AppPreferences async operation

Ionic Framework ionic-v3
1

I store a value on the service provider and then I get it from a component by using AppPreferences plugin.
the problem is async operation.

fetchKeysAppPreferences(){
  this.appPreferences.fetch("key1").then(
    (res) => {
        this.key.push(JSON.parse(res));
        console.log(this.key); //is definded
        return this.key;
      }
   );
   console.log(this.key); // undefined
}

I called the fetchKeysAppPreferences() from the home component. I get undefinded value because the async operation. How can I resolve this problem?