JavaScript - lesson 4 | HackIndia Skills

JavaScript - lesson 4 | HackIndia Skills | HackIndia

Example

Start with data

Keep the function focused on input and output. Do not make it depend on random outside variables unless it has to.

This example takes an order object and returns a display string for a UPI-style payment summary.

const order = {
  id: "ORD-104",
  customer: "Asha",
  amount: 249,
  paid: true
};

function formatPaymentStatus(order) {
  if (order.paid) {
    return `${order.customer} paid ₹${order.amount}`;
  }

  return `${order.customer} has not paid yet`;
}

console.log(formatPaymentStatus(order));

Common mistake

Avoid global leaks

A global variable is available everywhere in the file. It feels easy, but it can create bugs when two functions change the same value.

Prefer const and let inside the smallest useful scope. If a value belongs only inside a function, declare it inside that function.

let total = 0; // Avoid using this as shared state unless needed

function addToCart(price) {
  const updatedTotal = total + price;
  return updatedTotal;
}

console.log(addToCart(99));

Example

Scope in practice

Variables declared inside a function cannot be used outside it. That is function scope.

Variables declared inside if, for, or other blocks with const or let stay inside that block. That is block scope.

function getPassMessage(score) {
  const passingScore = 40;

  if (score >= passingScore) {
    const message = "Passed";
    return message;
  }

  return "Try again";
}

console.log(getPassMessage(72));
// console.log(message); // Error: message is not available here

Functions And Scope

One job only

Why functions matter

Start with data

Parameters beat guessing

Avoid global leaks

Return, do not print

Scope in practice

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Quick check