basic types you use

Simple typed values

When to use unions

Union for status

Do not reach for any

Narrow before use

Do this now

Quick check