This article only describes this specific case, if you want to know how to set up Typescript for Vue this is a good source.

On a recent client project I had wanted to reach prop values of my component and needed to use this, but that causes errors:

// example
mounted(): void {
  this.interval = setInterval(() => {
    doSomething()
  }, 1000)
},

// error
Property 'interval' does not exist on type 'CombinedVueInstance<vue, {}, unknown, Readonly<{poll: Poll; radius: string;}>>'

This caused quite a bit of annoyance before I realised the simply fix.

The solution is just to add a placeholder in data so the Vue instance has this value on initialisation. This allows Typescript to know that the value does indeed exist on the instance.

export default Vue.extend({
  name: 'PollingComponent',
  data() {
    return {
      interval: 0 as any,
    }
  },
  mounted(): void {
    this.interval = setInterval(() => {
      doSomething()
    }, 1000)
  },
})

Now there’s some interesting browser compatibility problems regarding the type of an Interval. I found that it could be both a Timer or a Number depending on the browser. So in the end I just ended up using any. I can’t see any hard in that since it’s a Javascript function.

Happy coding!