Why can using splice cause reactivity issues in Vue 2, and how is this resolved in Vue 3?

In Vue 2, reactivity is based on getter/setter properties created with Object.defineProperty. However, array methods like splice do not always track changes correctly if they modify the array directly, especially when adding new elements or changing the length.

The problem is that Vue 2 cannot detect changes if an element is added directly by index or if the array length is changed via splice, which leads to the interface not updating.

In Vue 3, reactivity is implemented through Proxy, which intercepts all operations on objects and arrays, including any changes via splice. This allows automatic and correct tracking of any array modifications without additional tricks.

Example in Vue 2, where Vue.set must be used for reactivity:

this.items.splice(index, 1, newItem); // may not trigger reactivity
Vue.set(this.items, index, newItem); // guarantees reactivity

In Vue 3, simply calling splice is enough, and reactivity will be preserved without additional actions.