Watch and watchEffect in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Difference in reactive triggers between watch and watchEffect

Consider a Vue 3 component using both watch and watchEffect. Which statement correctly describes when each watcher runs?

Vue

import { ref, watch, watchEffect } from 'vue';

const count = ref(0);
const double = ref(0);

watch(count, (newVal) => {
  double.value = newVal * 2;
});

watchEffect(() => {
  console.log(`Count is: ${count.value}`);
});

A <code>watch</code> runs immediately on setup; <code>watchEffect</code> runs only when dependencies change.

B <code>watch</code> runs whenever any reactive property changes; <code>watchEffect</code> runs only once on setup.

C <code>watch</code> runs only when <code>count</code> changes; <code>watchEffect</code> runs immediately and whenever dependencies change.

D <code>watch</code> and <code>watchEffect</code> both run only once on setup.

💡 Hint

Think about when each watcher first runs and how they track dependencies.

watch runs its callback only when the watched source changes, not immediately by default. watchEffect runs immediately on setup and then reruns whenever any reactive dependency inside it changes.

❓ state_output

intermediate

Output of watchEffect with multiple reactive sources

What will be logged to the console after the following code runs and count.value changes from 1 to 2?

Vue

import { ref, watchEffect } from 'vue';

const count = ref(1);
const message = ref('hello');

watchEffect(() => {
  console.log(`Count: ${count.value}, Message: ${message.value}`);
});

count.value = 2;

A

Count: 1, Message: hello
Count: 2, Message: hello

B Count: 1, Message: hello

C Count: 2, Message: hello

D

Count: 1, Message: hello
Count: 1, Message: hello

💡 Hint

Remember when watchEffect runs its function.

watchEffect runs immediately on setup, logging the initial values. Then it reruns when count.value changes, logging the updated values.

📝 Syntax

advanced

Correct syntax to watch multiple sources with watch

Which option shows the correct way to watch two reactive refs a and b and react when either changes?

Vue

import { ref, watch } from 'vue';

const a = ref(0);
const b = ref(0);

A watch(a, b, (newA, newB) => { console.log(newA, newB); });

B watch([a, b], ([newA, newB]) => { console.log(newA, newB); });

C watch(a && b, (newVal) => { console.log(newVal); });

D watch({a, b}, (newVals) => { console.log(newVals.a, newVals.b); });

💡 Hint

Think about how to pass multiple sources as an array.

To watch multiple sources, pass an array of refs as the first argument. The callback receives an array of new values.

🔧 Debug

advanced

Identify the error in this watchEffect usage

What error will this code produce when run in a Vue 3 component?

Vue

import { ref, watchEffect } from 'vue';

const count = ref(0);

watchEffect(() => {
  count.value++;
});

A TypeError: count.value is not a function

B SyntaxError: Unexpected token

C No error, runs fine resetting count after 5

D Maximum call stack size exceeded (infinite loop error)

💡 Hint

Consider what happens when you change a reactive value inside watchEffect.

Changing count.value inside watchEffect triggers the effect again, causing an infinite loop and stack overflow.

🧠 Conceptual

expert

Why prefer watch over watchEffect for expensive side effects?

In Vue 3, why might you choose watch instead of watchEffect when performing expensive operations based on reactive data?

A <code>watch</code> runs only when specific sources change, avoiding unnecessary runs; <code>watchEffect</code> runs immediately and on any dependency change.

B <code>watchEffect</code> caches results automatically; <code>watch</code> does not.

C <code>watch</code> runs synchronously; <code>watchEffect</code> runs asynchronously.

D <code>watchEffect</code> cannot access reactive data; <code>watch</code> can.

💡 Hint

Think about control over when the watcher runs.

watch lets you specify exactly which reactive sources to watch, so you avoid running expensive code unnecessarily. watchEffect runs immediately and reruns whenever any reactive dependency changes, which can cause extra runs.

Practice

1. What is the main difference between watch and watchEffect in Vue?

easy

A. watchEffect only runs once, watch runs continuously.

B. watch tracks specific reactive sources, watchEffect tracks all reactive dependencies automatically.

C. watch runs immediately on setup, watchEffect waits for changes.

D. watchEffect requires manual cleanup, watch does not.

Solution

1. Step 1: Understand watch behavior

   watch listens to specific reactive data sources you specify and runs a callback only when those change.

2. Step 2: Understand watchEffect behavior

   watchEffect runs immediately and tracks all reactive data accessed inside it, rerunning whenever any of those change.

