With Vue-cli, where do I declare my global variables?

You don't need to make the store a global variable like that, as every component (this.$store) and the Vue instance itself have access to the store after the initial declaration.

Take a look at the Quasar docs for App Vuex Store.

store.js

import Vue from 'vue'
import Vuex from 'vuex'

Vue.use(Vuex)

const store = new Vuex.Store({
  state: {
    count: 0
  },
  mutations: {
    updateCount(state) {
      state.count += 1
    }
  }
})

main.js

import App from './App.vue'
import store from '/path/to/store.js'

new Vue({
  el: '#app',
  store,
  render: h => h(App)
})

If you need to access the store from within a component you can either import it (as we did in main.js) and use it directly [note that this is a bad practice] or access using this.$store. You can read a bit more about that here.

---

In any case here's the official Getting Started guide from Vuex team

Yea, you can set those variables like this, in your entrypoint file (main.js):

Vue.store= Vue.prototype.store = 'THIS IS STORE VARIABLE';

and later access it in your vue instance like this:

<script>
export default {
  name: 'HelloWorld',
  methods: {
    yourMethod() {
        this.store // can be accessible here.
    }
  }
}
</script>

You can also see this in the vue-docs here.

Edit 1:

from the discussions in the comment sections about "no entrypoint file" in quasar's template.

what you can do is, to go to src/router/index.js, and there you will be able to get access to Vue, through which you can set a global variable like this:

...
import routes from './routes'

Vue.prototype.a = '123';

Vue.use(VueRouter)
...

and then if you console.log it in App.vue, something like this:

<script>
export default {
  name: 'App',
  mounted() {
        console.log(this.a);
  }
}
</script>

now, look at your console:

You can also do the same in App.vue file in the script tag.

We could add the Instance Properties

Like this, we can define instance properties.

Vue.prototype.$appName = 'My App'

Now $appName is available on all Vue instances, even before creation.

If we run:

new Vue({
  beforeCreate: function() {
    console.log(this.$appName)
  }
}) 

Then "My App" will be logged to the console!