What are some features that every frontend developer should know how to implement?

As the question suggests, what are some frontend features every frontend developer should know how to implement/optimize for, as a standard thing a frontend dev should know? A few things off the top of my head would be,

Working with APIs to

• Search
• Sort
• Filter

Optimizing by

• Throttling
• Debouncing
• Memoization

What else?

Comments

[Alvaro Saburido]

• Basic accessibility (aria labels, screen readers, focusable elements and tab navigation).
• How javascript works in the browser, the call stack and the event loop
• Basic UX

[Tanmay Naik]

• UI testing
• General testing
• Optimizing for page speed
• Knowledge of how SEO fits in