Lifecycle hooks in Composition API in Vue - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run a function when the component is mounted.

Vue

<script setup>
import { [1] } from 'vue'

[1](() => {
  console.log('Component mounted')
})
</script>

Drag options to blanks, or click blank then click option'

AonMounted

BonCreated

ConInit

DonStart

Attempts:

3 left

💡 Hint

Common Mistakes

Using hooks that don't exist like onCreated or onInit.

Forgetting to import the hook from 'vue'.

The onMounted hook runs a function after the component is inserted into the DOM.

2fill in blank

medium

Complete the code to run cleanup code when the component is about to be removed.

Vue

<script setup>
import { onBefore[1] } from 'vue'

onBefore[1](() => {
  console.log('Cleanup before component unmounts')
})
</script>

Drag options to blanks, or click blank then click option'

ADetach

BDestroy

CRemove

DUnmount

Attempts:

3 left

💡 Hint

Common Mistakes

Using onBeforeDestroy which is from Vue 2 options API.

Misspelling the hook name.

The onBeforeUnmount hook runs just before the component is removed from the DOM, perfect for cleanup.

3fill in blank

hard

Fix the error in the code to run code when the component updates.

Vue

<script setup>
import { on[1] } from 'vue'

on[1](() => {
  console.log('Component updated')
})
</script>

Drag options to blanks, or click blank then click option'

AUpdating

BUpdate

CUpdated

DUpdateded

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'onUpdate' instead of 'onUpdated'.

Typo in the hook name.

The onUpdated hook runs after the component has updated and the DOM has been re-rendered.

4fill in blank

hard

Fill both blanks to run code before the component updates and after it updates.

Vue

<script setup>
import { onBefore[1], on[2] } from 'vue'

onBefore[1](() => {
  console.log('Before update')
})
on[2](() => {
  console.log('After update')
})
</script>

Drag options to blanks, or click blank then click option'

AUpdate

BUpdated

CUnmount

DMounted

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up 'Unmount' and 'Update' hooks.

Using 'Mounted' instead of 'Updated' for after update.

onBeforeUpdate runs before the component updates, and onUpdated runs after the update.

5fill in blank

hard

Fill all three blanks to run code when the component is mounted, before unmount, and after update.

Vue

<script setup>
import { [1], onBefore[2], on[3] } from 'vue'

[1](() => {
  console.log('Mounted')
})
onBefore[2](() => {
  console.log('Before unmount')
})
on[3](() => {
  console.log('Updated')
})
</script>

Drag options to blanks, or click blank then click option'

AonMounted

BUnmount

CUpdated

DonUpdated

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'onBeforeDestroy' instead of 'onBeforeUnmount'.

Confusing import names with hook suffixes.

Import onMounted, then use onBeforeUnmount and onUpdated hooks for the respective lifecycle events.

Practice

1. Which lifecycle hook in Vue's Composition API runs code right after the component is added to the page?

easy

A. onUpdated

B. onCreated

C. onDestroyed

D. onMounted

Solution

1. Step 1: Understand the lifecycle phases

   The onMounted hook runs after the component is inserted into the DOM.

2. Step 2: Match the hook to the action

   Since the question asks for the hook that runs right after adding to the page, onMounted is the correct choice.

3. Final Answer:

   onMounted -> Option D

4. Quick Check:

   Mounted = after insert [OK]

Hint: Remember: mounted means component is on the page [OK]

Common Mistakes:

• Confusing onMounted with onCreated
• Thinking onUpdated runs after mounting
• Using onDestroyed for setup tasks

2. Which of these is the correct way to import and use the onUnmounted hook inside the setup() function?

easy

A. import onUnmounted from 'vue';setup() { onUnmounted(() => { /* cleanup */ }); }

B. import { onUnmounted } from 'vue';setup() { onUnmounted(() => { /* cleanup */ }); }

C. import { onUnmounted } from 'vue';setup() { onUnmounted = () => { /* cleanup */ }; }

D. import { onUnmounted } from 'vue';setup() { onUnmounted; }

Solution

1. Step 1: Check import syntax

   The correct import uses curly braces: import { onUnmounted } from 'vue';

2. Step 2: Check usage inside setup

   The hook is called as a function with a callback: onUnmounted(() => { /* cleanup */ });

3. Final Answer:

   Correct import and call syntax -> Option B

4. Quick Check:

   Import with braces + call hook [OK]

Hint: Import hooks with braces and call them as functions [OK]

Common Mistakes:

• Importing without braces
• Assigning hook to a variable instead of calling
• Calling hook without a callback function

3. What will be logged to the console when this Vue component runs?

import { ref, onMounted } from 'vue';
export default {
  setup() {
    const count = ref(0);
    onMounted(() => {
      count.value = 5;
      console.log('Mounted count:', count.value);
    });
    return { count };
  }
}

medium

A. Mounted count: 5

B. Mounted count: 0

C. Mounted count: undefined

D. No output

Solution

1. Step 1: Understand ref and onMounted

   count starts at 0, then inside onMounted it is set to 5 and logged.

