SOURCE

window.requestFileSystem  = window.requestFileSystem || window.webkitRequestFileSystem;

function getFileSystem(type, size) {
	return new Promise(function(resolve, reject) {
		requestFileSystem(type, size, resolve, reject);
	});
}

function getFile(dir, name, params) {
    return new Promise(function(resolve, reject) {
       dir.getFile(name,params, resolve); 
    });
}

function writeFile(file, content, type) {
    return new Promise(function(resolve, reject) {
       file.createWriter(function(writer) {
          // writer.addEventListener('error', function(){}, false); 
           writer.onerror = function(err) {
               console.log(err);
           };
           writer.onwriteend = function(err) {
               if(!err) {
                   console.log('Write end');
               }
               resolve();
           };
           writer.onwrite = function() {
               console.log('writing...');
           };
           if(Object.prototype.toString.call(content) !== '[object Array]') {
               content = [content.toString()];
           }
           var blob = new Blob(content, {type: type});
           writer.write(blob);
       });
    });
}

function readFile(fileEntry) {
    return new Promise(function(resolve, reject) {
    	fileEntry.file(function(file){
            var reader = new FileReader();
            reader.onloadend = function(e) {
                resolve(this.result);
            }
            reader.readAsText(file);
        });
    });
}
// Writer File
getFileSystem(window.TEMPORARY, 50*1024*1024).then(function(fs) {
    return getFile(fs.root,'log.txt',{create:true});
}).then(function(file) {
    return writeFile(file, ['This is a good day'], 'text/plain');
}).then(function(content) {
   console.log(content); 
    
    console.log("filesystem:http://"+window.location.hostname+"/temporary/log.txt");
    // var downloadbtn = document.createElement('a');
    // document.body.appendChild(downloadbtn);
    // downloadbtn.download = "log.txt";
    // downloadbtn.href = ("filesystem:http://"+window.location.hostname+"/temporary/answers.html");
    //downloadbtn.click();
});
// Reader File
getFileSystem(window.TEMPORARY, 50*1024*1024).then(function(fs) {
    return getFile(fs.root,'log.txt',{create:true});
}).then(function(file) {
    return readFile(file);
}).then(function(content) {
   console.log(content); 
});