I want convert this object

```
const obj= {
    book:{
        title:'Harry Potter and',
        subtitle:'goblet on Fire'
    },
    video:{
        title:'The Mechanic',
        subtitle:'resurrection'
    }
}

```

and i need a array of strings

```
const arr = ['book','title','Harry Potter and','subtitle','goblet on Fire','video','title','The Mechanic','subtitle','resurrection']

```

how can i convert to this?