Declaratively build Express applications with web components.
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What is this?

This repo contains web components built with the Google Polymer library that allow you to create Express.js applications. If you have not heard of web components, then please start learning today. Web components offer a way to modularize and package functionality into reusable components that can be easily shared and composed to create entire applications. Currently they are used mostly for front-end web development. Well, what about the back-end? Web components are not only useful for visual components, as the Polymer project has shown us. Now you can build APIs and other server-side applications, leveraging the same declarativeness of the front-end world. We are one step closer to true Universal JavaScript.

Mini Application

With boilerplate removed, and using Polymer 2, this is a preview of what you can expect server-side web component code to look like:

<express-app port="5000">
    <express-middleware method="get" path="/" callback="[[indexHandler]]"></express-middleware>
    <express-middleware callback="[[notFoundHandler]]"></express-middleware>

    class ExampleApp extends Polymer.Element {
        static get is() { return 'example-app'; }
        constructor() { super(); }

        indexHandler(req, res) {
            res.send('Hola mundo!');
        notFoundHandler(req, res) => {
            res.status(404).send('not found');


Here are some example Express.js apps that have been rewritten with these web components:

Here is a live example, built with web components on the front-end and the back-end.


These web components are meant to be run using Scram.js, which provides access to the Electron runtime, and Express.js, which is one of the most popular web frameworks running on top of Node.js. You must install these dependencies into your project separately:

bower install --save express-web-components
npm install --save express
npm install --save scram-engine

See here for information on how to use these components with Scram.js.


In addition to the documentation below, a great place to learn how to use the components is to view this repo's example.

Also, in the documentation I'm attempting to describe the API using TypeScript types. Not all of the types I'm using are real TypeScript types, I'm just hoping it helps describe what is expected. You do not need to use TypeScript to use these components.



Creates an Express application and calls the Express app.listen function. This component is the parent of all other components that you inted to be a part of the Express application created. As long as you use different ports, you can have multiple Express applications running for each instance of <express-app></express-app>. You can also nest <express-app></express-app> elements to create sub-apps.


port: string | number

The port the Express application will run on, as specified by app.listen.

path?: string

The path that this application instance will be mounted at on the parent app, used when creating a sub-app.

hostname?: string

An optional hostname the Express application will run on, as specified by app.listen.

backlog?: number

An optional backlog the Express application will use, as specified by app.listen, and for more info see server.listen.

callback?: (e: NodeListeningEvent) => any

An optional callback function to be invoked by app.listen, as specified by app.listen


Allows specifying a callback function to be invoked with the current pertinent objects of the parent Express application. This is useful for setting configurations, such as calling express.static(root, [options]).


callback: (app: express.Application, express: Express, router: express.Router, route: express.Route) => any

A callback function that will be invoked with the the current Express application, the current Express object (from require('express')), the current parent router, and the current parent route.


Allows hooking up Express middleware, i.e. performs the equivalent of app.use, app.get,, etc.


method?: string

An optional HTTP method to associate the middleware with, as defined by the Express app.METHOD. You must specify a path if you specify a method.

path?: string

The optional URL path that the middleware will be associated with. The path will default to / if no method is specified. You must specify a path if you specify a method.

callback: (req: express.Request, res: express.Response, next: express.NextFunction) => any

The callback function to be invoked on a matching request.

callbacks?: (req: express.Request, res: express.Response, next: express.NextFunction) => any[]

An optional list of callback functions to be invoked on a matching request.


Creates an Express router. All child components are hooked up to this router.


path?: string

The optional URL path that the router will be associated with. Defaults to /.


Creates an Express route. All child components are hooked up to this route.


path: string

The URL path that the route will be associated with.


Equivalent to app.param() or router.param(), depending on its immediate parent element.


name: string

The name of the parameter that the callback will be triggered for.

callback: (req: express.Request, res: express.Response, next: express.NextFunction, value: string, name: string): => any

The callback triggered on the corresponding route parameter. The parameters to the callback are the request object, the response object, the next middleware, the value of the route parameter, and the name of the route parameter.

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