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How to Build Reactive Declarative UI in Vanilla JavaScript

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Declarative Reactive Interfaces Without Frameworks

Modern UI frameworks offer abstraction layers that make user interfaces declarative and reactive. However, the web platform itself exposes primitives that can be composed to achieve similar patterns without introducing a dedicated UI library. This article demonstrates an experimental approach for creating a reactive, declarative UI flow using only vanilla JavaScript, Web APIs, and Proxy-based state tracking.

The purpose of the experiment is to examine how far native capabilities can be pushed without framework-level abstractions and to illustrate architectural benefits of declarative behavior in UI code: improved clarity, maintainability, and reduced coupling.

The Target Behavior

The experiment focuses on a practical business scenario:

Display a modal dialog that performs periodic polling of an API endpoint. The dialog should remain open until a specific condition is met, then resolve or reject accordingly.

The modal dynamically:

mounts itself into the DOM

starts and manages a polling process

exposes reactive internal state

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