Let's face it, algorithms are a useless test of a candidate's experience as a frontend developer. Someone who can construct a binary tree from preorder and inorder traversal doesn't mean they can create an amazing UI with an awesome user experience. It means they got lucky you asked them a question they've seen before.
Ask them to create a simple stopwatch. It should have buttons to start, pause, and stop the timer. I created a simple example.
Click the 'Start' button multiple times and see if multiple intervals are created. Most candidates make this mistake.
Ensure 'Pause' button is hidden when the stopwatch hasn't started.
style the stopwatch
show milliseconds, minutes and hours
Enough said, so many tutorials out there about this. Tweak the problem however you like.
make a todo editable
This is a simple polling tool, similar to Slido, that let's users ask questions and upvote the questions so the most popular ones are shown on top. See example.
Questions should be sorted by upvotes
When a question is upvoted past another question, the one with the highest upvotes should move to the top
Handle duplicate questions
Make questions editable
Add delete button next to each question
Style the app
Hope this helped!