The reflector

This tutorial presents a classic exercise: the reflector. A reflector is a simple web page that presents (reflects) all the data it receives: query strings, form data, HTTP headers, …

The references for this tutorial are

Prepare your project

This is going to be our first project with a server-side component. Consequently, its skeleton is going to be different from the one in the previous tutorials. Indeed, it is customary for a Node.js project to contain a file named package.json, describing the project, its dependencies, and the way to run it.

Start by remixing the project

  1. Execute the project by clicking the “Show” button. Explore the different urls.

  2. Create a new file named reflector.js. Edit package.json so that Glitch starts your reflector.js instead of example.js.

    Following the examples in example.js, make it show the message

    monde !

    for the URL /.

    The blank line is important. How to make sure that the browser shows it? (Tip: unless you tell it otherwise, the browser will interpret anything it receives as HTML code).

  3. In the handler for /, replace the previous code with a for ... in loop, printing the contents of the object

    let jours = { 'mon' : 'Lundi',
                  'tue' : 'Mardi',
                  'wed' : 'Mercredi',
                  'thu' : 'Jeudi',
                  'fri' : 'Vendredi',
                  'sat' : 'Samedi',
                  'sun' : 'Dimanche' };

    one key-value pair per line.

    Warning: a handler cannot execute res.send() more than once. We recall that string concatenation in JavaScript is performed by the plus operator (+).

The reflector, GET parameters

  1. We already saw in the first tutorial that web apps can accept parameters via URLs. It was the case, for example, for Google’s search, that we queried via a URL like thus

    The part of the URL following the question mark (?) is called the query string. Its syntax is partly defined by RFC 3986, partly by common practices. Express automatically analyses it and makes its contents accessible to the handler via the req.query object.

    Create a new handler for the URL /query_string that shows the contents of the query string, one key-value pair per line. Test your handler by passing various query strings in the addres bar, such as

  2. The unparsed query string is accessible via the req._parsedUrl.query property. Modify your handler so that, besides the parsed parameters, it also shows the unparsed query string. Visit now this URL

    /query_string?A=B=3&C=%26&X Y=W+Z&X%20Y=W%2BZ

    What do you notice? To better understand how the ULR has been interpreted, you can read this Wikipedia article.

Intermezzo: serving static files

Using JavaScript strings and res.send() allowed us to dynamically generate HTML. However, this is not efficient for large HTML files, especially if they are purely static (i.e., no content is generated on the fly).

The middleware express.static allows us to serve static files directly from a named folder. All you have to do is to add the following line to the configuration of your Express application.

app.use('/s', express.static('<name-of-the-folder>'));

The static files will be then accessible via the URL /s/<name-of-thefile>.

  1. Create a static file named, e.g., static/form.html. Add to it an HTML form that sends all its data to the /query_string URL (use the action attribute of <form>) using the get method (use the method attribute).

    Test the reflector via the form. In particular, test special charatecter, such as spaces, ampersands (&), etc.

The reflector, POST parameters

We now move on to data sent via the POST HTTP method. While parsing of the query string is automatically activated in Express, parsing of POST requests must be activated via the middleware body-parser. Add these lines to your configuration

const bodyP = require('body-parser');
app.use(bodyP.urlencoded({ extended: false }));

or, equivalently,

app.use(require('body-parser').urlencoded({ extended: false }));
  1. Modify the form from the previous exercise so that it sends its data via POST (method="POST") to a new URL /form_data.

    Create the handler for /form_data. The body-parser middleware makes the form data available in the req.body object. Present the form data one key-value pair per line, as done before.

    Note: your handler must handle POST requests, not GET requests. POST handlers are created by the call, instead of app.get().

    Test your handler using the form. Compare the results with the data presented by your browser’s dev tools “Network” tab (in Chrome or Firefox, open the dev tools with the F12 key).

The reflector, Cookies and headers

Two more interesting objects are attached to the request:

The cookie-parser middleware is not a standard one, and it must be installed. To install an Node.js package, go to package.json, and click on the “Add Package” menu (top-left of the editor). Start typing the name of the package you want to install, and then click on it. You will see that it is added to the dependencies section (you can also do this manually).

  1. Install the cookie-parser package, then activate the middleware in your application with

    const cookieP = require('cookie-parser');
  2. Create a new handler for the /headers URL, showing the contents of req.headers and req.cookies. Compare the results with the “Network” and “Storage/Application” tabs in the dev tools.

    Note: You can create a handler that anwers to all kinds of HTTP requests (GET, POST, etc.) by replacing app.get() with app.all().

  3. Now put everything together (you can copy-paste your code). Replace the handler for / with one that answers to all types of HTTP requests by showing:

    • the contents of the query string,
    • the request body, if it has one,
    • the HTTP headers,
    • the cookies.
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