


How does Vue's reactivity system (e.g., Proxies in Vue 3) track dependencies and trigger updates?
Jun 13, 2025 am 12:08 AMVue 3’s reactivity system uses JavaScript Proxies to efficiently track and update dependencies. 1. When reactive data is accessed, such as in a template or computed property, Vue records which effect (like a component render) depends on that data using Proxy’s get trap. 2. When the data changes via Proxy’s set trap, Vue notifies all dependent effects to update. 3. Updates are queued for the next tick to batch changes and improve performance. 4. Unlike Vue 2’s Object.defineProperty, Proxies allow intercepting all property accesses and mutations, enabling support for dynamic keys, arrays, Maps, and Sets without special handling. 5. Edge cases include losing reactivity when assigning reactive properties to local variables or destructuring, which can be resolved with .value or toRefs(). This approach makes reactivity more intuitive and robust for complex and dynamic data structures.
Vue's reactivity system is the backbone of how Vue updates the DOM when your data changes. In Vue 3, it uses JavaScript Proxies (along with Reflect) under the hood to track which parts of your data are used and to know when they change — so it can update only what’s necessary.
Here’s a breakdown of how this works in practice.
What happens when you access reactive data?
When you use a reactive object in your template or computed property, Vue tracks that usage. This tracking is done using Proxy
traps like get
. Each time a property is accessed, Vue records which effect (like a component render or computed property) is currently running.
For example:
const state = reactive({ count: 0 })
When state.count
is accessed inside a component or watcher, Vue knows that component or watcher depends on count
.
This creates a dependency relationship: "this component/watcher relies on this data".
How does Vue know when data changes?
Vue sets up watchers internally for things like templates and computed properties. When a reactive value changes, the set
trap in the Proxy notifies Vue that a change happened.
At that point, Vue knows something changed and marks any dependent effects (like a component) as needing an update.
It doesn’t immediately re-render — instead, it queues the update for the next tick, batching changes for performance.
So if you do:
state.count state.count
Only one update will be triggered, not two.
Why proxies make this easier than Object.defineProperty
In Vue 2, reactivity was built using Object.defineProperty
, which meant:
- Only existing properties could be made reactive.
- Adding new object keys wouldn't be tracked automatically.
With Proxies in Vue 3:
- You can intercept all property accesses and mutations, not just predefined ones.
- It’s easier to handle arrays, Maps, Sets, etc., because you don’t need special hacks for them.
Think of it like this:
Object.defineProperty
: patch each property individually.Proxy
: wrap the whole object and observe everything going in and out.
Some edge cases to be aware of
Even though Vue 3’s reactivity system is powerful, there are still a few gotchas:
- Assigning a reactive object to a local variable and mutating it breaks tracking unless you use
.value
withref()
. - Destructuring a reactive object also breaks reactivity — because you're pulling primitive values out.
- If you want to keep reactivity after destructuring, use
toRefs()
or stick with refs.
Example:
const { count } = useStore(); // ? loses reactivity const count = ref(useStore().count); // ? but not reactive to future changes const { count } = toRefs(useStore()); // ? keeps reactivity
That’s basically how Vue tracks dependencies and triggers updates. The Proxy system gives Vue more flexibility and makes reactivity feel more natural, especially with dynamic data structures.
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