Question

I’d like to make a node.js function that, when calls, reads a file, and returns the contents. I’m having difficulty doing this because ‘fs’ is evented. Thus, my function has to look like this:

function render_this() {
    fs.readFile('sourcefile', 'binary', function(e, content) {
        if(e) throw e;
        // I have the content here, but how do I tell people?
    });
    return /* oh no I can't access the contents! */;
};

I know that there might be a way to do this using non-evented IO, but I’d prefer an answer that allows me to wait on evented functions so that I’m not stuck again if I come to a situation where I need to do the same thing, but not with IO. I know that this breaks the “everything is evented” idea, and I don’t plan on using it very often. However, sometimes I need a utility function that renders a haml template on the fly or something.

Finally, I know that I can call fs.readFile and cache the results early on, but that won’t work because in this situation ‘sourcefile’ may change on the fly.

Answer 1

OK, so you want to make your development version to automatically load and re-render the file each time it changes, right?

You can use fs.watchFile to monitor the file and then re-render the template each time it changed, I suppose you’ve got some kind of global variable in your which states whether the server is running in dev or production mode:

var fs = require('fs');
var http = require('http');
var DEV_MODE = true;

// Let's encapsulate all the nasty bits!
function cachedRenderer(file, render, refresh) {
    var cachedData = null;
    function cache() {

        fs.readFile(file, function(e, data) {
            if (e) {
                throw e;
            }
            cachedData = render(data);
        });

        // Watch the file if, needed and re-render + cache it whenever it changes
         // you may also move cachedRenderer into a different file and then use a global config option instead of the refresh parameter
        if (refresh) {
            fs.watchFile(file, {'persistent': true, 'interval': 100}, function() {
                cache();
            });
            refresh = false;
        }
    }

    // simple getter
    this.getData = function() {
        return cachedData;
    }

    // initial cache
    cache();
}


var ham = new cachedRenderer('foo.haml',

    // supply your custom render function here
    function(data) {
        return 'RENDER' + data + 'RENDER';
    },
    DEV_MODE
);


// start server
http.createServer(function(req, res) {
    res.writeHead(200);
    res.end(ham.getData());

}).listen(8000);

Create a cachedRenderer and then access it’s getData property whenever needed, in case you’re in development mod it will automatically re-render the file each time it changes.