Error boundaries with onErrorCaptured in Vue - Performance & Optimization

Performance: Error boundaries with onErrorCaptured

MEDIUM IMPACT

This concept affects the rendering stability and responsiveness by catching errors in component trees without crashing the whole app.

Handling runtime errors in Vue components to avoid app crashes

Vue

import { defineComponent, onErrorCaptured } from 'vue';

const ErrorChild = defineComponent({
  setup() {
    throw new Error('Oops!');
  },
  template: `<div>Child</div>`
});

export default defineComponent({
  components: { ErrorChild },
  setup() {
    onErrorCaptured((err, instance, info) => {
      console.error('Captured error:', err);
      return false; // prevent further propagation
    });
  },
  template: `<div>Test with error boundary <ErrorChild /></div>`
});

Catches errors locally, prevents app crash, and allows fallback UI or graceful degradation.

📈 Performance GainAvoids full app re-render or crash, improving INP and user experience.

Handling runtime errors in Vue components to avoid app crashes

Vue

const MyComponent = {
  setup() {
    // No error boundary
    throw new Error('Oops!');
  },
  template: `<div>Test</div>`
}

Without error boundaries, any error crashes the entire Vue app, causing a blank screen and poor user experience.

📉 Performance CostBlocks rendering completely on error, causing poor INP and user frustration.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
No error boundary	Full tree re-render on error	Multiple reflows if error causes remount	High paint cost due to full blank screen	[X] Bad
Using onErrorCaptured	Error caught locally, limited re-render	Single reflow for fallback UI	Low paint cost, stable UI	[OK] Good

Rendering Pipeline

When an error occurs in a component, onErrorCaptured intercepts it during the rendering phase, preventing the error from propagating and crashing the app. This allows Vue to continue rendering other components or show fallback UI.

→Render

→Commit

→Error Handling

⚠️ BottleneckError handling logic adds slight overhead during render but prevents costly full re-renders or crashes.

Core Web Vital Affected

INP

This concept affects the rendering stability and responsiveness by catching errors in component trees without crashing the whole app.

Optimization Tips

1Use onErrorCaptured to catch errors locally and prevent full app crashes.

2Keep error handling logic lightweight to avoid slowing rendering.

3Test error boundaries with DevTools Performance panel to ensure minimal impact.

Performance Quiz - 3 Questions

What is the main performance benefit of using onErrorCaptured in Vue?

A Prevents full app crash and reduces re-rendering on errors

B Improves initial page load speed

C Reduces CSS paint cost

D Eliminates all JavaScript errors

DevTools: Performance

How to check: Record a session causing a component error, then inspect the flame chart for re-render and error handling duration.

What to look for: Look for reduced full app re-renders and stable frame times indicating error boundaries prevent crashes.

Practice

1. What is the main purpose of the onErrorCaptured hook in Vue?

easy

A. To prevent any rendering of child components

B. To automatically reload the page when an error occurs

C. To log errors only in the browser console without affecting UI

D. To catch errors from child components and handle them gracefully

Solution

1. Step 1: Understand the role of error boundaries

   Error boundaries catch errors in child components to prevent the whole app from crashing.

2. Step 2: Identify what onErrorCaptured does

   This hook lets you catch those errors and handle them, for example by showing fallback UI.

3. Final Answer:

   To catch errors from child components and handle them gracefully -> Option D

4. Quick Check:

   Catches child errors gracefully [OK]

Hint: Remember: onErrorCaptured catches child errors for graceful handling [OK]

Common Mistakes:

• Thinking it reloads the page automatically
• Believing it blocks all child rendering
• Assuming it only logs errors without UI impact

2. Which of the following is the correct way to use onErrorCaptured in a Vue 3 Composition API setup function?

easy

A. onErrorCaptured.handleError(error => { /* handle error */ })

B. onErrorCaptured((error, instance, info) => { /* handle error */ })

C. setup() { onErrorCaptured: function(error) { /* handle error */ } }

D. onErrorCaptured = (error) => { /* handle error */ }

Solution

1. Step 1: Recall the correct syntax for hooks in Composition API

   Hooks like onErrorCaptured are called as functions with a callback.

