nodejs

DesignPattern.js

// Pattern 
//Usually, you call a callback as the last thing you do inside a function. 
//You might consider it synonymous with return, only JavaScript does 
//not halt function execution when it hits it. It can be easy to accidentally 
//let execution continue after calling a callback, when you really expected it to end. 
//In order to make this easy to spot, and make sure execution stops in the way you expect, 
//I recommend returning the callback function call.

// wrong!

function writeCsvFile(target, data, callback) {
    convertToCsv(data, function (err, csv) {
        if (err) {
            callback(err); // oops! no return
        }
        // this line gets called even when theres an error
        writeFile(target, csv, callback);
    });
}


// right

function writeCsvFile(target, data, callback) {
    convertToCsv(data, function (err, csv) {
        if (err) {
            return callback(err); // execution stops here
        }
        writeFile(target, csv, callback);
    });
}
// Pattern 
// If you're calling a synchronous function that might throw an exception, 
// inside your asynchronous code, then calling your asynchronous 
// code might also result in an exception.

// wrong!

function readJson(filename, callback) {
    fs.readFile(filename, function (err, content) {
        if (err) {
            return callback(err);
        }

        // uh-oh! this line might throw an exception if the content
        // is not valid JSON
        var data = JSON.parse(content.toString());

        return callback(null, data);
    });
}


// right

function readJson(filename, callback) {
    fs.readFile(filename, function (err, content) {
        if (err) {
            return callback(err);
        }
        try {
            var data = JSON.parse(content.toString());
        }
        catch (e) {
            return callback(e);
        }
        return callback(null, data);
    });
}

// Pattern 2
// you can't use try-catch blocks around asynchronous code.
//Always, always, always pass errors back to the callback in asynchronous functions.
//As long as you're following the "Always check for errors in 
//callbacks" rule the errors will be handled in the appropriate place.

// wrong!

function getRecord(id, callback) {
    http.get('http://foo/' + id, function (err, doc) {
        if (err) {
            return callback(err);
        }
        if (doc.deleted) {
            // this will not get caught by the function calling getRecord
            throw new Error('Record has been deleted');
        }
        return callback(null, doc);
    });
}


// right

function getRecord(id, callback) {
    http.get('http://foo/' + id, function (err, doc) {
        if (err) {
            return callback(err);
        }
        if (doc.deleted) {
            return callback(new Error('Record has been deleted'));
        }
        return callback(null, doc);
    });
}