Vue: Reactivity not triggering for nested objects

Vue: Reactivity Not Triggering for Nested Objects – In-Depth Explanation and Fixes

When working with Vue’s reactivity system, you might encounter an issue where updates to deeply nested objects do not trigger reactivity. This happens because Vue’s reactivity system does not automatically detect changes inside deeply nested properties. Understanding why this occurs and how to fix it is crucial for maintaining a reactive UI.

Understanding Vue’s Reactivity System

Vue tracks state changes using a reactive dependency tracking system. When you assign a new value to a reactive property, Vue detects the change and updates the UI. However, Vue’s reactivity is shallow by default, meaning that only the top-level properties of an object are reactive, while nested objects are not deeply reactive.

Example of the Issue

const state = Vue.reactive({
  user: {
    name: "Alice",
    details: {
      age: 25,
      city: "New York"
    }
  }
});

// Attempt to modify a nested property
state.user.details.age = 30;

In the above example, Vue will not detect the change in state.user.details.age, and the UI will not update.

Why Does This Happen?

Vue uses Proxies (Vue 3) or Object.defineProperty (Vue 2) to track reactivity, but it only applies them to the first level of the object. Any deeply nested properties remain non-reactive unless explicitly made reactive.

Solutions to Make Nested Objects Reactive

1. Use Vue’s `reactive()` with a Deep Object (Vue 3)

Vue 3’s reactive() method works better for deep objects compared to ref(). However, updates to nested objects must be wrapped in a new reference.

import { reactive } from "vue";

const state = reactive({
  user: {
    name: "Alice",
    details: {
      age: 25,
      city: "New York"
    }
  }
});

// Correct way: Create a new reference for nested properties
state.user.details = { ...state.user.details, age: 30 };

✅ Vue will now detect the change and trigger a UI update.

2. Use `ref()` for Deep Objects (Vue 3)

In Vue 3, ref() can be used instead of reactive() for deeply nested properties.

import { ref } from "vue";

const user = ref({
  name: "Alice",
  details: {
    age: 25,
    city: "New York"
  }
});

// Correct way: Use `ref()` for nested properties
user.value.details.age = 30;

✅ The change will now be detected by Vue.

3. Use Vue’s `set()` in Vue 2

In Vue 2, you can use Vue.set() to ensure nested properties are reactive.

const state = Vue.observable({
  user: {
    name: "Alice",
    details: {
      age: 25,
      city: "New York"
    }
  }
});

// Correct way: Use Vue.set() for nested properties in Vue 2
Vue.set(state.user.details, "age", 30);

✅ Vue will now react to the change.

4. Use `watch()` for Nested Properties

If you need to track changes in nested objects, use Vue’s watch() method with deep observation.

import { watch } from "vue";

const state = reactive({
  user: {
    name: "Alice",
    details: {
      age: 25,
      city: "New York"
    }
  }
});

// Watch nested properties
watch(
  () => state.user.details,
  (newVal, oldVal) => {
    console.log("Details changed:", newVal);
  },
  { deep: true }
);

// Updating nested property
state.user.details.age = 30;

✅ Vue will detect changes due to the { deep: true } option.

5. Replace the Entire Object Reference

Another way to trigger reactivity is by replacing the entire nested object reference.

state.user.details = { ...state.user.details, age: 30 };

✅ Vue detects the change because the reference has changed.

Best Practices to Ensure Nested Reactivity

✅ Prefer ref() for deeply nested properties in Vue 3.
✅ Use reactive() with shallow updates (replacing objects rather than modifying them).
✅ Use watch() with { deep: true } when you need to track changes in deeply nested objects.
✅ In Vue 2, use Vue.set() for modifying nested properties dynamically.
✅ Replace entire object references instead of modifying properties directly.

Conclusion

Vue’s reactivity system does not automatically track changes to deeply nested properties. However, by using reactive(), ref(), watch(), or Vue.set() correctly, you can ensure that your Vue application remains fully reactive and updates as expected. Always follow best practices to avoid unexpected behavior and keep your code clean and efficient.