Error in data(): "TypeError: Cannot read property 'propsTitle' of undefined"

I cant initialize by props to data. it keeps showing me this error and I can't figure it out why? I read the vuejs tutorial it shows `title: this.propsTitle` this is the correct way. what am I missing? Thank everyone!

> 
> Error in data(): "TypeError: Cannot read property 'propsTitle' of
>  undefined"
> 
> 
> 

```
          props: {
            propsTitle: String, 
            propsLevel: Number,
            propsProgress: Number,
          },

          data: () => ({
            title: this.propsTitle,
            progress: this.propsLevel,
            level: this.propsLevel,
            activeBtnTxt: "Start",
            isStarted: false

          }),  
// watch: {
      //   progress(val) {
      //     this.progress = val
      //   }
      // },
      // mounted() {
      //   console.log(this.propsProgress)
      //   // this.progress = propsProgress
      //   // this.level = propsLevel
      //   // this.title = propsTitle
      // },

```