How to dynamically inject a mixin into component

You aren't going to be able to do this and it's actually by design, not accident, as stated by Evan You in this github issue:

I don't like the idea of runtime mixins, because it makes your components' behavior unpredictable depending on when mixins are applied.

This means that you can't assign a mixin after the fact and there is not any way to get the this context before the mixin loads.

I'm not entirely sure what you're aiming to do here, but if you simply don't want to init the timer mixin, then you can do the check in the mixins created hook before you take any further action (I personally would also add the hasTimer prop to the mixin aswell, but I'll keep it how you had it), so you get:

var timerMixin = {
  created(){
    if(this.hasTimer){
      this.initTimer();
    }
  },
  methods: {
    initTimer(){
      console.log('Init Timer Mixin')
    }
  }
}


export default {
    mixins: [timerMixin],
    props: ['hasTimer']
  });
}

Here's the JSFiddle: https://jsfiddle.net/gnkha6hr/

Unfortunately, there is no way, currently, to dynamically add or remove mixins for a component. The this var is not available at that scope. And in the earliest lifecycle hook, beforeCreate, mixins are already loaded.

In your case, depending on what's in the timerMixin mixin, it might make sense to make a separate timer component with related logic which could be loaded dynamically in the template of Component A.

Otherwise, always loading the mixin probably wouldn't be too bad (performance-wise), since the data will be reactive.