Gain insights into building real-world components using JavaScript. Delve into dropdown menus, auto-validating forms, autocomplete search, infinite scroll, and tooltips to tackle common frontend challenges seamlessly.

Learn to apply your introductory knowledge of HTML, CSS, and JavaScript to build beautiful, real-world components. In this course, you'll go through the process of developing several complex components, including:

- eBay's dropdown menu
- Gmail's auto-validating form
- Google's main autocomplete enabled search
- Twitter's infinitely scrolling list
- Medium's tooltips

You'll learn how to approach common frontend challenges, what questions to ask yourself, how to design solutions once you've thought of them, how to debug and work with mistakes, and more.

The end-goal is to empower you to look at anything you see on a website and be able to say, "I can build that!"

Intermediate JavaScript: Building Frontend Components

## Scroll action listener
​Let’s add an event listener to the scroll action, and when it reaches near the bottom of the page (say, 100px above), we’ll use that as a trigger to load. Here’s where the states will be used. If the state is in pending, we know not to trigger another request.



function onScroll(event) {
  const scrolledTo = window.innerHeight + window.pageYOffset;
  const scrollLimit = document.body.offsetHeight;
  const scrollThreshold = 30;
​
  if (scrollLimit - scrolledTo <= scrollThreshold) {
    // TODO update
  }
}

​
window.addEventListener('scroll', onScroll);

We can’t quite yet do the update, since we don’t have the lastTweetId.

In the loop for processing API responses, we’ll add this line.

```
lastTweetId = tweetResponse.id;
```



# Scroll action listener
​Let’s add an event listener to the scroll action, and when it reaches near the bottom of the page (say, 100px above), we’ll use that as a trigger to load. Here’s where the states will be used. If the state is in pending, we know not to trigger another request.



In this lesson we learn how to dynamically load more tweets.

Welcome

Building a Dropdown Menu

Building the Autocomplete Search

Building Tooltips

Building an Autovalidating Form

Building an Infinite List

From Toy Components to Production

Autopopulation

Scroll action listener #