Shallow ref and shallow reactive in Vue - Performance & Optimization

Performance: Shallow ref and shallow reactive

MEDIUM IMPACT

This affects how Vue tracks changes and updates the DOM, impacting rendering speed and responsiveness.

Tracking changes on a large nested object where only top-level properties change

Vue

const state = shallowReactive({ user: { name: 'Alice', age: 30, address: { city: 'NY', zip: '10001' } } });

Vue tracks only top-level properties, ignoring nested changes, reducing watchers and update overhead.

📈 Performance Gainsingle reflow per top-level change, faster rendering

Tracking changes on a large nested object where only top-level properties change

Vue

const state = reactive({ user: { name: 'Alice', age: 30, address: { city: 'NY', zip: '10001' } } });

Vue deeply tracks all nested properties, causing many watchers and slow updates even if only top-level changes occur.

📉 Performance Costtriggers multiple reflows and recalculations on any nested change

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Deep reactive/ref	Many nested watchers	Multiple reflows on nested changes	High paint cost due to frequent updates	[X] Bad
Shallow reactive/ref	Watchers only on top-level	Single reflow per top-level change	Lower paint cost, fewer updates	[OK] Good

Rendering Pipeline

Vue's reactivity system tracks changes and triggers updates that flow through style calculation, layout, paint, and composite stages. Shallow refs/reactive reduce the number of tracked dependencies, lowering the cost in these stages.

→Dependency Tracking

→Style Calculation

→Layout

→Paint

⚠️ BottleneckDependency Tracking and Layout due to deep watchers and reflows

Core Web Vital Affected

INP

This affects how Vue tracks changes and updates the DOM, impacting rendering speed and responsiveness.

Optimization Tips

1Use shallowReactive or shallowRef when you only need to track top-level changes.

2Avoid deep reactive tracking on large or complex nested objects to reduce reflows.

3Check performance with DevTools to identify expensive reactive updates.

Performance Quiz - 3 Questions

What is the main performance benefit of using shallowReactive over reactive in Vue?

A It tracks only top-level properties, reducing watchers and reflows

B It automatically caches computed properties

C It disables reactivity completely

D It increases bundle size for better debugging

DevTools: Performance

How to check: Record a performance profile while interacting with the component. Look for long tasks caused by reactive updates and frequent layout recalculations.

What to look for: High number of reactive dependency recalculations and layout thrashing indicate deep reactive overhead; fewer and shorter tasks indicate shallow reactive efficiency.

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