Error Handling by display static HTML page - node.js

Question

I am trying to add functionality to my error handler by not only logging the message to the console, but by redirecting the client's browser to a static HTML page that would display some simple text content. Here is the existing handler:
var sql = require('msnodesql');
//store a connection to MS SQL Server-----------------------------------------------------------------------------------
sql.open(connStr, function(err, sqlconn){
if(err){
console.error("Could not connect to sql: ", err);
}
else
conn = sqlconn; //save the sql connection globally for all client's to use
});
I'm using express.js to create my web server. This is server side code. I want this to happen in realtime, as soon as the error occurs the client's web browser gets redirected.
EDIT: I guess what I really want to know is how to redirect the client's browser to a page from inside if(err).

Answer 1

You can use a middleware that checks the state of the connection for each request and renders an appropriate template depending on the state (instead of using res.render, you can also use res.redirect or res.sendfile, of course):
var sql = require('msnodesql');
var conn = null;
sql.open(connStr, function(err, sqlconn) {
if (err) {
console.error("Could not connect to sql: ", err);
conn = false;
} else {
conn = sqlconn;
}
});
// Express middleware that checks the connection state of the database
// connection: active, not yet active, or failed.
app.use(function(req, res, next) {
// database connection not active yet
if (conn === null || conn === undefined) {
res.status(503);
return res.render('not-active-yet');
}
// database connection failed
if (conn === false) {
res.status(500);
return res.render('db-connection-failed');
}
// everything seems okay
next();
});
EDIT: forgot to mention that you need to include this middleware very early in the middleware chain, but certainly before any of your routes.