middl

A generic middleware library, inspired by Express and suitable for anything

Note all examples below use ES2015 syntax but the library should be able to run even in browsers back to IE9, as long as you're polyfilling the Promise object.

Why?

Because the middleware pattern is super useful! With this module you can even BYOE (Build Your Own Express) quite easily (see example below).

It also supports promises so your middleware can be async (see module usage for an example).

Installation

Install middl using npm:

npm install --save middl

Usage

Module usage

const middl = require('middl');
const app = middl();
 
// a sync middleware
app.use((input, output) => {
    output.prop = 1;
});
 
// an async middleware
app.use((input, output) => {
    // Faking a time consuming task...
    return new Promise(resolve => {
        setTimeout(() => {
            output.prop += 1;
            resolve();
        }, 10);
    });
});
 
// a time measuring logger:
app.use((input, output, next) => {
    var start = new Date();
    next()
        .then(() => {
            var ms = new Date() - start;
            console.log('Done in %s ms', ms);
        });
});
// or even prettier with generator functions:
app.use(function *(input, output, next) {
    var start = new Date();
    yield next();
    var ms = new Date() - start;
    console.log('Done in %s ms', ms);
});
// or when using Babel and async/await:
app.use(async (input, output, next) => {
    var start = new Date();
    await next();
    var ms = new Date() - start;
    console.log('Done in %s ms', ms);
});
 
 
// an error handling middleware
app.use((err, input, output, next) => {
    if (err) {
        console.error(err);
        // by not calling next() we abort the chain
    } else {
        next();
    }
});
 
// conditional middleware
// this will be run because the conditions ({val: 'hello'}) matches the input:
app.match({val: 'hello'}, (input, output) => {
    output.response = 'world';
});
// this will *not* be run because the former middleware did not call next:
app.match({val: 'hello'}, (input, output) => {
    output.response = 'you';
});
// this won't run because the conditions ({val: 'hi'}) does not match the input
app.match({val: 'hi'}, (input, output) => {
    output.response = 'there';
});
 
// pass in the initial `input` and `output` objects
// and run the middleware stack:
app.run({val: 'hello'}, {})
    .then(output => {
        // output.prop === 2
        // output.response = 'world'
    });

Examples

Example BYOE

BYOE - Build Your Own Express

With Middl all it takes to create an Express like web server is the following code snippet. Off course this is a quite limited feature set and API compared to the full Express library, but hopefully it shows how to use Middl in a creative way.

const http = require('http');
const middl = require('middl');
 
// Make middl more Express like by using `url` as the property to match paths with:
const app = middl({pathProperty: 'url'});
 
// Adding all app.METHOD() functions à la Express:
http.METHODS.forEach(method => {
    app[method.toLowerCase()] = app.match({method});
});
// Also the app.all():
app.all = (path, fn) => {
    http.METHODS.forEach(method => {
        app[method.toLowerCase()](path, fn);
    });
    return app;
};
 
// A route handler for requests to: GET /test
app.get('/test', (req, res) => {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('ok\n');
});
// A route handler for requests to: POST /test
app.post('/test', (req, res) => {
    res.writeHead(202, {'Content-Type': 'text/plain'});
    res.end('accepted\n');
});
 
// Make the middle app web server listen on port 3000:
http.createServer(app).listen(3000);

API

middl(options)

Name	Type	Description
options	Object	Options

Returns: app

options.pathProperty

Type: String

Used to enable the possibility to use a mount path when adding middleware (see example above).

app(input, output)

Name	Type	Description
input	Object	The input to the middleware stack, will be used as the first argument of each middleware (think of Express' req)
output	Object	The output to/from the middleware stack, will be used as the second argument of each middleware (think of Express' res)

The Express like app function.

Returns: Promise which is resolved when the whole stack of middleware have been run through, or the flow have been aborted by throwing an error/returning a rejected Promise or by not calling next.

app.use([path, ], ...fn)

Name	Type	Description
path	String	An optional mount path for the middleware (only applicable if options.pathProperty is set)
...fn	Function	The middleware function(s)

The middleware function signature should be similar to that of Express, i.e: function ([err, ] input, output [, next]) { ... }.

Returns: app for chaining.

app.match(conditions [, path [, ...fn]])

Name	Type	Description
conditions	Object	The middleware will only be run if the current input matches this object
path	String	An optional mount path for the middleware (only applicable if options.pathProperty is set)
...fn	Function	The middleware function(s)

The middleware function signature should be similar to that of Express, i.e: function ([err, ] input, output [, next]) { ... }.

Returns: app for chaining if ...fn is provided, otherwise a partially applied function will be returned, e.g:

// this:
app.match({method: 'GET'}, '/test', (req, res) => { ... });
// is the same as this:
const get = app.match({method: 'GET'});
get('/test', (req, res) => { ... });

How the conditions are matched

Each own property of the conditions object are matched with the property with the same name in input, in the following way:

If conditions[property] is a:

• regular expression: input[property] must match the regular expression
• function: the function will be called with input[property] as argument and should return a boolean indicating if the property matches (true) or not (false)
• otherwise: input[property] must equal (===) conditions[property]

License

MIT © Joakim Carlstein