Develop Reference

Read/Write and store data internelly in a Local App (no server) with JavaScript

So I am making a local App using Javascript , React and Electron and I want it to be able to work just fine without internet.
I can't use 'localStorage' because the data might get deleted if the user deletes the cache.
I tried reading/writing using differant Modules, none of them worked mostly because of CROS. Using XMLHTTPrequests and Ajax doesn't work either and am running out of time.
When I use them on the test server, they return the index.html for the mainpage (They can at least access that ... and still they can't read the data) but when I try it on the build I get CORS the error.
My Idea for now is to enable CORS on my webpage since I have no worries about security : The App will run ONLY offline so there is no danger.
But After many hours...I didn't find a solution to do it on the client side.
If anyone has an idea or suggestion I would be grateful.
I tried : fs,FileReader,FileSaver, $.ajax,XMLHTTPrequests
//using $ajax
var test = $.ajax({
crossDomain:true,
type: 'GET',
url:'../data/DefaultCategorie.txt',
contentType: 'text/plain',
success: function(data){
console.log(data);
},
error: function(){
alert('failed');
},
})
//using fs
fs.readFile('../data/DefaultCategorie.txt', 'utf8', (err, data) => {
if (err) {
console.log("Failed");
throw err
}
console.log(data);
fs.close(data, (err) => {
if (err) throw err;
});
});

This article covers the 3 most common ways to store user data: How to store user data in Electron
The Electron API for appDatadoes what you want. It is very easy to use.
From the above article:
const userDataPath = (electron.app || electron.remote.app).getPath('userData');
this.path = path.join(userDataPath, opts.configName + '.json')
this.data = parseDataFile(this.path, opts.defaults);
function parseDataFile(filePath, defaults) {
try {
return JSON.parse(fs.readFileSync(filePath));
} catch(error) {
// if there was some kind of error, return the passed in defaults instead.
return defaults;
}
}
Docs
app.getPath(name)
name String
Returns String - A path to a special directory or file associated with
name. On failure, an Error is thrown.
You can request the following paths by the name:
appData - Per-user application data directory, which by default points to:
%APPDATA% on Windows
$XDG_CONFIG_HOME or ~/.config on Linux
~/Library/Application Support on macOS
userData - The directory for storing your app's configuration files,
which by default it is the appData directory appended with your app's
name.

NeDB not loading or storing to file

I cannot get the simplest example of NeDB to run properly. My code only works in-memory, persistence to file keeps failing without any error messages.
The error callbacks for the loaddatabase and insert events always pass a null reference as error, so no information there. Oddly it seems no one else has this issue, so I guess I'm missing something here. All help is much appreciated.
Here is the code:
var Datastore = require('nedb'), db = new Datastore({ filename: 'test.db' });
db.loadDatabase(function (err) {
alert(err); // err is null, with the autoload flag no error is thrown either
});
var doc = { hello: 'world'};
db.insert(doc, function (err, newDoc) {
alert(err); // err is null here as well. Doc will be in the memory storage but no persisted to file
});

Although this question is pretty old, I'd like to share my experience for anyone facing a similar issue.
NeDB API does not allow JSON input. You have to put in a javascript object. When you use JSON input, no error is returned and nothing will be persisted.
'null' is returned as error in callback to signal that no problem occurred. When saving the first JSON document it is indexed with 'undefined' key, because NeDB calls 'key = obj[fieldname[0]]' which returns 'undefined', when the obj is just a (JSON) string. No error is returned unfortunately. Inserting a second document will cause a unique constraint violation error in the callback as the key 'undefined' has already been taken. Anyhow, nothing will be persisted.
Try
var Datastore = require('nedb'), db = new Datastore({ filename: 'test.db' });
db.loadDatabase(function (error) {
if (error) {
console.log('FATAL: local database could not be loaded. Caused by: ' + error);
throw error;
}
console.log('INFO: local database loaded successfully.');
});
// creating the object with new, just to make it clear.
// var doc = {hello: 'world'}; should work too.
function myDoc(greeting)
{
this.hello=greeting;
}
var doc = new myDoc('world');
db.insert(doc, function (error, newDoc) {
if (error) {
console.log('ERROR: saving document: ' + JSON.stringify(doc) + '. Caused by: ' + error);
throw error;
}
console.log('INFO: successfully saved document: ' + JSON.stringify(newDoc));
});
Maybe it helps someone. :)

This question is quite old but since I had very similar problem I thought that I'll write my resolution for anyone facing similar issues.
In my case I was writing Electron app using electron-webpack as an application builder. It turns out that NeDB loaded by Webpack was running in browser mode without access to file system.
To get it working I had to change import statement from:
import DataStore from 'nedb';
to this:
const DataStore = require('nedb');
Also I had to add NeDB to Webpack configuration as external module (in package.json):
"electronWebpack": {
"externals": {
"nedb": "commonjs nedb"
}
}
I have found this resolution on NeDB github page: https://github.com/louischatriot/nedb/issues/329

All I had to do to fix this was delete the .db file and let the program make one for me by running it one more time.
The other thing I did that could have fixed it was making sure my package.json had all the required information. this can be easily done with a quick "npm init" in the terminal.

