Hydration is the process of making a server-rendered web page interactive. After the browser receives and displays the initial HTML, JavaScript runs to attach event listeners, restore application state, and connect the static markup to the client-side framework. Until hydration completes, elements may appear fully rendered but not yet respond to user interactions.
For software testers, hydration explains why an element can exist in the DOM and be visible on screen, yet still fail to respond to clicks or other actions. Tests that interact with the page before hydration finishes may become flaky, even though the application is behaving as designed. Understanding hydration helps testers distinguish between structural readiness and true application readiness, allowing them to use more reliable waiting strategies.
