Listing an array how to do?

Question

React Native

countryData = ["India", "Pakistan", "USA"];

    countryList = () => {
      return (this.countryData.map((x, i) => {
        return (<Picker.Item label={x} key={i} value={x} />)
      }));
    }

Why doesn't it work? Located in componentDidMount . Says countryList is not a function.