TypeScript - lesson 7 | HackIndia Skills

TypeScript - lesson 7 | HackIndia Skills | HackIndia

type Expense = {
  title: string;
  amount: number;
};

function addExpense(expense: Expense) {
  console.log(`${expense.title}: ₹${expense.amount}`);
}

const amountFromInput = "499";

addExpense({
  title: "Domain name",
  amount: amountFromInput,
});

// TypeScript error:
// Type 'string' is not assignable to type 'number'.

Your to-do

Fix the mismatch

Do each one yourself, then tap it to tick it off. The ticks are only a checklist for you: they are not marked or scored.

Read what you passed: amountFromInput is a string.
Read what was expected: amount must be a number.
Convert at the boundary, where the input enters your app.
Handle bad input before calling the typed function.

0 of 4 done

const rawAmount = "499";
const amount = Number(rawAmount);

if (Number.isNaN(amount)) {
  throw new Error("Enter a valid amount");
}

addExpense({
  title: "Domain name",
  amount,
});

Common mistake

The any shortcut

This silences the error, but it also removes protection. Now amount could be a string, object, or undefined, and TypeScript will not warn you.

Use any only when you have a clear reason and a comment explaining it. In a hackathon repo, avoid it in normal app code.

const amountFromInput: any = "499";

addExpense({
  title: "Domain name",
  amount: amountFromInput,
});

Tip

Use unknown first

When data comes from localStorage, fetch, a form, or an AI-generated mock file, you may not trust its shape. Use unknown, then check it before using it.

This keeps TypeScript strict without pretending you know the data.

function parseExpense(data: unknown): Expense {
  if (
    typeof data === "object" &&
    data !== null &&
    "title" in data &&
    "amount" in data &&
    typeof data.title === "string" &&
    typeof data.amount === "number"
  ) {
    return {
      title: data.title,
      amount: data.amount,
    };
  }

  throw new Error("Invalid expense data");
}

Read errors before using any

One rule

How errors speak

Read this error

Fix the mismatch

The any shortcut

Use unknown first

Use AI carefully

Do this now

Quick check