Not much is known about Uber's front end interview process.
- Implement a rate limiter attribute/decoration/annotation on top of an API endpoint. Caps to N requests per minute with a rolling window. Source A and Source B
User interface coding
- Create a button that when clicked, adds a progress bar onto the page. The progress bar would then fill up in a given amount of time (think 3 to 5 seconds). If you get past the first part, you will be asked to do throttling how many progress bars can be running at once. For example, if the limit is 3 progress bars, and the user clicks on the button 4 times, the fourth progress bar only starts after the very first one finishes. Source
- Overlapping circles app. Source 80/Rate-Limiter)