Function failing to return a value - node.js

Question

So I'm making a simple steganography tool (encrypting messages within images) and exposing it as a web service via Node.js. I am very new to Javascript and Node.js in particular. The app first converts a text string into a binary string by changing each character into an 8-bit ASCII encoding, resulting in one large binary string. I then encrypt the message within the pixels. Even values of pixels represent 0s from the binary, and odd values represent 1s. The end of the string is marked as 3 pixels of value 100 in a row (this is temporary, until I figure out a better way to mark the end). I'm using a node.js library called 'pngjs' that gives me pixel-level access to png images.
So I have a problem with the decodeMessage function. It builds up the string message, and is then meant to return it, however the return call at the end results in undefined.
How can I fix it?
Thanks in advance for the help!
function encodeMessage(image, mes) {
var message = mes;
var fs = require('fs'),
PNG = require('pngjs').PNG;
fs.createReadStream(image)
.pipe(new PNG({
filterType: 4
}))
.on('parsed', function() {
for (var y = 0; y < this.height; y++) {
for (var x = 0; x < this.width; x++) {
var idx = (this.width * y + x);// << 2;
//console.log(idx);
if (idx < message.length) {
var item = message.charAt(idx);
/* if the character in the encoded string is 0 */
if (item == 0) {
/* if the pixel is odd, we want it to be even */
if (this.data[idx] % 2 == 1) {
/* if the pixel is 0, add 1 to it */
if (this.data[idx] == 0) {
this.data[idx] = this.data[idx] + 1;
} else {
/* else substract 1 */
this.data[idx] = this.data[idx] - 1;
}
}
} else {
/* if the character in the encoded string is 1 */
if (this.data[idx] % 2 == 0) {
if (this.data[idx] == 0) {
this.data[idx] = this.data[idx] + 1;
} else {
this.data[idx] = this.data[idx] - 1;
}
}
}
//console.log(this.data[idx]);
} else if (idx === message.length) {
/* do something to the first pixel following the end of the string */
this.data[idx] = 100;
this.data[idx+1] = 100;
this.data[idx+2] = 100;
//console.log(this.data[idx]);
} else {
/* do something to the remaining pixels */
}
}
}
this.pack().pipe(fs.createWriteStream('encoded_' + image));
});
}
function decodeMessage(image) {
var message = "";
var fs = require('fs'),
PNG = require('pngjs').PNG;
fs.createReadStream(image)
.pipe(new PNG({
filterType: 4
}))
.on('parsed', function() {
dance:
for (var y = 0; y < this.height; y++) {
for (var x = 0; x < this.width; x++) {
var idx = (this.width * y + x);// << 2;
if (this.data[idx] == 100 && this.data[idx+1] == 100 && this.data[idx+2] == 100) {
break dance;
} else {
if (this.data[idx] % 2 == 0) {
message += "0";
} else {
message += "1";
}
}
}
}
/* the message outputs correctly over here */
console.log(message);
//return message;
});
/* but the return of the variable here doesn't work */
return message;
}
exports.encodeMessage = encodeMessage;
exports.decodeMessage = decodeMessage;

Answer 1

The parsed event is fired asynchronously, so you cannot return a value from decodeMessage.
function decodeMessage(image, cb) {
// Code
.on('parsed', function() {
// Code
console.log(message);
cb(message);
});
}
Then you must pass a callback to your decodeMessage function.
decodeMessage(image, function(decoded){
// Here is the decoded data.
});
The same is true for your encodeMessage function. The function will return before encoding has finished. If you want to know when it is done, you need to pass a callback the same way.