1# Async Component Error Handling
2 
3## Rule
4 
5Always configure error handling for async components using `errorComponent` and/or `onError` callback. Without proper error handling, failed component loads can leave the UI in an undefined state with no user feedback.
6 
7## Why This Matters
8 
9Network failures, timeouts, and server errors are common in production. Without error handling, users see blank spaces or broken UIs with no indication of what went wrong or how to recover.
10 
11## Bad Code
12 
13```vue
14<script setup>
15import { defineAsyncComponent } from 'vue'
16 
17// No error handling - fails silently
18const AsyncWidget = defineAsyncComponent(() =>
19  import('./Widget.vue')
20)
21</script>
22```
23 
24```vue
25<script setup>
26import { defineAsyncComponent } from 'vue'
27 
28// isLoading never becomes false on error - infinite spinner
29const isLoading = ref(true)
30const Widget = defineAsyncComponent({
31  loader: () => import('./Widget.vue').finally(() => {
32    isLoading.value = false  // Only runs on success
33  })
34})
35</script>
36```
37 
38## Good Code
39 
40```vue
41<script setup>
42import { defineAsyncComponent } from 'vue'
43import LoadingSpinner from './LoadingSpinner.vue'
44import ErrorDisplay from './ErrorDisplay.vue'
45 
46const AsyncWidget = defineAsyncComponent({
47  loader: () => import('./Widget.vue'),
48  loadingComponent: LoadingSpinner,
49  errorComponent: ErrorDisplay,
50  delay: 200,    // Prevent loading flicker
51  timeout: 10000 // Show error after 10 seconds
52})
53</script>
54```
55 
56```vue
57<script setup>
58import { defineAsyncComponent } from 'vue'
59 
60// With retry logic using onError
61const AsyncWidget = defineAsyncComponent({
62  loader: () => import('./Widget.vue'),
63  loadingComponent: LoadingSpinner,
64  errorComponent: ErrorDisplay,
65  onError(error, retry, fail, attempts) {
66    if (attempts <= 3) {
67      // Retry up to 3 times
68      retry()
69    } else {
70      // Give up and show error component
71      fail()
72    }
73  }
74})
75</script>
76```
77 
78```vue
79<script setup>
80import { defineAsyncComponent } from 'vue'
81 
82// Fallback component pattern - catch in loader
83const AsyncWidget = defineAsyncComponent(() =>
84  import('./Widget.vue').catch(() => import('./WidgetFallback.vue'))
85)
86</script>
87```
88 
89## onError Callback Parameters
90 
91The `onError` callback receives four arguments:
92 
93| Parameter | Type | Description |
94|-----------|------|-------------|
95| `error` | `Error` | The error that caused the load to fail |
96| `retry` | `Function` | Call to retry loading the component |
97| `fail` | `Function` | Call to give up and show errorComponent |
98| `attempts` | `number` | Number of load attempts so far |
99 
100## Key Points
101 
1021. Always provide an `errorComponent` for production applications
1032. Use `timeout` to prevent indefinite loading states
1043. Consider retry logic with `onError` for transient network issues
1054. The `delay` option (default 200ms) prevents loading flicker on fast networks
1065. Use the fallback pattern (`.catch()` in loader) when you want a seamless degradation
107 
108## SSR Warning
109 
110Using `onError` with SSR can cause issues in some configurations, potentially leading to infinite loading. Test thoroughly in SSR environments.
111 
112## References
113 
114- [Vue.js Async Components Documentation](https://vuejs.org/guide/components/async)
115- [Handling Async Components' loading errors](https://awad.dev/blog/handling-async-component-loading-errors/)
116