Title Slide

Mastering setInterval() in JavaScript

Interactive guide to timing, control, and clean coding practices.

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Slide 1: What is setInterval()?

• A built-in JavaScript function

• Repeatedly executes a callback function at specified intervals (in milliseconds)

• Syntax: setInterval(callback, delay, ...args)

Visual: Alarm clock ringing every X seconds

Example:

setInterval(() => {
  console.log("Tick");
}, 1000); // Prints "Tick" every second

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Slide 2: How Does setInterval() Work Under the Hood?

• Added to the Web API task queue

• Callback executes after the delay — not guaranteed to be precise

• Depends on JS event loop & current call stack

Visual:
Event loop diagram with setInterval being queued

Speaker Note: Great for non-blocking repeated tasks but NOT for time-sensitive precision.

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Slide 3: Real-World Use Case #1: Live Clock

setInterval(() => {
  const now = new Date();
  document.getElementById("clock").innerText = now.toLocaleTimeString();
}, 1000);

Visual: Live clock UI

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Slide 4: Real-World Use Case #2: Polling an API

setInterval(async () => {
  const response = await fetch("/api/status");
  const data = await response.json();
  updateStatus(data);
}, 5000);

Tip: Add retry logic and error handling!

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Slide 5: Real-World Use Case #3: UI Animation (Basic)

let pos = 0;
const box = document.getElementById("box");
const interval = setInterval(() => {
  if (pos >= 300) clearInterval(interval);
  else box.style.left = `${pos++}px`;
}, 10);

Visual: Moving box demo

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Slide 6: Common Issue #1 - Memory Leaks

• Forgetting to clearInterval()

• Interval keeps running even after it's no longer needed

Example:

useEffect(() => {
  const interval = setInterval(doSomething, 1000);
  return () => clearInterval(interval);
}, []);

Tip: Always clear intervals on unmount in frameworks like React

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Slide 7: Common Issue #2 - Overlapping Intervals

• If callback takes longer than delay

• Can cause stacking and crashes

Bad:

setInterval(async () => {
  await fetchHeavyData();
}, 1000);

Fix: Use recursive setTimeout

async function poll() {
  await fetchHeavyData();
  setTimeout(poll, 1000);
}
poll();

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Slide 8: Common Issue #3 - Incorrect Clearing

• Forgetting to track interval ID

const id = setInterval(...);
clearInterval(id); // Correct

• Avoid using clearInterval() without the right ID

Tip: Store IDs in variables or refs

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Slide 9: Common Issue #4 - Browser Throttling

• Inactive tabs slow down intervals

• setInterval becomes unreliable in background

Best Practice: Use requestAnimationFrame() for animations, setTimeout for better control

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Slide 10: Best Practices Recap

• Always store and clear interval IDs

• Use setTimeout recursively for async control

• Avoid for precise timing (e.g., games, metronomes)

• Prefer requestAnimationFrame() for animations

• Clean up in componentWillUnmount / useEffect

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Slide 11: Visual Analogy - The Kitchen Timer
setInterval: Set a repeating timer, even if the chef is still cooking
setTimeout (recursive): Set a new timer only after cooking ends

Visual: Cartoon of chef and timers

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Slide 12: Quick Quiz (Audience Participation!)

1. What happens if you don’t clear an interval?

2. Why use recursive setTimeout over setInterval?

3. When should you avoid using setInterval()?

Visual: Polling buttons (if interactive)

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Slide 13: Summary Slide

• setInterval() is powerful but easy to misuse

• Understand the event loop and potential side effects

• Always clean up intervals

• Prefer setTimeout for async and precise control

• Use visuals and IDs to track timers

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Slide 14: Cheat Sheet 📚 (Screenshot This!)

// Start
const id = setInterval(fn, ms);

// Stop
clearInterval(id);

// Safe async polling
async function loop() {
  await fetchSomething();
  setTimeout(loop, 1000);
}
loop();

// React cleanup
useEffect(() => {
  const id = setInterval(fn, 1000);
  return () => clearInterval(id);
}, []);

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Slide 15: Closing Tips ✨

• Practice with small projects (live clock, API checkers)

• Don’t fear timers—embrace the control they offer

• Keep code clean, timed, and well-documented

• Screenshot the cheat sheet!

• Questions? Let’s talk!

Thank you! 👋