JavaScript Callback Functions: Passing Functions as Arguments

function repeatTask(times, callback) {
  for (let i = 1;
  i <= times;
  i++) {
    callback(i);
    // 📞 calling back with the round number!  }}
    repeatTask(3, function(round) {
      console.log(`Round ${round} complete ✅`);
    });
    // Round 1 complete ✅// Round 2 complete ✅// Round 3 complete ✅

console.log("1. Ordering food 🍔");
setTimeout(function() {
  console.log("3. Food arrived! 🔔");
  // callback — runs LAST}, 1000);
  console.log("2. Watching TV 📺");

1. Ordering food 🍔2. Watching TV 📺3. Food arrived! 🔔 ← 1 second later

// ⏰ Timers (Lesson 32):
setTimeout(() => console.log("Later!"), 1000);
// 👆 Events (Lesson 30):
btn.addEventListener("click",() => console.log("Clicked!"));
// 🌐 Network (the old-school style):
fetch(url).then(response => console.log("Data's here!"));

function greetUser(name, formatter) {
  const formatted = formatter(name);
  // 📞 call back to format
  console.log(`Welcome, ${formatted}!`);
}greetUser("priya", name => name.toUpperCase());
// Welcome, PRIYA!
greetUser("amit", name => `Mr. ${name}`);
// Welcome, Mr. amit!
greetUser("riya", name => `🌟 ${name} 🌟`);
// Welcome, 🌟 riya 🌟!

function downloadFile(filename, onSuccess, onError) {
  console.log(`
⬇
️ Downloading ${filename}…`);
  setTimeout(() => {
    const success = Math.random() > 0.3;
    // simulate flakiness
    if (success) {
      onSuccess(filename);
      // ✅ success callback
    } else {
      onError(filename);
      // ❌ error callback
    }
  }, 1500);
}downloadFile( "report.pdf", file => console.log(`✅ ${file} downloaded!`), file => console.log(`❌ ${file} failed — retry?`));

⬇️ Downloading report.pdf…✅ report.pdf downloaded!(or ❌ failed)

// 💀 The infamous pyramid:
getUser(userId, function(user) {
  getOrders(user.id, function(orders) {
    getOrderDetails(orders[0].id, function(details) {
      getProduct(details.productId, function(product) {
        getReviews(product.id, function(reviews) {
          console.log(reviews);
          // 5 levels deep…
          // 😵 error handling at EVERY level…
          // 😵 closing braces piling up…
        });
      });
    });
  });
});

// Fix 1 — Promises flatten the pyramid (Lesson 26):
getUser(userId) .then(user => getOrders(user.id)) .then(orders => getOrderDetails(orders[0].id)) .then(details => console.log(details)) .catch (error => console.log(error));
// ONE error handler! ✅
// Fix 2 — async/await makes it read like sync code (Lesson 24):
async function showOrder(userId) {
  try {
    const user = await getUser(userId);
    const orders = await getOrders(user.id);
    const details = await getOrderDetails(orders[0].id);
    console.log(details);
  } catch (error) {
    console.log(error);
  }
}

// 🎯 Quiz Engine — callbacks control the flow
function askQuestion(question, options, correctIndex, onResult) {
  console.log(`\n❓ ${question}`);
  options.forEach((opt, i) => console.log(`   ${i + 1}. ${opt}`));
  // Simulate the player answering option 2:
  const playerAnswer = 2;
  const correct = playerAnswer - 1 === correctIndex;
  onResult(correct, options[correctIndex]);
  // 📞 report via callback!
}let score = 0;
askQuestion( "Which keyword creates a block-scoped variable?",["var", "let", "function"], 1,(correct, answer) => {
  if (correct) {
    score++;
    console.log(`✅ Correct! Score: ${score}`);
  } else {
    console.log(`❌ Wrong! It was "${answer}"`);
  }
});
askQuestion( "What does a callback mean?",["A function called immediately", "A function passed to be called later", "A loop"], 1,(correct, answer) => {
  console.log(correct ? `✅ Correct! Final score: ${++score} 🏆` : `❌ It was "${answer}". Final score: ${score}`);
});

❓ Which keyword creates a block-scoped variable?1. var2. let3. function✅ Correct! Score: 1❓ What does a callback mean? …✅ Correct! Final score: 2 🏆