DefineEmits Macro Only Works When Assigned To Variable

Apr 25, 2025 by ADMIN 55 views

Introduction

In Vue 3, the defineEmits macro is used to define the emit functions for a component. However, when trying to use the result of defineEmits as a function argument without assigning it to a variable first, a ReferenceError is thrown. This article will explore this issue and provide a solution.

Problem Statement

The problem arises when trying to use the result of defineEmits as a function argument without assigning it to a variable first. This is demonstrated in the following code snippet:

<script setup lang="ts">
// Error
const transformed = fn(defineEmits<{
  validChange: [boolean]
}>());

// Works
const emit = defineEmits<{
  validChange: [boolean]
}>();
const transformed = fn(emit);
</script>

Expected Behavior

The expected behavior is that the result of defineEmits can be used as a function argument without assigning it to a variable first. This would allow for more concise and readable code.

Actual Behavior

However, the actual behavior is that a ReferenceError is thrown when trying to use the result of defineEmits as a function argument without assigning it to a variable first. This is demonstrated in the following error message:

ReferenceError: defineEmits is not defined
    setup about:srcdoc line 63 > injectedScript:20
    callWithErrorHandling https://cdn.jsdelivr.net/npm/@vue/runtime-dom@3.5.13/dist/runtime-dom.esm-browser.js:2334
    setupStatefulComponent https://cdn.jsdelivr.net/npm/@vue/runtime-dom@3.5.13/dist/runtime-dom.esm-browser.js:9982
    setupComponent https://cdn.jsdelivr.net/npm/@vue/runtime-dom@3.5.13/dist/runtime-dom.esm-browser.js:9943
    mountComponent https://cdn.jsdelivr.net/npm/@vue/runtime-dom@3.5.13/dist/runtime-dom.esm-browser.js:7293
    processComponent https://cdn.jsdelivr.net/npm/@vue/runtime-dom@3.5.13/dist/runtime-dom.esm-browser.js:7259
    patch https://cdn.jsdelivr.net/npm/@vue/runtime-dom@3.5.13/dist/runtime-dom.esm-browser.js:6788
    render https://cdn.jsdelivr.net/npm/@vue/runtime-dom@3.5.13/dist/runtime-dom.esm-browser.js:8056
    mount https://cdn.jsdelivr.net/npm/@vue/runtime-dom@3.5.13/dist/runtime-dom.esm-browser.js:6056
    mount https://cdn.jsdelivr.net/npm/@vue/runtime-dom@3.5.13/dist/runtime-dom.esm-browser.js:12239
    _mount about:srcdoc line 63 > injectedScript:18
    <anonymous> about:srcdoc line 63 > injectedScript:23

System Information

The system information is as follows:

System:
    OS: Windows 10 10.0.19045
    CPU: (4) x64 Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz
    Memory: 21.30 GB / 39.85 GB
  Binaries:
    Node: 22.14.0 - C:\Program Files\nodejs\node.EXE
    npm: 10.3.0 - C:\Program Files\nodejs\npm.CMD
    pnpm: 9.11.0 - C:\Program Files\nodejs\pnpm.CMD
  Browsers:
    Chrome: 129.0.6668.70
    Edge: Chromium (127.0.2651.105)
    Internet Explorer: 11.0.19041.4355

Solution

To solve this issue, we need to assign the result of defineEmits to a variable first. This is demonstrated in the following code snippet:

<script setup lang="ts">
const emit = defineEmits<{
  validChange: [boolean]
}>();
const transformed = fn(emit);
</script>

By assigning the result of defineEmits to a variable first, we can use it as a function argument without any issues.

Conclusion

In conclusion, the defineEmits macro only works when assigned to a variable. This is a limitation of the current implementation of defineEmits in Vue 3. To work around this issue, we need to assign the result of defineEmits to a variable first. This will allow us to use the result of defineEmits as a function argument without any issues.

Additional Comments

I ran into this bug when I tried to create a wrapper for defineEmits that converts the emit function to individual functions. This wrapper is necessary to make the code more readable and maintainable. However, due to the limitation of defineEmits, I had to assign the result of defineEmits to a variable first, which makes the code less concise.

const emit = defineEmits<{
  validChange: [valid: boolean];
}>();
const { validChange } = useEmitters(emit); // This sadly has to be on separate line

const valid = computed(() => ...);
watch(valid, (value) => validChange(value));

Q: What is the `defineEmits` macro in Vue 3?

A: The defineEmits macro in Vue 3 is used to define the emit functions for a component. It allows you to specify the names and types of the emit functions that a component can emit.

Q: Why does the `defineEmits` macro only work when assigned to a variable?

A: The defineEmits macro only works when assigned to a variable because it returns an object that contains the emit functions. When you try to use the result of defineEmits as a function argument without assigning it to a variable first, a ReferenceError is thrown because the object is not yet defined.

Q: How can I work around this limitation?

A: To work around this limitation, you need to assign the result of defineEmits to a variable first. This will allow you to use the result of defineEmits as a function argument without any issues.

Q: What is the difference between using `defineEmits` and not using it?

A: When you use defineEmits, you can specify the names and types of the emit functions that a component can emit. This makes the code more readable and maintainable. When you don't use defineEmits, you need to define the emit functions manually, which can make the code more verbose and harder to maintain.

Q: Can I use `defineEmits` with TypeScript?

A: Yes, you can use defineEmits with TypeScript. In fact, defineEmits is designed to work seamlessly with TypeScript. You can use the defineEmits macro to define the emit functions and then use TypeScript to type-check the emit functions.

Q: Are there any other limitations of `defineEmits`?

A: Yes, there are a few other limitations of defineEmits. For example, defineEmits only works with function components, not with class components. Additionally, defineEmits only works with the setup function, not with other lifecycle hooks.

Q: How can I report a bug or issue with `defineEmits`?

A: If you encounter a bug or issue with defineEmits, you can report it on the Vue.js GitHub repository. Make sure to include a minimal reproducible example and a clear description of the issue.

Q: Can I use a wrapper function to convert the emit function to individual functions?

A: Yes, you can use a wrapper function to convert the emit function to individual functions. However, due to the limitation of defineEmits, you need to assign the result of defineEmits to a variable first, which makes the code less concise.

const emit = defineEmits<{
  validChange: [valid: boolean];
}>();
const { validChange } = useEmitters(emit); // This sadly has to be on separate line

const valid = computed(() => ...);
watch(valid, (value) => validChange(value));

I hope this Q&A article helps to clarify the issue and provides a solution. If you have any further questions or concerns, please feel free to ask.