Write data to file using cordova file plugin. This snippet uses promise using q.js

cordova-file-io.js

var fnCreateDirectoryInFileSystem = function (rootPath, newFolderName) {
    var deferred = Q.defer();

    var fnDirFail = function (err) {
        //Root path missing
        deferred.reject();
    };

    var fnGotDir = function (dirEntry) {
        //Create the first directory
        dirEntry.getDirectory(newFolderName, {
            create: true,
            exclusive: false
        }, function () {
            deferred.resolve({
                data: rootPath + '/' + newFolderName
            });
        }, fnDirFail);
    };

    window.resolveLocalFileSystemURL(rootPath, fnGotDir, fnDirFail);

    return deferred.promise;
};

var fnWriteToFileSystem = function (fileName, fileData) {
    var deferred = Q.defer();
    var rootPath = cordova.file.dataDirectory;
    var dirPath = rootPath + 'my-dir';
    var fnFail = function (err) {
        deferred.reject(err);
    };

    var _fnWriter = function (dirEntry) {
        var gotFileWriter = function (writer) {
            writer.onwrite = function (evt) {
                deferred.resolve({
                    data: evt
                });
            };
            writer.write(fileData);
        }

        var fnGotFile = function (fileEntry) {
            fileEntry.createWriter(gotFileWriter, fnFail);
        }

        dirEntry.getFile(relativePath, {
            create: true,
            exclusive: false
        }, fnGotFile, fnFail);

    };

    var fnCreateDir = function () {
        fnCreateDirectoryInFileSystem(rootPath, 'my-dir').then(function (data) {
            var dirPath = data.data;
            window.resolveLocalFileSystemURL(dirPath, _fnWriter, fnFail);
        });
    }

    //Check if dirPath exists
    window.resolveLocalFileSystemURL(dirPath, _fnWriter, fnCreateDir);

    return deferred.promise;
}

document.addEventListener('deviceready', function(e){
    fnWriteToFileSystem('mydata.txt', 'ABC PQR MNO');
});