To conditionally skip a middleware when a condition is met, we can use the express-unless package.

In this article, we’ll take a look at how to use it.

Installation

We can install it by running:

npm i express-unless --save

Usage

After installing it, we can assign the unless function exposed by the express-unless and assign it to the unless property of our middleware functions.

For example, we can write:

const express = require('express');  
const unless = require('express-unless');  
   
const static = express.static(__dirname + '/public');  
static.unless = unless;const app = express();  
app.use(static.unless({ method: 'OPTIONS' }));  
app.listen(3000, () => console.log('server started'));

In the code above, we set the unless property of static to the unless function of the express-unless package.

Then we write:

app.use(static.unless({ method: 'OPTIONS' }));

to exclude the static route from running on OPTIONS requests.

If we’re writing our own middleware, we can assign the unless property of our middleware function as follows:

const express = require('express');  
const unless = require('express-unless');  
   
const static = express.static(__dirname + '/public');
const logHostname = (req, res, next) =>{  
  console.log(req.hostname);  
  next();  
}  
logHostname.unless = unless;const app = express();  
app.use(logHostname.unless({ method: 'OPTIONS' }));  
app.listen(3000, () => console.log('server started'));

In the code above, we defined the logHostman middleware to log the hostname. Then we set the unless property of the function to the unless function exposed in the express-unless package.

Options

In the object that we pass into unless , we can pass in the following options:

• method — this can be a string or an array of strings. If the request method matches then the middleware won’t run.
• path — this can be a string, regex or an array of string or regex. It can also be an array of object which is URL and method key-value pairs. If the request path or path and method match, then the middleware won’t run.
• ext — this is a string or array of strings. If the path ends with those extensions then a middleware won’t run.
• custom — a function that accepts req and returns a boolean. If the function returns true for the given request, then the middleware won’t run.
• useOriginalUrl — this should be a boolean. Defaults to true . If it’s false , the path will match against req.url instead of req.originalUrl .

Advanced Examples

We can exclude calling the express.static middleware when we have the jpg , html , css or .js extensions as follows:

const express = require('express');  
const unless = require('express-unless');  
const url = require('url');  
const static = express.static(__dirname + '/public');
static.unless = unless;
const app = express();  
app.use(static.unless((req) => {  
  const ext = url.parse(req.originalUrl).pathname.substr(-4);  
  return !['.jpg', '.html', '.css', '.js'].includes(ext);  
}));  
app.listen(3000, () => console.log('server started'));

In the code above, we check the extensions of the req.originalUrl property to see if they the request have the extensions listed in the array and skip the static middleware if they’re there.

We can also mix paths and request methods together as follows:

const express = require('express');  
const unless = require('express-unless');
const static = express.static(__dirname + '/public');  
const logHostname = (req, res, next) => {  
  console.log(req.hostname);  
  next();  
}  
logHostname.unless = unless;  
const app = express();  
app.use(logHostname.unless({  
  path: [  
    '/index.html',  
    { url: '/', methods: ['OPTIONS', 'PUT'] }  
  ]  
}));  
app.listen(3000, () => console.log('server started'));

Then we stop the logHostname middleware from running when we go to index.html or / with the PUT or OPTION request methods.

Conclusion

We can stop middleware from running on certain routes or request methods by using the express-unless package.

To use it, we just set the unless function exposed from the package and set it to the unless property of our middleware function.

Then we can exclude routes by using various conditions by checking the URL and/or request method.