Callbacks and error handling in an asynchronous function

I'm trying to improve my understanding of callbacks/error handling/async structure of Node.js working on challenges at nodeschool.io.
I have one local module file and one actual program file. Here they are:
my_module.js
module.exports = function(path, extension, callback) {
var fs = require('fs');
if (path) {
fs.readdir(path, function(err, list) {
if (err) return callback(err);
var filtered = list.filter(function(item) {
//console.log(item.split("."));
if (item.split(".")[1] == extension) return item
})
callback(null, filtered);
})
} else {
callback("===> Please provide a directory path.")
}
}
program.js
var my_module = require('./my_module');
var path = process.argv[2];
var extension = process.argv[3];
my_module(path, extension, function(err, data) {
if (err) return new Error(err);
console.log(data.join("\n"));
})
This program works just fine. BUT when it should give an error, it doesn't.
Inside my_module.js, if path variable is empty I want to give the error "Please provide a directory path.". The weird thing is, when I write console.log instread of callback, I can show the message. But when I call the callback as callback("===> Please provide a directory path.") nothing happens. No errors, it's silent.
Why is this? And how to fix that?
Thanks

It is silent because in your program.js, you are not consuming the error.
Think program.js is going to consume your module. module.js doesn't know what to do with error. it just passes the err and data to your program.js which consumes it. So its the responsibility of program.js to do whatever it wants to with the error. Something like log it on console, etc.
Try this:
my_module(path, extension, function(err, data) {
if (err) return console.log(err);
console.log(data.join("\n"));
})

You may want to throw that error rather than return it, depending on the error handling pattern for your application.
var my_module = require('./my_module');
var path = process.argv[2];
var extension = process.argv[3];
my_module(path, extension, function(err, data) {
if (err) throw new Error(err);
console.log(data.join("\n"));
})
As a recommendation, since process.arg[] contains user-specified input, I'd actually recommend doing the parameter checking with user-friendly (pretty) error messages (rather than an exception) BEFORE calling my_module(). my_module() itself can then be modified to check and throw an exception if any bogus parameters are passed in.

How to make javascript code to work witn node.js?

I have the following code and I know that if I use it in the terminal (node test.js, in the case the file is called test.js) but how do I make this code work in javascript with HTML? I mean, how do I make possible to click a button and execute the code? Thank you!
var SerialPort = require("serialport").SerialPort
var serialPort = new SerialPort("/dev/ttyACM0", {
baudrate: 9600
}, false);
serialPort.on('error', function(err) {
console.log(err);
});
serialPort.open(function(err) {
if (err) {
console.log(err);
return;
}
console.log('open');
serialPort.on('data', function(data) {
console.log('data received: ' + data);
});
serialPort.write('1', function(err, results) {});
});
}

You can't execute this in a browser (which wouldn't let you access the serial port, for example) but there are various solutions to package some HTML code with nodejs.
The best solution today for a local all-including "desktop-type" architecture is probably node-webkit which has a good support and traction.
Another standard architecture is to simply make nodejs act as a server serving an HTML page including your button. That might be more suited for piloting an Arduino.

JSONP call with server-side language as Javascript

I've been trying to use JSONP to get a JSON object from a server via a client-side call (on a different port). However, because my server is implemented in javascript using Node.js and Express, I haven't been able to find much on JSONP with Javascript on the server as most sites I found used php for server-side code.
I believe the issue is with how I set up the url with respect to the callback, which I'm a bit fuzzy on cause it's new to me.
On the server:
//Setting up server stuff
var express = require('express'),
app = express.createServer();
app.use(express.logger());
//Making a connection to the mongoDB to get the data that I want to display in the JSON object
new Db('prism',
new Server("127.0.0.1", 27017, {auto_reconnect: false}), {}).open(function(err, db) {
app.get('/getData', function(req, res) {
console.log('JSONPCALLBACK CALLED WITH RETURNDATA PASSED IN; SERVER SIDE');
if (typeof callback == 'function') {
console.log('callback is defined');
callback(returnData);
}
else {
console.log('callback is not defined');
}
}
});
And on the client:
$.ajaxSetup({ cache : false });
$.getJSON('http://127.0.0.1:1337/getData&callback=?', function(rtndata) {
console.log('SUCCESS');
console.log(rtndata);
});
embedded by the standard tags.
But I get the error:
GET http://127.0.0.1:1337/getData&callback=jQuery16108897686484269798_1311007334273?_=1311007334342 404 (Not Found)
The server is on port 1337 while the client is run through MAMP on localhost:8888. I'm not sure if its even a localhost related issue as I've been trying to get this setup running for a few days now.
I believe the issue has something to do with not writing this line, which is in php, into my server-side Javascript. Most of the JSONP examples I found had something like this. But I'm not sure.
if ($GET['callback'] != '')
$json = $GET['callback']."( $json )";
return $json;
Any help would be greatly appreciated. I apologize ahead of times for being super verbose.
Bests,
Cong

I think you have two problems. First is the 404. Completely separate from getting the client-side jQuery code to work, you need to make sure that you can issue a regular browser request (i.e. paste in that URL) and get back what you expect. I haven't used express, so it's hard for me to comment on why you'd be getting that, except to say that I don't see 1337 anywhere in your server-side code, just what appears to be the port number 27017.
The second problem is that you don't actually want to execute the callback on the server, just build the JSON response including the callback (string) prefixed to it.
So instead of this ...
if (typeof callback == 'function') {
console.log('callback is defined');
callback(returnData);
}
else {
console.log('callback is not defined');
}
try this:
if (callback) {
console.log('callback is defined');
res.write(callback + '(' + JSON.stringify(returnData) + ')');
}
else {
console.log('callback is not defined');
}
Hope this helps!

From http://api.jquery.com/jQuery.getJSON/ there is an example that includes 2 '?' in the URL.
you only have one, so try
$.getJSON('http://127.0.0.1:1337/getData?callback=?', function(rtndata) {
and see if that gets rid of your 404
then look #jimbojw suggestion for returning a proper jsonp formated responce.

Use this:
var express = require("express");
var server = express.createServer();
server.enable("jsonp callback");
server.get("/foo", function(req, res) {
// this is important - you must use Response.json()
res.json("hello");
});
jsonp with node.js express