Custom v-model on components in Vue - Performance & Optimization

Performance: Custom v-model on components

MEDIUM IMPACT

This affects how component state updates propagate and how often the DOM updates due to two-way binding.

Binding a custom input component with two-way data binding

Vue

<CustomInput v-model="inputValue" />

// Inside CustomInput emits 'update:modelValue' event only

Vue handles syncing automatically with a single event, reducing re-renders.

📈 Performance Gainsingle re-render per input event

Binding a custom input component with two-way data binding

Vue

<CustomInput v-model="inputValue" @input="val => inputValue = val" />

Manually syncing input events causes double updates and extra re-renders.

📉 Performance Costtriggers 2 re-renders per input event

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Manual event sync with extra listeners	Multiple reactive updates	2+ reflows per input	Higher paint cost due to repeated DOM patches	[X] Bad
Single 'update:modelValue' event emission	Single reactive update	1 reflow per input	Lower paint cost with minimal DOM patches	[OK] Good

Rendering Pipeline

Custom v-model updates trigger component state changes that flow through Vue's reactive system, causing reactivity tracking, virtual DOM diffing, and DOM patching.

→Reactivity Tracking

→Virtual DOM Diffing

→DOM Patching

⚠️ BottleneckExcessive or redundant event emissions cause multiple reactivity updates and DOM patches.

Core Web Vital Affected

INP

This affects how component state updates propagate and how often the DOM updates due to two-way binding.

Optimization Tips

1Emit only one 'update:modelValue' event per input change.

2Avoid manually syncing input events alongside v-model.

3Use Vue's built-in v-model syntax for automatic efficient binding.

Performance Quiz - 3 Questions

What causes extra re-renders when using custom v-model on Vue components?

A Binding with v-bind instead of v-model

B Using only 'update:modelValue' event emission

C Manually syncing input events alongside v-model

D Using computed properties inside the component

DevTools: Performance

How to check: Record a performance profile while interacting with the input. Look for multiple component updates or repeated DOM patches.

What to look for: Fewer component updates and shorter scripting times indicate better custom v-model performance.

Practice

1. What is the default prop name used in a Vue component to work with v-model?

easy

A. inputValue

B. value

C. vModel

D. modelValue

Solution

1. Step 1: Understand Vue's default v-model prop

   Vue uses modelValue as the default prop name to bind data from parent to child.

2. Step 2: Recall the default event name

   The event emitted to update the parent is update:modelValue, matching the prop name.

3. Final Answer:

   modelValue -> Option D

4. Quick Check:

   Default prop for v-model = modelValue [OK]

Hint: Default v-model prop is always 'modelValue' [OK]

Common Mistakes:

• Using 'value' as default prop
• Confusing event name with prop name
• Assuming 'vModel' is default prop

2. Which of the following is the correct syntax to use a custom v-model named checked on a component?

easy

A. <MyCheckbox v-model:checked="isChecked" />

B. <MyCheckbox v-model.checked="isChecked" />

C. <MyCheckbox v-model_checked="isChecked" />

D. <MyCheckbox v-modelChecked="isChecked" />

Solution

1. Step 1: Recall custom v-model syntax

   Vue uses v-model:customName syntax to bind custom model names.

2. Step 2: Match syntax with options

   Only <MyCheckbox v-model:checked="isChecked" /> uses the correct colon syntax for custom v-model binding.

3. Final Answer:

   <MyCheckbox v-model:checked="isChecked" /> -> Option A

4. Quick Check:

   Custom v-model uses colon syntax [OK]

Hint: Use colon for custom v-model names: v-model:customName [OK]

Common Mistakes:

• Using dot instead of colon
• Using underscore instead of colon
• Using camelCase without colon

3. Given this child component code:

props: ['modelValue'],
methods: {
  updateValue(val) {
    this.$emit('update:modelValue', val)
  }
}

And parent usage:

<ChildComponent v-model="parentData" />

What happens when updateValue('new') is called inside the child?

medium

A. The child's modelValue prop changes to 'new'.

B. Nothing happens because event name is wrong.

C. The parent's parentData updates to 'new'.

D. An error occurs because updateValue is not defined.

Solution

1. Step 1: Understand event emission in child

   The child emits update:modelValue with the new value, which Vue listens to for v-model updates.

2. Step 2: Effect on parent data

   The parent bound to v-model receives the event and updates parentData to 'new'.

3. Final Answer:

   The parent's parentData updates to 'new'. -> Option C

4. Quick Check:

   Child emits update:modelValue -> parent updates [OK]

Hint: Child emits update:modelValue event to update parent data [OK]

Common Mistakes:

• Thinking prop changes inside child
• Confusing event name with prop name
• Assuming no update without v-model modifier

4. Identify the error in this child component code for custom v-model named checked:

props: ['checked'],
methods: {
  updateChecked(val) {
    this.$emit('update:checked', val)
  }
}

Parent uses:

<MyCheckbox v-model:checked="isChecked" />

medium

A. Event name should be update:modelValue for custom v-model.

B. Prop name and event name are correct for custom v-model checked.

C. Prop name should be modelValue instead of checked.

D. The method should emit checked event, not update:checked.

Solution

1. Step 1: Check prop and event names for custom v-model

   For v-model:checked, the prop must be checked and event update:checked.

2. Step 2: Verify code matches custom v-model pattern

   The code uses props: ['checked'] and emits update:checked, which is correct.

3. Final Answer:

   Prop name and event name are correct for custom v-model checked. -> Option B

4. Quick Check:

   Custom v-model uses matching prop and update:event [OK]

Hint: Custom v-model uses prop and event names matching the model name [OK]

Common Mistakes:

• Using default modelValue prop for custom v-model
• Emitting wrong event name
• Confusing event and prop names

5. You want to create a component ToggleSwitch that uses a custom v-model named active. Which of these code snippets correctly implements the child component's props and emits to support this?

hard

A. props: ['active'],
methods: {
toggle() {
this.$emit('update:active', !this.active)
}
}

B. props: ['modelValue'],
methods: {
toggle() {
this.$emit('update:modelValue', !this.modelValue)
}
}

C. props: ['active'],
methods: {
toggle() {
this.$emit('update:modelValue', !this.active)
}
}

D. props: ['modelValue'],
methods: {
toggle() {
this.$emit('update:active', !this.modelValue)
}
}

Solution

1. Step 1: Match prop and event names to custom v-model name

   For v-model:active, the prop must be active and event update:active.

2. Step 2: Check each option for correct prop and event

   props: ['active'],
   methods: {
     toggle() {
       this.$emit('update:active', !this.active)
     }
   }

   uses props: ['active'] and emits update:active, matching the custom v-model name.

3. Final Answer:

   props: ['active'], methods: { toggle() { this.$emit('update:active', !this.active) } } -> Option A

4. Quick Check:

   Custom v-model prop and event must match model name [OK]

Hint: Custom v-model prop and event must match the model name exactly [OK]

Common Mistakes:

• Mixing default modelValue with custom names
• Emitting wrong event name
• Using wrong prop name for custom v-model