watch() in Vue 3 runs a side effect in response to reactive state changes — fetching data when a filter changes, saving state to localStorage, or triggering animations when a value updates. As the creator of CoreUI with Vue development experience since 2014, I use watch() whenever I need to react to a change with a side effect, and computed() when I just need a derived value. The key difference from watchEffect() is that watch() is explicit — you declare exactly what to watch and receive the old and new values in the callback. This makes it easier to understand what triggers the watcher and to compare values before and after.

Watching arrays for changes is essential when you need to react to modifications in list data, whether items are added, removed, or updated. As the creator of CoreUI, a widely used open-source UI library, I’ve implemented array watchers extensively throughout my 11 years of Vue development. The most reliable approach is using the watch function with the deep option set to true, which monitors nested changes within array items. This method ensures all array mutations trigger your callback function.

Watching deeply nested objects is essential for Vue applications that need to react to changes in complex data structures. As the creator of CoreUI with over 11 years of Vue development experience since 2014, I’ve implemented deep watchers in countless form validation and state management scenarios. The most effective solution is to use the watch function with the deep option to monitor all nested property changes. This approach ensures reactivity for complex objects with multiple levels of nesting.