Asynchronous JavaScript Callbacks and Promises
Continue your JavaScript Tutorial learning path with hands-on explanation, code practice, and quiz.
Course
JavaScript Tutorial
Estimated Time
8 min
Progress
21%
Track Position
15 / 73
Lesson Overview
Simple Explanation
Synchronous code line-by-line run hota hai. Asynchronous tasks (timer, network) background me run hote hain. Callback hell nested callbacks ki readability issue hai. Promises is problem ko better structure deti hain.
Code Explanation
setTimeout async behavior dikhata hai. Promise resolve/reject aur then/catch/finally chain handle karta hai.
Output Description
Logs se execution order samajh aayega.
Practice Exercise
3 fake async steps banao: callback version aur Promise version compare karo.
Extra Explanation
Why This Matters
Asynchronous JavaScript Callbacks and Promises is core to dynamic behavior. Strong fundamentals here help you build forms, API flows, and interactive UI with confidence.
Real-World Workflow
Complex features are usually split into small functions, then verified through focused logging and tiny test cases.
Common Mistakes to Avoid
Avoid uncontrolled globals, weak error handling, and ignored async states. Define clear input-output expectations for each function.
Example + Live Practice
<script>
console.log("Start");
setTimeout(() => {
console.log("Async callback after 1s");
}, 1000);
const task = new Promise((resolve, reject) => {
const ok = true;
if (ok) resolve("Task completed");
else reject(new Error("Task failed"));
});
task
.then((msg) => console.log(msg))
.catch((err) => console.error(err.message))
.finally(() => console.log("Promise finished"));
console.log("End");
</script>Try It Yourself
Test Your Knowledge
Quiz Coming Soon
Quiz for this lesson is not added yet.
Save Your Work
Lesson ke end par apna code save karein. Dashboard me aap kabhi bhi is saved code ko dobara dekh sakte hain.