2. Step 2: Match the syntax with options

   onErrorCaptured((error, instance, info) => { /* handle error */ }) correctly shows calling onErrorCaptured with a callback function receiving error details.

3. Final Answer:

   onErrorCaptured((error, instance, info) => { /* handle error */ }) -> Option B

4. Quick Check:

   Hook usage = function call with callback [OK]

Hint: Hooks are functions called with callbacks in setup() [OK]

Common Mistakes:

• Assigning onErrorCaptured instead of calling it
• Using object syntax inside setup incorrectly
• Calling non-existent methods on onErrorCaptured

3. Given this Vue component snippet, what will be the console output when ChildComponent throws an error?

setup() {
  onErrorCaptured((err) => {
    console.log('Error caught:', err.message)
    return false
  })
}

medium

A. Error caught: [error message]

B. No output, error is ignored

C. Error caught: [error message] and error propagates further

D. Runtime error: onErrorCaptured not defined

Solution

1. Step 1: Understand onErrorCaptured behavior with return false

   Returning false stops error propagation and prevents further bubbling.

2. Step 2: Analyze console.log call

   The callback logs the error message, so console shows 'Error caught:' plus the error message.

3. Final Answer:

   Error caught: [error message] -> Option A

4. Quick Check:

   Return false stops propagation, logs error [OK]

Hint: Return false in onErrorCaptured stops error bubbling [OK]

Common Mistakes:

• Assuming error propagates despite return false
• Expecting no console output
• Confusing syntax causing runtime errors

4. Identify the error in this Vue 3 setup function using onErrorCaptured:

setup() {
  onErrorCaptured = (error) => {
    console.error(error)
    return false
  }
}

medium

A. Incorrect assignment instead of calling onErrorCaptured as a function

B. Missing import of onErrorCaptured from Vue

C. Returning false is not allowed in onErrorCaptured

D. console.error cannot be used inside setup

Solution

1. Step 1: Check how onErrorCaptured is used

   It should be called as a function with a callback, not assigned to a function.

2. Step 2: Identify the incorrect syntax

   The code assigns a function to onErrorCaptured instead of calling it, causing an error.

3. Final Answer:

   Incorrect assignment instead of calling onErrorCaptured as a function -> Option A

4. Quick Check:

   Hooks are called, not assigned [OK]

Hint: Use onErrorCaptured(callback), not assignment [OK]

Common Mistakes:

• Assigning hook instead of calling it
• Forgetting to import onErrorCaptured
• Misunderstanding return false usage

5. You want to create an error boundary component in Vue 3 that catches errors from its children and shows a fallback message. Which approach correctly uses onErrorCaptured to stop error propagation and display fallback UI?

hard

A. Wrap children in try-catch inside template to catch errors

B. Use onErrorCaptured but do not return anything, so errors propagate

C. Use onErrorCaptured to set a reactive error state and return false to stop propagation

D. Use onErrorCaptured and always return true to propagate errors

Solution

1. Step 1: Understand error boundary behavior

   An error boundary catches errors and shows fallback UI by tracking error state.

2. Step 2: Use onErrorCaptured to update error state and stop propagation

   Returning false in onErrorCaptured stops error bubbling, allowing fallback UI to show.

3. Final Answer:

   Use onErrorCaptured to set a reactive error state and return false to stop propagation -> Option C

4. Quick Check:

   Set error state + return false = error boundary [OK]

Hint: Return false in onErrorCaptured to stop error bubbling [OK]

Common Mistakes:

• Not returning false, so errors keep propagating
• Trying to catch errors in template with try-catch
• Returning true which continues error propagation