Shallow ref and shallow reactive in Vue

Introduction

Shallow ref and shallow reactive help you track changes only on the top level of an object or array. This makes your app faster when you don't need to watch deep changes.

You have a big object but only care if the whole object changes, not its inner parts.

You want to improve performance by avoiding deep tracking of nested data.

You receive data from an API and only want to react when the main object changes.

You want to store a complex object but don't want Vue to make all nested properties reactive.

Syntax

Vue

import { shallowRef, shallowReactive } from 'vue';

const myShallowRef = shallowRef(initialValue);
const myShallowReactive = shallowReactive(initialObject);

shallowRef tracks changes only when the reference itself changes, not inside the object.

shallowReactive makes the top-level properties reactive but does not make nested objects reactive.

Examples

This example shows shallowRef holding an object. Changing properties inside user.value won't trigger updates, but replacing user.value with a new object will.

Vue

import { shallowRef } from 'vue';

const user = shallowRef({ name: 'Anna', age: 25 });

// Changing user.value to a new object triggers updates
user.value = { name: 'Ben', age: 30 };

Here, settings.theme change is reactive, but changing options.fontSize inside the nested object is not reactive.

Vue

import { shallowReactive } from 'vue';

const settings = shallowReactive({ theme: 'dark', options: { fontSize: 14 } });

settings.theme = 'light'; // reactive
settings.options.fontSize = 16; // NOT reactive

Sample Program

This Vue app shows how shallowRef and shallowReactive behave. Only changes to the top-level object or properties trigger the watchers and console logs.

Vue

import { createApp, shallowRef, shallowReactive, watch } from 'vue';

const App = {
  setup() {
    const shallowUser = shallowRef({ name: 'Anna', age: 25 });
    const shallowSettings = shallowReactive({ theme: 'dark', options: { fontSize: 14 } });

    watch(() => shallowUser.value, () => {
      console.log('shallowUser changed');
    });

    watch(() => shallowSettings.theme, () => {
      console.log('Theme changed');
    });

    watch(() => shallowSettings.options.fontSize, () => {
      console.log('Font size changed');
    });

    // Change nested property inside shallowRef (no trigger)
    shallowUser.value.name = 'Ben';
    // Replace shallowRef value (triggers)
    shallowUser.value = { name: 'Cara', age: 28 };

    // Change top-level shallowReactive property (triggers)
    shallowSettings.theme = 'light';
    // Change nested shallowReactive property (no trigger)
    shallowSettings.options.fontSize = 16;

    return {};
  },
  template: `<div>Check console for watch logs.</div>`
};

createApp(App).mount('#app');

Important Notes

Use shallowRef when you want to track changes only when the whole object changes, not its inner parts.

Use shallowReactive when you want reactivity on top-level properties but want to ignore nested objects for performance.

Remember that nested changes inside shallow reactive objects or shallow refs do not trigger updates.

Summary

Shallow ref tracks changes only when the reference changes, not inside the object.

Shallow reactive tracks changes on top-level properties only, ignoring nested objects.

Use them to improve performance when deep reactivity is not needed.

Practice

1. What does shallowRef in Vue track reactively?

easy

A. All nested properties deeply inside the object

B. Only primitive values inside the object

C. Only changes to the reference itself, not nested properties

D. Changes only to arrays, not objects

Solution

1. Step 1: Understand shallowRef behavior

   shallowRef tracks reactivity only when the reference changes, not inside nested objects.

2. Step 2: Compare with deep reactivity

   Unlike ref or reactive, it does not track nested property changes.

3. Final Answer:

   Only changes to the reference itself, not nested properties -> Option C

4. Quick Check:

   shallowRef tracks reference, not deep changes [OK]

Hint: shallowRef tracks only the outer reference change [OK]

Common Mistakes:

• Thinking shallowRef tracks nested property changes
• Confusing shallowRef with reactive
• Assuming shallowRef tracks primitive values inside

2. Which of the following is the correct way to create a shallow reactive object in Vue?

easy

A. const state = reactive({ count: 0, nested: { value: 1 } })

B. const state = shallowReactive({ count: 0, nested: { value: 1 } })

C. const state = shallowRef({ count: 0, nested: { value: 1 } })

D. const state = ref({ count: 0, nested: { value: 1 } })

Solution

1. Step 1: Identify shallow reactive syntax

   shallowReactive is used to create a shallow reactive object that tracks only top-level properties.

