NodeJS Callback and Return value confusion - node.js

Question

I need to find a way return a string to digest in my primary code block as well as a callback or something to start working on the rest of the code in my primary code block once the value for digest is returned.
Please Help!
Here is my current code that does not work.
var digest = checkIntegrity(filePath, res[3]);
//digest always come back undefined and never matches res[2] so file always deletes
if (digest === 0){
console.log('File Inaccessible');
} else {
if (digest === res[2]){
createNewFile();
} else {
console.log('File hash doesn't match');
delBadFile();
}
}
function checkIntegrity(filePath, algorithm, cb){
console.log('in checkIntegrity');
var hash = crypto.createHash(algorithm);
var digest;
//see if file is there
fs.stat(filePath, function(fileErr, fileStats){
if(fileErr){
//error accessing file, most likely file does not exist
return 0;
} else {
//file exists
var fileIn = fs.createReadStream(filePath);
fileIn.on('data', function(chunk){
if (chunk) {
hash.update(chunk);
}
});
fileIn.on('end', function(){
return hash.digest('hex');
});
}
});
}

Answer 1

You're checkIntegrity function is asynchronous, i.e. it accepts a callback. Any value that you wish to pass as a result of that function should be passed as an argument to that callback function. What is happening in your example is that checkIntegrity is calling out to fs.stat (which is also asynchronous) and then returns undefined straightaway - before fs.stat has chance to complete.
You have a choice:
Change the call from fs.stat to fs.statSync. That is a synchronous version of the stat function.
Change your code to use callbacks properly:
checkIntegrity(filePath, res[3], function (err, digest) {
if (err) return console.error(err);
if (digest === 0) {
console.log('File Inaccessible');
} else {
if (digest === res[2]){
createNewFile();
} else {
console.log('File hash doesn\'t match');
delBadFile();
}
}
});
function checkIntegrity(filePath, algorithm, cb){
console.log('in checkIntegrity');
var hash = crypto.createHash(algorithm);
var digest;
//see if file is there
fs.stat(filePath, function(fileErr, fileStats) {
if(fileErr){
//error accessing file, most likely file does not exist
return cb(fileErr);
} else {
//file exists
var fileIn = fs.createReadStream(filePath);
fileIn.on('data', function(chunk){
if (chunk) {
hash.update(chunk);
}
});
fileIn.on('end', function() {
cb(null, hash.digest('hes'));
});
}
});
}
In my opinion, asynchronous code and callbacks are such a fundamental part of Node.js I would encourage you to go for option 2. It is definitely worthwhile learning. There are hundreds of sites out there like callbackhell.com that will do a much better job at explaining callbacks.