3. Final Answer:

   watch tracks specific reactive sources, watchEffect tracks all reactive dependencies automatically. -> Option B

4. Quick Check:

   watch vs watchEffect difference = watch tracks specific reactive sources, watchEffect tracks all reactive dependencies automatically. [OK]

Hint: Remember: watch tracks specific data, watchEffect tracks all used data [OK]

Common Mistakes:

• Thinking watchEffect only runs once
• Assuming watch runs immediately on setup
• Confusing cleanup requirements

2. Which of the following is the correct syntax to watch a reactive variable count in Vue 3 Composition API?

easy

A. watch(count, (newVal, oldVal) => { console.log(newVal) })

B. watch(count, () => { console.log(count) })

C. watchEffect(() => { console.log(count) })

D. watch(() => count, (newVal, oldVal) => { console.log(newVal) })

Solution

1. Step 1: Identify correct watch syntax

   In Vue 3 Composition API, watch accepts a ref directly as the first argument, which automatically watches its .value.

2. Step 2: Check callback parameters

   The callback receives newVal and oldVal as parameters, correctly used in watch(count, (newVal, oldVal) => { console.log(newVal) }) to log the new value.

3. Final Answer:

   watch(count, (newVal, oldVal) => { console.log(newVal) }) -> Option A

4. Quick Check:

   Correct watch syntax uses ref directly = watch(count, (newVal, oldVal) => { console.log(newVal) }) [OK]

Hint: Pass ref directly to watch [OK]

Common Mistakes:

• Using getter returning ref object like () => count
• Confusing watch with watchEffect syntax
• Omitting callback parameters

3. Given the code below, what will be logged to the console after count.value = 2 is executed?

import { ref, watch } from 'vue'
const count = ref(1)
watch(() => count.value, (newVal, oldVal) => {
  console.log(`Changed from ${oldVal} to ${newVal}`)
})
count.value = 2

medium

A. SyntaxError

B. Changed from undefined to 2

C. No output

D. Changed from 1 to 2

Solution

1. Step 1: Understand watch callback parameters

   The watch callback receives old and new values of the watched reactive source.

2. Step 2: Trace the change

   Initially, count.value is 1. When changed to 2, the callback logs the change from 1 to 2.

3. Final Answer:

   Changed from 1 to 2 -> Option D

4. Quick Check:

   watch logs old and new values correctly = Changed from 1 to 2 [OK]

Hint: watch callback shows old and new values on change [OK]

Common Mistakes:

• Expecting no output on change
• Confusing initial value with undefined
• Misunderstanding callback parameters

4. What is wrong with this Vue 3 code using watch? How to fix it?

import { ref, watch } from 'vue'
const count = ref(0)
watch(count, (newVal) => {
  console.log(newVal)
})

medium

A. No error; code works fine

B. Missing cleanup function; fix by adding cleanup inside callback

C. Callback missing oldVal parameter; fix by adding it

D. Passing reactive variable directly; fix by passing a getter function like () => count

Solution

1. Step 1: Identify watch argument type

   In Vue 3, watch(count, callback) is valid; it automatically tracks count.value changes.

2. Step 2: Callback parameters

   The callback receives newVal and optional oldVal; omitting oldVal is fine.

3. Final Answer:

   No error; code works fine -> Option A

4. Quick Check:

   watch accepts ref directly = No error; code works fine [OK]

Hint: watch accepts refs directly [OK]

Common Mistakes:

• Thinking ref must be wrapped in getter
• Assuming oldVal always required
• Believing code has error

5. You want to run a side effect immediately and re-run it whenever any reactive data inside changes, but also want to stop the effect manually later. Which Vue 3 API and pattern should you use?

hard

A. Use watch without immediate option and no cleanup

B. Use watch with immediate: true option and no stop function

C. Use watchEffect and store the returned stop function to call later

D. Use watchEffect but it cannot be stopped manually

Solution

1. Step 1: Identify immediate reactive effect

   watchEffect runs immediately and tracks all reactive dependencies automatically.

2. Step 2: Understand stopping effect

   watchEffect returns a stop function that can be called to stop the effect manually.

3. Final Answer:

   Use watchEffect and store the returned stop function to call later -> Option C

4. Quick Check:

   watchEffect runs immediately and can be stopped = Use watchEffect and store the returned stop function to call later [OK]

Hint: watchEffect returns stop function to control effect lifecycle [OK]

Common Mistakes:

• Thinking watchEffect cannot be stopped
• Using watch without immediate for immediate run
• Ignoring stop function usage