Alternative to waiting for child_process to quit node.js - node.js

Question

I've got these 2 codes running in 1 script:
The ping script:
app.get("/ping", function(req, res) {
res.send("Pong!");
});
And a work in progress youtube downloader:
app.post("/nodedl", function(req, res) {
res.write("===Gradyncore listnener===\n")
res.write("Recived POST request from "+req.ip+" to /nodedl\n")
res.write("POST(url): "+req.body.url+"\n")
res.write("checking key...\n")
if (req.body.key==="<Insert key here>"){
res.write("Key is valid! Skipping pre-download script...\n")
} else {
res.write("Key is invalid. Running pre-download script...\n")
exec("/home/gradyn/website/projects/nodeDL/scripts/check.sh", function (error, results) {
if (results != null) {
res.write(results+"\n");
} else if (error != null) {
res.write("Error: " + error+"\n");
}
});
}
res.end();
});
The problem is, by the time check.sh finishes, res.end(); has allredy been emitted, causing this error followed by a crash
events.js:160
throw er; // Unhandled 'error' event
^
Error: write after end
at ServerResponse.OutgoingMessage.write (_http_outgoing.js:439:15)
at /home/gradyn/listener/app.js:29:13
at ChildProcess.exithandler (child_process.js:213:5)
at emitTwo (events.js:106:13)
at ChildProcess.emit (events.js:191:7)
at maybeClose (internal/child_process.js:877:16)
at Socket.<anonymous> (internal/child_process.js:334:11)
at emitOne (events.js:96:13)
at Socket.emit (events.js:188:7)
at Pipe._handle.close [as _onclose] (net.js:498:12)
I'm fairly new to node.js, but from what I understand, If i wait for the child process to complete before calling res.end(), the entire script (including the ping listener) will not work until the child process completes.
How should I go about doing this?

Answer 1

Problem is that your exec is asynchronous, code below it won't wait to for completion of exec and get executed... You have to end response inside callback function:
app.post("/nodedl", function(req, res) {
res.write("===Gradyncore listnener===\n")
res.write("Recived POST request from "+req.ip+" to /nodedl\n")
res.write("POST(url): "+req.body.url+"\n")
res.write("checking key...\n")
if (req.body.key==="<Insert key here>"){
res.write("Key is valid! Skipping pre-download script...\n")
} else {
res.write("Key is invalid. Running pre-download script...\n")
exec("/home/gradyn/website/projects/nodeDL/scripts/check.sh", function(error, results) {
if (results != null) {
res.write(results+"\n");
res.end();
} else if (error != null) {
res.write("Error: " + error+"\n");
res.end();
}
});
}
});