Vue 3 fully supports reactive array mutations including index assignment and length changes due to Proxy, while Vue 2 requires using Vue.set() or mutating methods because it relies on Object.defineProperty and array method interception; 1. In Vue 2, direct index assignment and length modification are not reactive — use Vue.set() or splice(); 2. In Vue 3, all array mutations are reactive including arr[index] = value and arr.length = 0; 3. Non-mutating methods like filter, map, slice return new arrays and require reassignment to trigger updates in both versions; 4. Always use mutating methods like push, splice for predictable reactivity; 5. Vue 3 internally wraps mutating methods for dependency tracking but detects all changes via Proxy, making array reactivity more reliable overall.
Vue's reactivity system handles arrays with special care because of the limitations in JavaScript’s ability to detect certain types of mutations. Here's how it works under the hood and what you need to know as a developer.
Vue 2 vs Vue 3: Different Approaches
The way arrays are made reactive differs between Vue 2 and Vue 3 due to the underlying reactivity mechanisms.
In Vue 2: Object.defineProperty with Array Augmentation
Vue 2 uses Object.defineProperty to make object properties reactive. However, this method cannot detect direct index assignment (e.g., arr[0] = newValue) or changes to the array's length (e.g., arr.length = 0).
To work around this, Vue 2 intercepts array mutation methods. It wraps the array’s prototype methods that do change the array (mutating methods) so they trigger view updates.
The intercepted methods include:
pushpopshiftunshiftsplicesortreverse
When you call one of these, Vue knows the array has changed and triggers a re-render.
However, non-mutating methods like filter, concat, and slice don’t change the original array, so they don’t trigger updates — but that’s okay because you’re expected to assign the returned new array back to your data property.
?? Gotchas in Vue 2:
- You can't add a new item with direct index assignment:
vm.items[0] = newValuewon’t trigger an update. - You can't change array length:
vm.items.length = 0won’t be reactive.
? Solutions:
- Use
Vue.set(vm.items, indexOfItem, newValue) - Or replace the whole array with a new one (e.g., using
splice,push, or reassignment)
In Vue 3: Proxy-based Reactivity
Vue 3 uses ES6 Proxy, which can observe all kinds of property access and mutations, including direct index access and length changes.
This means in Vue 3:
-
arr[0] = 'new value'is reactive ? -
arr.length = 0is reactive ? - You can add items to any index directly
No need for special interception of array methods — the Proxy observes the mutations naturally.
However, Vue 3 still wraps the array’s mutating methods (like push, splice, etc.) internally to ensure proper dependency tracking and event triggering, but the underlying detection is more robust.
Key Takeaways
-
Vue 3 handles arrays more reliably due to
Proxy, supporting all mutation types. - In Vue 2, avoid direct index assignment or length modification — use
Vue.set()or array methods instead. - Non-mutating methods (e.g.,
map,filter) always return a new array — assign the result to trigger reactivity. - Vue does not make every array method reactive by rewriting logic, but ensures that when the array changes, the view updates.
Practical Tips
When working with arrays in Vue:
- Prefer using mutating methods (
push,splice) when possible. - When replacing values by index:
- Vue 2: Use
Vue.set(array, index, value)orarray.splice(index, 1, value) - Vue 3: Direct assignment is fine:
array[index] = value
- Vue 2: Use
- When clearing an array:
- Vue 2: Use
splice(0)instead of settinglength = 0 - Vue 3: Both
length = 0andspliceare reactive
- Vue 2: Use
Basically, Vue’s array reactivity is robust in Vue 3, while Vue 2 requires being mindful of its limitations.
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