2. Step 2: Differentiate from other APIs

   reactive creates deep reactivity, shallowRef and ref create refs, not shallow reactive objects.

3. Final Answer:

   const state = shallowReactive({ count: 0, nested: { value: 1 } }) -> Option B

4. Quick Check:

   shallowReactive creates shallow reactive objects [OK]

Hint: Use shallowReactive() for shallow reactive objects [OK]

Common Mistakes:

• Using reactive() instead of shallowReactive()
• Confusing shallowRef with shallowReactive
• Using ref() for reactive objects

3. Given the code:

const state = shallowReactive({ count: 0, nested: { value: 1 } });

state.nested.value = 2;
console.log(state.nested.value);

What will be logged to the console?

medium

A. Error: Cannot assign to nested property

B. 1

C. undefined

D. 2

Solution

1. Step 1: Understand shallowReactive behavior

   shallowReactive tracks only top-level properties reactively, but nested objects still exist and can be mutated normally.

2. Step 2: Check nested property assignment

   Assigning state.nested.value = 2 changes the nested object property directly, so the value updates.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Nested objects update but are not reactive [OK]

Hint: shallowReactive allows nested mutation but no reactivity inside [OK]

Common Mistakes:

• Assuming nested changes are not possible
• Expecting nested changes to trigger reactivity
• Thinking assignment causes error

4. What is the problem with this code snippet?

const state = shallowRef({ count: 0, nested: { value: 1 } });

state.value.nested.value = 5;
console.log(state.value.nested.value);

medium

A. Nested property change won't trigger reactivity

B. Syntax error due to incorrect property access

C. state.value is undefined

D. Cannot assign to nested property inside shallowRef

Solution

1. Step 1: Analyze shallowRef behavior

   shallowRef tracks only changes to the reference itself, not nested properties inside the object.

2. Step 2: Understand nested mutation effect

   Changing state.value.nested.value updates the nested property but does not trigger Vue reactivity updates.

3. Final Answer:

   Nested property change won't trigger reactivity -> Option A

4. Quick Check:

   shallowRef ignores nested property changes [OK]

Hint: shallowRef ignores nested changes, only tracks reference [OK]

Common Mistakes:

• Expecting nested changes to trigger updates
• Confusing shallowRef with reactive
• Thinking this causes syntax or runtime errors

5. You want to optimize performance by tracking only top-level changes in a complex object with nested data. Which approach correctly combines shallow reactivity with a watch to detect nested changes manually?

const state = shallowReactive({ user: { name: 'Alice', age: 30 } });

watch(() => state.user, (newVal, oldVal) => {
  console.log('User changed:', newVal);
});

state.user.name = 'Bob';

What is the expected behavior?

hard

A. The watch callback runs only if state.user is replaced entirely

B. The watch callback runs immediately when state.user.name changes

C. The watch callback will run when state.user.name changes

D. The watch callback runs twice for nested changes

Solution

1. Step 1: Understand shallowReactive and watch

   shallowReactive tracks only top-level properties reactively. The watch on state.user triggers only if the user object reference changes.

2. Step 2: Analyze nested property mutation

   Changing state.user.name mutates a nested property but does not replace the user object, so the watch callback does not run.

3. Final Answer:

   The watch callback runs only if state.user is replaced entirely -> Option A

4. Quick Check:

   shallowReactive watches top-level refs only [OK]

Hint: Watch triggers only on reference changes with shallowReactive [OK]

Common Mistakes:

• Expecting watch to detect nested property changes
• Assuming watch runs on every nested mutation
• Confusing shallowReactive with deep reactive