Event Loop
The Event Loop is how JavaScript handles asynchronous operations despite being single-threaded.
Learning Goals
The Core Concept
JavaScript is a single-threaded language, meaning it can only do one thing at a time. However, it can perform asynchronous operations (like downloading a file) without blocking the main thread.
It does this using the Event Loop. When JavaScript encounters an async task, it offloads it to the browser (Web APIs). When the task finishes, it gets pushed to a Callback Queue. The Event Loop constantly checks if the main thread is empty. If it is, it pushes the callback onto the main thread to run.
Visual guide
JavaScript concept flow
A simple original diagram to connect the lesson idea with real project flow.
Code & Implementation
console.log("1. Start");
setTimeout(() => {
console.log("2. Timeout Callback");
}, 0);
Promise.resolve().then(() => {
console.log("3. Promise Resolved");
});
console.log("4. End");
// Output Order: 1, 4, 3, 2Expected Output
1. Start 4. End 3. Promise Resolved 2. Timeout Callback
Practical Project: Event Loop Implementation
Hands-on practice task
The Challenge
Apply your knowledge of Event Loop to build a real-world feature. This project helps you move beyond theory and understand how JavaScript works in professional settings.
Helpful Hints
- •Refer back to the 'Steps' section for the correct sequence.
- •Check the 'Tips' for common optimization patterns.
- •Look at the 'Code Highlights' to ensure you're using the right syntax.
Quick Knowledge Check
Why does the Promise run before setTimeout even if timeout is 0?
Continue Learning
Next steps after this lesson
Apply your knowledge of Event Loop to build a real-world feature. This project helps you move beyond theory and understand how JavaScript works in professional settings.
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