defineExpose is used in Vue 3's
defineExpose is a macro in Vue 3's <script setup></script> syntax that allows you to explicitly define which variables or functions should be exposed when using the component as a ref. Normally, when you use <script setup></script> , everything declared inside it is not automatically exposed—you have to opt-in to what you want to expose.
Why Use defineExpose ?
When you're working with child components and need to access their internal methods or properties from a parent component (via a ref ), you might find that those values ??aren't available by default. That's because <script setup></script> doesn't expose them unless told to do so.
For example:
<!-- ChildComponent.vue -->
<script setup>
const someData = ref(42)
function updateData() {
someData.value
}
defineExpose({ someData, updateData })
</script>Now, if you create a ref to this component in the parent:
<!-- ParentComponent.vue -->
<template>
<ChildComponent ref="childRef" />
</template>
<script setup>
import { ref } from 'vue'
const childRef = ref()
</script>You can then access the exposed values ??like this:
childRef.value.someData // 42 childRef.value.updateData() // increases value to 43
Without defineExpose , someData and updateData wouldn't be accessible on childRef.value .
How to Use It
Using defineExpose is straightforward:
- Just pass an object where keys are the names you want to expose, and values ??are the actual variables or functions.
Here's a quick breakdown:
- You don't need to import anything—it's a compile-time macro.
- You can expose as many items as needed.
- You can even expose calculated properties or methods.
Example:
const count = ref(0)
const double = computed(() => count.value * 2)
function increment() {
count.value
}
defineExpose({ count, double, increment }) This makes count , double , and increment available on the component instance via a ref.
When Do You Actually Need It?
Most of the time, especially in smaller apps or when props/events handle communication, you won't need defineExpose . But it becomes useful in cases like:
- Building reusable UI components (like custom input fields) that need to expose methods such as
.focus()or.validate(). - Working with important logic that needs direct access to child state.
- Integrating with third-party libraries that expect certain methods to be present on DOM elements or component instances.
If you're trying to reach into a child component and pull out data or call a method directly, that's when defineExpose comes into play.
A Few Things to Keep in Mind
- It only works in
<script setup></script>: If you're using the Options API (setup()function orexport default {}style), you don't needdefineExpose. Instead, you use theexposeoption. - No automatic reactivity transfer : Exposing a
refdoesn't make it reactive outside the component. You still need to access.valueif it's a ref. - Avoid overuse : Like any important pattern, exposing internals tightly couples your components. Try to favor props and events first.
So yeah, defineExpose gives you control over what gets exposed from a <script setup></script> component. Not something you'll use every day, but definitely handy when you need it.
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