Meaning of v-slot:activator="{ on }"

Looking at the Vuetify example code for v-toolbar, what is the purpose of v-slot:activator="{ on }"? For example:

<template v-slot:activator="{ on }">
  <v-toolbar-title v-on="on">
    <span>All</span>
    <v-icon dark>arrow_drop_down</v-icon>
  </v-toolbar-title>
</template>

<script>
  export default {
    data: () => ({
      items: [
        'All', 'Family', 'Friends', 'Coworkers'
      ]
    })
  }
</script>

As far as I can see, on is not a defined variable anywhere, so I don't see how this is working. When I try it in my project, Internet Explorer throws an error on the <template v-slot:activator="{ on }">, but if I remove it, the page renders.

You're likely referring to this example:

<v-toolbar color="grey darken-1" dark>
  <v-menu :nudge-width="100">
    <template v-slot:activator="{ on }">
      <v-toolbar-title v-on="on">
        <span>All</span>
        <v-icon dark>arrow_drop_down</v-icon>
      </v-toolbar-title>
    </template>

    ...
  </v-menu>
</v-toolbar>

The following line declares a scoped slot named activator, and it is provided a scope object (from VMenu), which contains a property named on:

<template v-slot:activator="{ on }">

This uses destructuring syntax on the scope object, which IE does not support.

For IE, you'd have to dereference on from the scope object itself:

<template v-slot:activator="scope">
  <v-toolbar-title v-on="scope.on">

But the ideal solution IMO is to use a Vue CLI generated project, which includes a Babel preset (@vue/babel-preset-app) to automatically include the transforms/polyfills needed for the target browsers. In this case, babel-plugin-transform-es2015-destructuring would be automatically applied during the build.

Details on the activator slot

VMenu allows users to specify a slotted template named activator, containing component(s) that activate/open the menu upon certain events (e.g., click). VMenu provides listeners for those events via an object, passed to the activator slot:

<v-menu>
  <template v-slot:activator="scopeDataFromVMenu">
    <!-- slot content goes here -->
  </template>
</v-menu>

The slot content can access VMenu's event listeners like this:

<v-menu>
  <template v-slot:activator="scopeDataFromVMenu">
    <button v-on="scopeDataFromVMenu.on">Click</button>
  </template>
</v-menu>

For improved readability, the scoped data can also be destructured in the template:

<!-- equivalent to above -->
<v-menu>
  <template v-slot:activator="{ on }">
    <button v-on="on">Click</button>
  </template>
</v-menu>

The listeners from the scope object are passed to the <button> with v-on's object syntax, which binds one or more event/listener pairs to the element. For this value of on:

{
  click: activatorClickHandler // activatorClickHandler is an internal VMenu mixin
}

...the button's click handler is bound to a VMenu method.

I think the original question is about understanding the "on" object. It is best explained here:

https://github.com/vuetifyjs/vuetify/issues/6866

Essentially "on" is a prop passed in from the activator. What v-on="on" does is bind that on prop to the component. "on" itself is all of the event listeners passed from the activator.

To call out a readability tip, it's possible to use this syntax:

<v-menu>
    <template v-slot:activator="{ on: activationEvents }">
        <v-btn v-on="activationEvents">
            I like turtles 🐢
        </v-btn>
    </template>
</v-menu>

In my brain this has a more fluent readability than v-on="on", which to me is like observing a conversation consisting solely of:

• Person 1: "Hey"
• Person 2: "Yep"

Understand? ;)

By the way, activationEvents could be any alias, like "slotEvents", "listeners", "anyOldEvent", or whatever makes more sense to the reader as a renaming of the mysterious on.