reactjs - Access props inside a closure

calling this.props.navigation.dispatch(resetAction); inside componentWillMount() works fine for me, but if I try this code

componentWillMount(){
    Keychain
    .getGenericPassword()
    .then(function(credentials) {
         this.props.navigation.dispatch(resetAction);
    })
}

I get an error telling me that this.props.navigation is undefined. I suppose that the problem is that I am calling it inside .then. Do you know how can I solve this?