TypeScript, our most-used language

// JavaScript: an amount that changes unnoticed
const price = 20;
const shippingInput = "5";

// A number + text joins the values together.
const wrongTotal = price + shippingInput;

console.log(wrongTotal);
// "205", although the customer expects 25 euros.

// Suppose the payment code reads this as euros:
const wrongPaymentRequest = {
  amount: wrongTotal,
  currency: "EUR",
};

console.log(wrongPaymentRequest.amount);
// Still "205". JavaScript reports no error.

// TypeScript: rules for the calculation
function calculateTotal(
  price: number,
  shipping: number,
): number {
  const total = price + shipping;
  return total;
}

const amountAsText = "5";

// TypeScript would reject this call:
// calculateTotal(20, amountAsText);
// Text (string) does not belong in a number field.

// With two numbers, addition works as intended.
const exampleTotal = calculateTotal(20, 5);
console.log(exampleTotal);
// 25

// Convert and check the input
function readShipping(input: string): number {
  const amount = Number(input);

  if (
    input.trim() === "" ||
    !Number.isFinite(amount) ||
    amount < 0
  ) {
    throw new Error("Enter a valid shipping cost.");
  }

  return amount;
}

const shipping = readShipping("5");
const correctTotal = calculateTotal(20, shipping);

const correctPaymentRequest = {
  amount: correctTotal,
  currency: "EUR",
};

console.log(correctPaymentRequest.amount);
// 25: the amount we expect.

// Input such as "free" fails the input check:
// readShipping("free");
// This stops us calculating with an invalid amount.