The difference between emitting and passing functions as props in Vue

Imagine having a versatile button component that is utilized in various other components. Instead of tying the child components to specific functionalities triggered by this button, you want to keep those logics flexible and customizable within each component using the button.

There appear to be two main strategies for achieving this:

  1. Have the child component emit an event to its parent components, allowing the parents to determine the appropriate handler function.

  2. Alternatively, define the handlers in the parent components and pass them down to the button component as props.

In React, I typically opt for the latter approach. Is there a preferred method or best practice in Vue for handling this kind of situation?

Answer №1

In the world of Vue, the guiding principle is passing props down and handling events up. The preferred approach aligns closely with this philosophy by emitting the event (up) to the parent component for it to be managed.

Additionally, when working within a Vue Single File Component (SFC), you have the advantage of using a v-on (or @) prefix for bound attributes to indicate that they are events being sent upwards, rather than using v-bind (or :) which may imply they are props, when in reality they are callbacks triggered by events.

Answer №2

Vue.js events are callbacks, not actual DOM events. This can be confirmed by the custom name added to the event listener instead of a standard DOM event name like click or focus. Additionally, there is no event object passed to the function unless specifically specified with an $event argument in the $emit call.

Events

Advantages

  • For libraries: provides a lighter option and gives clients more flexibility in method usage
  • Useful Vue devtools event logging feature
  • Allows global listeners (this.$root.on), although this aspect could be improved with Vuex.js
  • Different syntax: : for props and @ for events/methods

Disadvantages

  • Less explicit, making debugging harder (fail silently if there are no listeners or if the event name is misspelled)

Props

Advantages

  • More explicit, declarative, defaultable, required, and validated, which makes them easier to debug (runtime errors or compilation errors in TypeScript)

Disadvantages

  • Need to include props validation to avoid checking if a function() prop exists before calling it (although using props validation is a good practice regardless)

Conclusion

Seems like the choice between approaches comes down to convention and personal preference. If Vue.js documentation didn't favor the events approach, many would likely opt for using props exclusively, which I believe is clearer and better.

Props can accomplish everything that events do, except for a few scenarios (like the $root event listening pattern - where Vuex.js is recommended for scalability). The advantage of using props lies in their explicitness, debuggability, and error-checking capabilities.

Refined from: https://forum.vuejs.org/t/events-vs-callback-props/11451

Answer №3

Coming from a React background, I find it puzzling why the @event attribute even exists (or why it is considered standard as mentioned in the previous answers). It's hard for me to determine which events a component will $emit, but identifying which props are being passed down is much clearer to me. With proper naming conventions, I can easily distinguish which ones are callback events.

Answer №4

Recommended Approach

The best practice in this scenario would be to go with option number 1. This approach is clearly outlined in the official Vue.js documentation under the section: Sending Messages to Parents with Events

Performance

When passing a reference to a function for execution either through an event bus or as a prop, you should not experience any noticeable performance drawbacks.

Illustrative Example of Option 1

To implement option 1, use the following syntax:

this.$emit('eventName', dataToSend, ...)
to transmit data to the parent component which will then listen for the event like so:
<my-component @eventName="yourHandler" />
. This allows for the differentiation of logic based on each button function.

For demonstration purposes, I have put together a fiddle showcasing a multi-select component that applies this methodology: Multi-Select Component Fiddle

// HTML
<div id="app">
  <multi-choice :items="myItems" @selected="alert($event)"></multi-choice>
  <multi-choice :items="myItems" @selected="sayIsCool"></multi-choice>
</div>

// JavaScript
const multiChoice = {
    template: '<div class="multi-choice"><span v-for="item in items" @click="select(item)">{{ item }}</span></div>',
  props: ['items'],
  methods: {
    select(item) {
        this.$emit('selected', item);
    }
  }
};

new Vue({
  el: "#app",
  data() {
    return {
        myItems: [
        'Homer',
        'Marge',
        'Bart'
      ],
    }
  },
  components: {
    multiChoice: multiChoice
  },
  methods: {
    sayIsCool(item) {
        alert(item + ' is cool!')
    }
  }
})

Answer №5

Are you in search of "See-through Packaging"?

The way Vue's custom event functions is distinct from a traditional DOM event. To make it work, you must attach the .native property to the event.

If your goal is for the event to take place on the child element, then create a computed property that will return an object of listeners. This way, you won't have to worry about...

By default, any attributes not specifically defined as props will be applied to the main element of the view.

To change this behavior, set inheritAttrs: false and then connect the $attrs to the child component so it becomes the recipient of those attributes.

This approach eliminates the need to determine the root component.

Chris Fritz offers an excellent explanation of these concepts in his presentation on the 7 secret patterns. His insights begin at 21:44 here.

Answer №6

In my opinion, the outcome of this scenario hinges on whether or not we provide a more comprehensive context. Comparing props to events is akin to comparing pull versus push, much like any pub-sub system.

When passing props, we essentially inject (push) the parent context's dependencies into the child context, resulting in potential contamination of the child context by the parent context. Instead of just holding a weak reference to the parent, any changes made in the parent context will also affect the child context. This creates a tight coupling between the two.

On the other hand, with event pulling, the parent listens for events from the child, and each event data is preferably treated as a copy value rather than a reference. This eliminates the coupling issue between parent and child contexts. By using a queue or custom modifier for events, we can make it easier to control how parents interact with children (for example, avoiding the need to manage debounce or throttle functions in the parent context, and handling them within the child context instead - such as within the Button component).

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