Node.js - Loading Files in Runtime

Question

I'm fairly new to Node.js and am having trouble understanding the way to go about loading libraries or files, in runtime. Apparently, it is a bad idea to load files in runtime using Node.js's native "require" function because it is blocking i/o, and therefore should not be used from within request handlers. So, I'm assuming something like this is to be avoided:

var http = require('http').createServer(function(req, res) { 
  var file = require('./pages/'+req.url);
  res.end();
}).listen(8080);

So then is there a way to require files in runtime, in a non-blocking/asynchronous way?

I don't think it would always be possible to load files in "boot time" rather than runtime because like in the example above, the only way to know what file to load/require is by getting the name through the req.url property.

So that seems like the only option. Unless, all the files in the folder are preloaded and then called upon by name, in the callback (By using fs.readdirSync or something to iterate through all the files in the folder and compare the gotten files' names to the req.url property), but that seems wasteful. "Preloading" all the files in the folder (maybe around 50 files) and then only using 1 of them, doesn't seem like a good idea. Am I wrong?

Either way, I would just like to know if there is a way to require files in runtime in a better, non-blocking/asynchronous way.

Thank you!

hexacyanide · Accepted Answer · 2013-09-14 19:33:27Z

7

The function require() is generally used for caching modules or configuration files before most of your application runs. You can think of using require() somewhat like this:

var file = fs.readFileSync('/path');

// depending on the file type
eval(file);
JSON.parse(file);

The reason it is done this way is so that dependencies are loaded in order. If you want to read a file after initializing the application, you should use a asynchronous read, and respond in the callback like this:

var http = require('http');
http.createServer(function(req, res) {
  fs.readFile('./pages/' + req.url, function(err, data) {
    res.end(data);
  });
}).listen(8080);

If you needed to check if a file existed, then you could use fs.stat() to check the existence of a file, rather than querying the directory.

var http = require('http');
http.createServer(function(req, res) {
  var file = './pages/' + req.url;
  fs.stat(file, (err, stats) {
    if (err || !stats.isFile()) {
      res.writeHead(404);
      res.send();

      return;
    }

    fs.readFile(file, function(err, data) {
      res.writeHead(200);
      res.end(data);
    });
  });
}).listen(8080);

edited Sep 14, 2013 at 19:33

answered Sep 14, 2013 at 19:22

hexacyanide

92.4k31 gold badges168 silver badges162 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

user2779891 Over a year ago

Thank you! This works great for files like static HTML files, CSS files etc. But what if it's a Javascript file? How should I evaluate the Javascript code that I get from the file? eval isn't really a good idea so how should I do it?

user2779891 Over a year ago

But isn't that considered bad practice? I mean since require() is blocking?

hexacyanide Over a year ago

I deleted my previous comment because it is misleading. I meant, if you have anything to execute, then you should require() it at the beginning of the application. That is how require() is and should be used. You shouldn't load functions into your application after it has started.

user2779891 Over a year ago

Oh I see. But that is really the problem, that I can't use require() inside the request handler, but that's where I need to include the file from. I can only know which file to require by getting the name through req.url. But the req.url is only accessible inside the request handler. I can't use require() anywhere before that because I won't know the name of the file until I get it through req.url.

hexacyanide Over a year ago

If that's what you're looking for, then it isn't really possible. There once was an asynchronous require(), but it was seldom used and eventually removed. However, you could fork the main process and run code in another process. There are also modules which do similar things to eval(), such as the vm module, vm.runInNewContext() and vm.runInThisContext().

|

Collectives™ on Stack Overflow

Node.js - Loading Files in Runtime

1 Answer 1

8 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

8 Comments

Your Answer

Sign up or log in

Post as a guest

Related