Node.js - Best practices

Best Practices

Felix Geisendörfer Munich Node.js User Group 01.12.2011 (v1)

(@)felixge(.de)

Twitter / GitHub / IRC

Felix Geisendörfer
(Berlin, Germany)

A terrible Example:

var fs = require('fs');

function readJSON(path, cb) {
fs.readFile(path, 'utf8', function(err, data) {
cb(JSON.parse(data));
});
}

Error Delegation


if (err) return cb(err);

});
}

Exception Handling


try {
} catch (err) {
cb(err);
}
});
}

Error parameter ﬁrst


try {
cb(null, JSON.parse(data));
} catch (err) {
cb(err);
}
});
}

Not your error


try {
var json = JSON.parse(data);
} catch (err) {
return cb(err);
}

cb(null, json);
});
}

Another common mistake
function readJSONFiles(files, cb) {
var results = {};
var remaining = files.length;

files.forEach(function(file) {
readJSON(file, function(err, json) {

results[file] = json;
if (!--remaining) cb(null, results);
});
});
}

Only call back once
function readJSONFiles(files, cb) {
var results = {};
var remaining = files.length;

files.forEach(function(file) {
readJSON(file, function(err, json) {
if (err) {
cb(err);
cb = function() {};
return;
}

results[file] = json;
if (!--remaining) cb(null, results);
});
});
}

db.query('SELECT A ...', function() {
db.query('SELECT B ...', function() {
db.query('SELECT C ...', function() {
db.query('SELECT D ...', function() {
db.query('SELECT E ...', function() {
db.query('SELECT F ...', function() {
db.query('SELECT G ...', function() {
db.query('SELECT H ...', function() {
db.query('SELECT I ...', function() {

});
});
});
});
});
});
});
});
});

Control Flow Libs
var async = require('async');

async.series({
queryA: function(next) {
db.query('SELECT A ...', next);
},
queryB: function(next) {
},
queryC: function(next) {
}
// ...
}, function(err, results) {

});

Split code into more
methods

Maybe node.js is not a
good tool for your
problem?

Exceptions
(Dealing with Death)

Exceptions

throw new Error('oh no');

Explicit, useful to crash your program

Exceptions
node.js:134
throw e; // process.nextTick error, or 'error' event on ﬁrst tick
^
Error: oh no
at Object.<anonymous> (/Users/felix/explicit.js:1:69)
at Module._compile (module.js:411:26)
at Object..js (module.js:417:10)
at Module.load (module.js:343:31)
at Function._load (module.js:302:12)
at Array.<anonymous> (module.js:430:10)
at EventEmitter._tickCallback (node.js:126:26)

Output on stderr

Exceptions
function MyClass() {}

MyClass.prototype.myMethod = function() {
setTimeout(function() {
this.myOtherMethod();
}, 10);
};

MyClass.prototype.myOtherMethod = function() {};

(new MyClass).myMethod();

Implicit, usually undesirable / bugs

Exceptions

/Users/felix/implicit.js:5
this.myOtherMethod();
^
TypeError: Object #<Object> has no method 'myOtherMethod'
at Object._onTimeout (/Users/felix/implicit.js:5:10)
at Timer.callback (timers.js:83:39)

Incomplete stack trace : (

Global Catch

process.on('uncaughtException', function(err) {
console.error('uncaught exception: ' + err.stack);
});

Please be careful
with this!

Global catch gone wrong
// Deep inside the database driver you are using
cb(null, rows);

this._executeNextQueuedQuery();

// In your code
db.query('SELECT ...', function(err, rows) {
console.log('First row: ' + row[0].id);
});

If you have to:

process.on('uncaughtException', function(err) {
// You could use node-airbake for this
sendErrorToLog(function() {
// Once the error was logged, kill the process
console.error(err.stack);
process.exit(1);
});
});

Even Better

Processes die.

Accept it.

Deal with it on a higher level.

Advanced Beginner

$ node server.js &

Competent

$ screen
$ node server.js

Proﬁcient

#!/bin/bash

while :
do
node server.js
echo "Server crashed!"
sleep 1
done

Expert
#!upstart

description "myapp"
author "felix"

start on (local-filesystems and net-device-up
IFACE=eth0)
stop on shutdown

respawn # restart when job dies
respawn limit 5 60 # give up restart after 5
respawns in 60 seconds

script
exec sudo -u www-data /path/to/server.js >> /
var/log/myapp.log 2>&1
end script

Innovation

$ git push joyent master
$ git push nodejitsu master
$ git push heroku master

Node.js - Best practices

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