Error boundaries with onErrorCaptured in Vue - Step-by-Step Execution

Concept Flow - Error boundaries with onErrorCaptured

Child Component Throws Error

↓

onErrorCaptured Hook in Parent

↓

Return false

↓

Stop Propagation

↓

Parent Handles Error or Passes Up

When a child component throws an error, the parent's onErrorCaptured hook runs. Returning false stops error propagation; otherwise, it continues up.

Execution Sample

Vue

import { defineComponent, ref } from 'vue';

export default defineComponent({
  setup() {
    const errorMsg = ref('');
    function onErrorCaptured(err) {
      errorMsg.value = err.message;
      return false;
    }
    return { errorMsg, onErrorCaptured };
  }
});

A parent component uses onErrorCaptured to catch errors from children and store the error message.

Execution Table

Step	Event	Error Thrown?	onErrorCaptured Called?	Return Value	Error Propagation	State Change
1	Child renders normally	No	No	N/A	N/A	errorMsg = ''
2	Child throws error 'Oops!'	Yes	Yes	false	Stopped	errorMsg = 'Oops!'
3	Parent renders error message	No	No	N/A	N/A	errorMsg = 'Oops!'
4	Another error thrown	Yes	Yes	false	Stopped	errorMsg = 'Another error!'
5	Error propagation stopped, no further handling	N/A	N/A	N/A	N/A	errorMsg holds last error

💡 Error propagation stops when onErrorCaptured returns false, preventing further error bubbling.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
errorMsg	''	'Oops!'	'Another error!'	'Another error!'

Key Moments - 2 Insights

Why does returning false in onErrorCaptured stop error propagation?

Returning false tells Vue the error is handled here, so it stops sending the error to parent components, as shown in step 2 of the execution_table.

What happens if onErrorCaptured returns true or nothing?

The error keeps propagating up to higher parents or global handlers, because only false stops propagation. This is implied by the flow in concept_flow.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of errorMsg after step 2?

A 'Oops!'

B ''

C 'Another error!'

D undefined

💡 Hint

Check the 'State Change' column at step 2 in the execution_table.

At which step does onErrorCaptured stop error propagation?

A Step 3

B Step 1

C Step 2

D Step 5

💡 Hint

Look at the 'Error Propagation' column in the execution_table.

If onErrorCaptured returned true instead of false, what would change in the execution_table?

A errorMsg would not update

B Error propagation would continue to parents

C Error propagation would stop immediately

D Child would not throw error

💡 Hint

Refer to the 'Return Value' and 'Error Propagation' columns in the execution_table.

Concept Snapshot

Error boundaries in Vue use onErrorCaptured hook.
onErrorCaptured runs when a child error occurs.
Return false to stop error propagation.
Return true or nothing to continue bubbling.
Use to catch and handle errors gracefully.

Full Transcript

In Vue, error boundaries are created using the onErrorCaptured hook in a parent component. When a child component throws an error, Vue calls onErrorCaptured with the error. If the hook returns false, Vue stops sending the error to higher parents, effectively catching it. If it returns true or nothing, the error continues to propagate upwards. This lets you handle errors locally and prevent app crashes. The error message can be stored in a reactive variable to show feedback in the UI. This visual trace shows how errorMsg updates when errors occur and how returning false stops propagation.

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