2. Step 2: Predict console output

   The console logs 'Mounted count:' followed by the updated value 5.

3. Final Answer:

   Mounted count: 5 -> Option A

4. Quick Check:

   Value updated before log in onMounted [OK]

Hint: onMounted runs after setup, so updated values show [OK]

Common Mistakes:

• Assuming count is still 0 inside onMounted
• Thinking onMounted runs before setup returns
• Confusing ref.value with ref itself

4. Identify the error in this Vue Composition API code snippet:

import { onMounted } from 'vue';
export default {
  setup() {
    onMounted = () => {
      console.log('Mounted');
    };
  }
}

medium

A. onMounted is incorrectly assigned instead of called

B. Missing import of onUnmounted

C. setup() should return a value

D. Console.log syntax is wrong

Solution

1. Step 1: Check how onMounted is used

   The code assigns a function to onMounted instead of calling it with a callback.

2. Step 2: Understand correct usage

   The correct usage is onMounted(() => { ... }), not assignment.

3. Final Answer:

   onMounted is incorrectly assigned instead of called -> Option A

4. Quick Check:

   Hooks are called, not reassigned [OK]

Hint: Use onMounted(callback), don't assign it like a variable [OK]

Common Mistakes:

• Assigning hook instead of calling it
• Forgetting to pass a function to onMounted
• Thinking setup must always return something

5. You want to fetch data when a component mounts and cancel a timer when it unmounts using Composition API. Which code snippet correctly uses lifecycle hooks to do this?

hard

A. import { ref, onUnmounted } from 'vue';
export default {
setup() {
const data = ref(null);
let timer = setInterval(() => console.log('tick'), 1000);
onUnmounted(() => {
clearInterval(timer);
});
return { data };
}
}

B. import { ref, onMounted } from 'vue';
export default {
setup() {
const data = ref(null);
onMounted(() => {
fetch('/api/data').then(res => res.json()).then(json => data.value = json);
setInterval(() => console.log('tick'), 1000);
});
return { data };
}
}

C. import { ref, onMounted, onUnmounted } from 'vue';
export default {
setup() {
const data = ref(null);
let timer;
onMounted(() => {
fetch('/api/data').then(res => res.json()).then(json => data.value = json);
timer = setInterval(() => console.log('tick'), 1000);
});
onUnmounted(() => {
clearInterval(timer);
});
return { data };
}
}

D. import { ref } from 'vue';
export default {
setup() {
const data = ref(null);
fetch('/api/data').then(res => res.json()).then(json => data.value = json);
let timer = setInterval(() => console.log('tick'), 1000);
return { data };
}
}

Solution

1. Step 1: Check data fetching on mount

   import { ref, onMounted, onUnmounted } from 'vue';
   export default {
     setup() {
       const data = ref(null);
       let timer;
       onMounted(() => {
         fetch('/api/data').then(res => res.json()).then(json => data.value = json);
         timer = setInterval(() => console.log('tick'), 1000);
       });
       onUnmounted(() => {
         clearInterval(timer);
       });
       return { data };
     }
   }

   uses onMounted to fetch data and start the timer, which is correct.

2. Step 2: Check cleanup on unmount

   import { ref, onMounted, onUnmounted } from 'vue';
   export default {
     setup() {
       const data = ref(null);
       let timer;
       onMounted(() => {
         fetch('/api/data').then(res => res.json()).then(json => data.value = json);
         timer = setInterval(() => console.log('tick'), 1000);
       });
       onUnmounted(() => {
         clearInterval(timer);
       });
       return { data };
     }
   }

   uses onUnmounted to clear the timer, preventing memory leaks.

3. Final Answer:

   Correctly uses onMounted and onUnmounted hooks -> Option C

4. Quick Check:

   Fetch on mount + cleanup on unmount [OK]

Hint: Use onMounted for setup, onUnmounted for cleanup [OK]

Common Mistakes:

• Starting timers outside lifecycle hooks
• Not clearing timers on unmount
• Fetching data outside onMounted