Vue: default value for prop function

I'd say, always provide a default value that matched the type you specified: makes things much more easier to reason about. Here, what you are looking for would be a noop function (a.k.a. a function that does nothing):

props: {
  clickFunction: {
    type: Function
    default: () => {}
  }
}

You can then use this.clickFunction() in your code without checking it defensively first: it is always a function. Be sure you don't get mixed up with the default value for an Object prop, that would be an empty object:

props: {
  someObject: {
    type: Object
    default: () => ({})
  }
}

In Vue, Object and Array props must have their default value returned by a function to ensure they are distinct for every instance of the component (which does not seem to be required for Function props, but docs indeed aren't crystal clear about this).

Yes:

Vue.component('foo', {
  template: '<div>{{ num }}</div>',
  props: {
    func: {
      type: Function,
      default: () => 1,
    },
  },
  data() {
    return { 
      num: this.func()
    }
  }
})

new Vue({
  el: '#app',
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="app">
  <foo></foo>
</div>

This way should work with Vue 2.0

props: {
    clickFunction: {
        type: 'Function'
        default(){
          return () => true
        }
    }
}