Effective scope for cleanup in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens to the cleanup function in this Vue 3 component?

Consider this Vue 3 component using the Composition API. What will happen when the component unmounts?

Vue

import { ref, onMounted, onUnmounted } from 'vue';

export default {
  setup() {
    const count = ref(0);

    onMounted(() => {
      const interval = setInterval(() => {
        count.value++;
      }, 1000);

      onUnmounted(() => {
        clearInterval(interval);
      });
    });

    return { count };
  }
}

A The component will throw an error because onUnmounted cannot be called inside onMounted.

B The interval will be cleared properly when the component unmounts, stopping the count increment.

C The interval will never be cleared because onUnmounted is nested inside onMounted, so cleanup won't run.

D The count will reset to zero when the component unmounts due to the cleanup function.

💡 Hint

Think about when lifecycle hooks run and if nesting affects their execution.

In Vue 3, lifecycle hooks like onUnmounted can be called inside onMounted. The cleanup function registered with onUnmounted will run when the component unmounts, clearing the interval properly.

❓ lifecycle

intermediate

When is the cleanup function called in Vue 3's watchEffect?

In Vue 3, watchEffect can return a cleanup function. When exactly is this cleanup function executed?

Vue

import { ref, watchEffect } from 'vue';

export default {
  setup() {
    const count = ref(0);

    watchEffect((onCleanup) => {
      console.log('Effect runs with count:', count.value);

      onCleanup(() => {
        console.log('Cleanup for count:', count.value);
      });
    });

    return { count };
  }
}

A The cleanup function runs after the effect runs each time.

B The cleanup function runs only once when the component unmounts.

C The cleanup function runs before the effect re-runs and when the component unmounts.

D The cleanup function runs only if the watched value changes to null.

💡 Hint

Think about how watchEffect manages reactive dependencies and cleanup.

The cleanup function in watchEffect runs before the effect re-runs to clean up previous side effects, and also when the component unmounts to avoid memory leaks.

🔧 Debug

advanced

Why does this Vue 3 cleanup function not run on unmount?

Examine the code below. Why does the cleanup function not execute when the component unmounts?

Vue

import { ref, onMounted, onUnmounted } from 'vue';

export default {
  setup() {
    const timer = ref(null);

    onMounted(() => {
      timer.value = setTimeout(() => {
        console.log('Timer done');
      }, 1000);
    });

    onUnmounted(() => {
      clearTimeout(timer.value);
    });

    return {};
  }
}

A Because setTimeout inside onMounted delays the cleanup registration until after unmount.

B Because onUnmounted is not imported from 'vue', so the hook never runs.

C Because the timer variable is declared with ref, which is not reactive enough for cleanup.

D Because clearTimeout is called with a ref object instead of the timer ID, so the timeout is not cleared.

💡 Hint

Check the argument passed to clearTimeout carefully.

The timer is a ref object, so timer itself is an object, not the timer ID. The correct call should be clearTimeout(timer.value) to clear the actual timer.

📝 Syntax

advanced

Which option correctly registers a cleanup function in Vue 3's onMounted hook?

Choose the code snippet that correctly registers a cleanup function to run when the component unmounts.

A

onMounted(() => {
  const id = setInterval(() => {}, 1000);
  onUnmounted(() => clearInterval(id));
});

B

onMounted(() => {
  const id = setInterval(() => {}, 1000);
  return () => clearInterval(id);
});

C

onMounted(() => {
  const id = setInterval(() => {}, 1000);
  onCleanup(() => clearInterval(id));
});

D

onMounted(() => {
  const id = setInterval(() => {}, 1000);
  cleanup(() => clearInterval(id));
});

💡 Hint

Remember the correct lifecycle hook for cleanup in Vue 3.

In Vue 3, onUnmounted is the correct hook to register cleanup functions. Returning a function from onMounted or using onCleanup or cleanup are not valid Vue 3 APIs.

🧠 Conceptual

expert

Why is it important to scope cleanup functions inside the setup function in Vue 3?

In Vue 3's Composition API, why should cleanup functions be registered inside the setup() function rather than outside it?

A Because cleanup functions registered inside <code>setup()</code> are tied to the component instance lifecycle, ensuring proper cleanup on unmount.

B Because registering cleanup outside <code>setup()</code> causes Vue to run cleanup multiple times per component instance.

C Because Vue only allows cleanup functions inside <code>setup()</code> to access reactive state variables.

D Because cleanup functions outside <code>setup()</code> run immediately when the component mounts, not on unmount.

💡 Hint

Think about how Vue tracks component lifecycles and reactive scopes.

Cleanup functions must be registered inside setup() so they are linked to the component's lifecycle. This ensures cleanup runs exactly when the component unmounts, preventing memory leaks and stale effects.