Lead Frontend Interview Experience: Designing a Global Modal Architecture

I Was Asked to Build a Global Modal System in a Lead Frontend Interview — Here’s How I Designed It In a recent interview for a Lead Frontend Engineer role, I got a live system design + implementation challenge: “Design a global modal architecture that can be used from anywhere in the app.” What looked like a simple UI task quickly became a great discussion on architecture, state boundaries, accessibility, and API design. I was also asked to write custom CSS for the modal UI, so the exercise covered both behavior and presentation. This post is a breakdown of my approach, the follow-up questions I got, and the refinements I made afterward. The Requirements I Wrote Down First Before coding, I confirmed the contract: Open a modal from anywhere in the app. Close an open modal. Support stacking multiple modals. Keep last-opened modal active; when it closes, reveal the previous one. One thing I intentionally added was custom content support . It was not explicitly stated, but I read between t