When you mutate reactive state, it may trigger both Vue component updates and watcher callbacks created by you.īy default, user-created watcher callbacks are called before Vue component updates. This is more convenient and typically results in terser code, but makes its reactive dependencies less explicit. It automatically tracks every reactive property accessed during its synchronous execution. WatchEffect, on the other hand, combines dependency tracking and side effect into one phase. watch separates dependency tracking from the side effect, giving us more precise control over when the callback should fire. In addition, the callback only triggers when the source has actually changed. It won't track anything accessed inside the callback. maybe have multiple styles files, and call the one you need. Watch only tracks the explicitly watched source. Their main difference is the way they track their reactive dependencies: Watch and watchEffect both allow us to reactively perform side effects. When using it with an async callback, only properties accessed before the first await tick will be tracked. WatchEffect only tracks dependencies during its synchronous execution. In addition, if you need to watch several properties in a nested data structure, watchEffect() may prove more efficient than a deep watcher, as it will only track the properties that are used in the callback, rather than recursively tracking all of them. But for watchers that have multiple dependencies, using watchEffect() removes the burden of having to maintain the list of dependencies manually. Language files containing the translations are. You can check out this example of watchEffect() and reactive data-fetching in action.įor examples like these, with only one dependency, the benefit of watchEffect() is relatively small. Vue.js offers the possibility to translate a page or app into several languages via the NPM package vue-i18n. With watchEffect(), we no longer need to pass todoId explicitly as the source value. Whenever todoId.value changes, the callback will be run again. During its execution, it will automatically track todoId.value as a dependency (similar to computed properties). Here, the callback will run immediately, there's no need to specify immediate: true.
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