Component registration (global vs local) in Vue - Performance Comparison

Performance: Component registration (global vs local)

MEDIUM IMPACT

This affects initial page load speed and runtime rendering performance by controlling how many components are loaded and parsed upfront versus on demand.

Using components in a Vue app

Vue

import { createApp } from 'vue';
import App from './App.vue';

const app = createApp(App);

// Register locally inside component that uses it (e.g. in App.vue):
// import MyButton from './MyButton.vue'
// export default {
//   components: { MyButton }
// }

app.mount('#app');

Local registration loads components only when needed, reducing initial bundle size and speeding up first paint.

📈 Performance GainSaves 100-300ms on LCP by reducing initial bundle size and parsing time

Using components in a Vue app

Vue

import { createApp } from 'vue';
import MyButton from './MyButton.vue';
import App from './App.vue';

const app = createApp(App);
app.component('MyButton', MyButton); // global registration
app.mount('#app');

Global registration loads all components upfront, increasing bundle size and delaying initial render.

📉 Performance CostAdds extra KBs to initial bundle, increasing LCP by 100-300ms depending on component size

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Global registration	All components parsed upfront	Multiple reflows if many components render	Higher paint cost due to larger DOM	[X] Bad
Local registration	Only used components parsed	Fewer reflows as fewer components render initially	Lower paint cost with smaller DOM	[OK] Good

Rendering Pipeline

Global registration adds components to the app instance before mounting, increasing initial JavaScript parsing and bundle size. Local registration scopes components to where they are used, allowing tree shaking and smaller bundles. This reduces the time spent in script evaluation and layout calculation during initial render.

→JavaScript Parsing

→Style Calculation

→Layout

→Paint

⚠️ BottleneckJavaScript Parsing and Initial Layout

Core Web Vital Affected

LCP

This affects initial page load speed and runtime rendering performance by controlling how many components are loaded and parsed upfront versus on demand.

Optimization Tips

1Avoid global registration for many components to keep initial bundle small.

2Use local registration to load components only when needed.

3Combine local registration with lazy loading for best load performance.

Performance Quiz - 3 Questions

What is a main performance downside of globally registering many Vue components?

A Causes more CSS recalculations

B Increases initial bundle size and delays first paint

C Prevents lazy loading of images

D Blocks user input events

DevTools: Performance

How to check: Record page load and look at scripting and rendering times. Check bundle size in Network panel.

What to look for: Long scripting times and large JS bundles indicate global registration impact; faster scripting and smaller bundles indicate local registration.

Practice

1. What is the main difference between global and local component registration in Vue?

easy

A. Global registration requires importing components manually, local does not.

B. Local registration makes a component available everywhere, global limits it to one component.

C. Global registration makes a component available everywhere, local limits it to one component.

D. Local registration automatically registers components globally.

Solution

1. Step 1: Understand global registration

   Global registration means the component can be used in any template without importing it again.

2. Step 2: Understand local registration

   Local registration means the component is only available inside the component where it is registered.

3. Final Answer:

   Global registration makes a component available everywhere, local limits it to one component. -> Option C

4. Quick Check:

   Global = everywhere, Local = one component [OK]

Hint: Global = everywhere, local = one place only [OK]

Common Mistakes:

• Confusing global with local availability
• Thinking local registration is automatic
• Assuming global requires manual import every time

2. Which of the following is the correct syntax to register a component locally inside a Vue component?

easy

A. export default { globalComponents: { MyComponent } }

B. Vue.component('MyComponent', MyComponent)

C. import MyComponent from './MyComponent.vue';

D. components: { MyComponent }

Solution

1. Step 1: Identify local registration syntax

   Local registration uses the components option inside the component definition object.

2. Step 2: Match correct syntax

   The syntax is components: { MyComponent } to register locally.

3. Final Answer:

   components: { MyComponent } -> Option D

4. Quick Check:

   Local registration uses components: { } [OK]

Hint: Local registration uses components: { ComponentName } [OK]

Common Mistakes:

• Using Vue.component() for local registration
• Confusing import statement with registration
• Trying to use globalComponents option which doesn't exist

3. Given this code, what will be the output when rendering <ParentComponent/>?

const ChildComponent = {
  template: '
Child here
'
}

const ParentComponent = {
  template: '
',
  components: { ChildComponent }
}

app.component('parent-component', ParentComponent)
app.mount('#app')

medium

A. Child here

B. Error: Unknown custom element

C.

D. Child here

Solution

1. Step 1: Check local registration in ParentComponent

   ChildComponent is registered locally inside ParentComponent using components: { ChildComponent }.

2. Step 2: Understand template rendering

   The tag inside ParentComponent's template will render ChildComponent's template

   Child here

   only if the tag name matches the registered component name in kebab-case.

3. Step 3: Identify the issue

   The component is registered as ChildComponent but used as , which Vue treats as a different tag. Since 'child-component' is not globally registered, Vue will throw an unknown custom element error.

4. Final Answer:

   Error: Unknown custom element <child-component> -> Option B

5. Quick Check:

   Component names are case-sensitive and must match usage [OK]

Hint: Component names must match tag names exactly or use kebab-case [OK]

Common Mistakes:

• Assuming PascalCase component names automatically match kebab-case tags
• Forgetting to register components globally when using kebab-case tags
• Expecting local registration to work with mismatched tag names

4. What is wrong with this Vue code snippet?

const app = Vue.createApp({})

app.component('MyButton', {
  template: 'Click me'
})

const AnotherComponent = {
  template: ''
}

app.component('AnotherComponent', AnotherComponent)

app.mount('#app')

medium

A. The tag inside AnotherComponent template should be lowercase .

B. MyButton component is not registered globally.

C. AnotherComponent is missing local registration of MyButton.

D. Vue.createApp() cannot register components.

Solution

1. Step 1: Check component registration

   MyButton is registered globally using app.component('MyButton', ...), so it is available everywhere.

2. Step 2: Check template tag casing

   Vue requires component tags in templates to be kebab-case or PascalCase but HTML is case-insensitive, so should be .

3. Final Answer:

   The <MyButton /> tag inside AnotherComponent template should be lowercase <my-button />. -> Option A

4. Quick Check:

   Component tags must be kebab-case in templates [OK]

Hint: Use kebab-case for component tags in templates [OK]

Common Mistakes:

• Using PascalCase tags directly in templates
• Thinking global registration needs import in every component
• Assuming Vue.createApp() can't register components

5. You have a button component used in many places and a modal component used only inside one page component. How should you register them for best practice?

hard

A. Register the button component globally and the modal component locally inside the page component.

B. Register both components globally for consistency.

C. Register both components locally inside each component that uses them.

D. Register the modal component globally and the button component locally.

Solution

1. Step 1: Consider usage frequency

   The button is used many places, so global registration avoids repeated imports and registrations.

2. Step 2: Consider component scope

   The modal is used only inside one page component, so local registration keeps scope limited and reduces global clutter.

3. Final Answer:

   Register the button component globally and the modal component locally inside the page component. -> Option A

4. Quick Check:

   Global for many uses, local for few uses [OK]

Hint: Global for shared, local for limited use components [OK]

Common Mistakes:

• Registering rarely used components globally unnecessarily
• Registering widely used components locally repeatedly
• Ignoring app size and performance considerations