gistfile1.js

var {
    MCFileWriterUtil
} = require('NativeModules');

// Step #1
MCFileWriterUtil.writeFile(
    'MyFile.txt',                       // File name
    'Some Text',                        // File contents
    function errorCallback(results) {
        alert('Error: ' + results);
    },
    function successCallback(results) {
        alert('Success : ' + results);
    }
);