Both run code in response to reactive changes, but they differ in how you tell Vue what to watch and when they first run.

watch: you specify what to observe

watch takes an explicit source (a ref, a getter, or an array of sources) and only runs when that specific source changes. It gives you both the old and new values.

import { ref, watch } from 'vue'

const count = ref(0)

watch(count, (newVal, oldVal) => {
  console.log(`Changed from ${oldVal} to ${newVal}`)
})

// Watch multiple sources
const firstName = ref('Ana')
const lastName = ref('García')

watch([firstName, lastName], ([newFirst, newLast], [oldFirst, oldLast]) => {
  console.log(`Name changed to ${newFirst} ${newLast}`)
})

watch is lazy by default. It doesn't run until the source actually changes. Add { immediate: true } if you need it to run once immediately.

watchEffect: Vue figures out the dependencies

watchEffect runs your callback immediately and automatically tracks every reactive value you read inside it. When any of those values change, it re-runs.

import { ref, watchEffect } from 'vue'

const query = ref('')
const page = ref(1)

watchEffect(async () => {
  // Vue sees you reading query.value and page.value
  // It will re-run this whenever either changes
  const results = await fetch(`/api/search?q=${query.value}&page=${page.value}`)
  // ...
})

You don't list dependencies anywhere. Vue detects them at runtime by observing which reactive values your code actually reads.

When to use which

Use watch when:

• You need the previous value (e.g., comparing old vs new to decide what to do)
• You want to watch something specific and ignore other reactive data in the callback
• You want lazy execution (don't run until the first change)
• You need { deep: true } to watch nested object changes

watch(route, (newRoute, oldRoute) => {
  if (newRoute.path !== oldRoute.path) {
    analytics.track('page_view', { path: newRoute.path })
  }
})

Use watchEffect when:

• You have a simple side effect that depends on reactive data and should run right away
• You don't care about the previous value
• You want dependencies tracked automatically (less code, fewer mistakes)

watchEffect(() => {
  document.title = `${count.value} items — MyApp`
})

The auto-tracking caveat

watchEffect only tracks dependencies that are read synchronously during execution. If you read a reactive value after an await, Vue won't track it:

watchEffect(async () => {
  // ✅ tracked — read before await
  const url = `/api/users?role=${role.value}`

  const data = await fetch(url)

  // ❌ NOT tracked — read after await
  console.log(filter.value)
})

If you hit this, either read all reactive values before the first await, or switch to watch with explicit sources.

See also: What's the difference between computed and watch? · How does Vue batch DOM updates?

References

• watch() - Vue.js docs
• watchEffect() - Vue.js docs
• Watchers - Vue.js docs