Have you encountered the concept of loss of reactivity in Vue?

Yes, I have encountered this. Loss of reactivity in Vue 2 mainly occurred in two cases:

1. Adding a new property directly to an existing object. Vue 2 only tracks changes to properties that were present in the object at the time of its creation.
2. Modifying an array by index or changing its length. Vue 2 could not reactively track such changes directly.

To solve these issues, methods like $set for objects and mutator methods (push, pop, shift, unshift, splice, sort, reverse) or splice-based methods for replacing elements/changing length were used.

// Loss of reactivity when adding a property
const obj = { a: 1 };
// obj.b = 2; // Not reactive in Vue 2

// Solution with $set
// Vue.$set(obj, 'b', 2);

// Loss of reactivity when changing an array by index
const arr = [1, 2, 3];
// arr[0] = 4; // Not reactive in Vue 2

// Solution with mutator methods
// arr.splice(0, 1, 4);

// Solution with $set for arrays (less commonly used but possible)
// Vue.$set(arr, 0, 4);

In Vue 3, with the Composition API and reactive/ref, as well as with an improved proxy-based reactive engine, these problems are largely resolved. Adding properties to reactive objects and changing arrays by index or length become reactive by default.

// Vue 3
import { reactive } from 'vue';

const obj = reactive({ a: 1 });
obj.b = 2; // Reactive in Vue 3

const arr = reactive([1, 2, 3]);
arr[0] = 4; // Reactive in Vue 3
arr.length = 1; // Reactive in Vue 3

Nevertheless, understanding the reasons for loss of reactivity in Vue 2 is useful when working with legacy code or transitioning between versions.