WebFund 2013F: Tutorial 4

In this lab you will be learning the basics of debugging Node-based web applications. All of the following assumes you have form-demo setup and running from Assignment 1.

Browser-based debugging

• Firefox: Tools->Web Developer->Toggle Tools
• Chrome/Chromium: Tools->Developer Tools

Select the Network tab to see HTTP traffic. Select Inspector (Firefox) or Elements (Chrome/Chromium) to see the HTML document (DOM).

Node debugger

Node has a built-in debugger. Start it by running node debug app.js. This will stop on the first line of the file. Type n to step to the next line of the file. Type c to continue to the next breakpoint. Breakpoints are set by adding a debugger; statement to the javascript source.

At any time you can type repl into the debugger to drop into a read-eval-print loop where you can evaluate JavaScript statements in the current context. Ctrl-C will get you out of the REPL.

For example, consider this source for app.js.

var x = 5;
var y = 10;

debugger;

You can run node debug app.js to start the debugger.

It which will stop on the first line of the file (var x = 5;). If you enter c node will continue executing until the debugger; statement where it will stop. From here if you enter repl you can execute Javascript in the current context. In the repl prompt if you enter x; it will return 5. If you enter x + y; it will return 15, etc.

More commands:

• s step in
• o step out
• list(x) show x number of lines around current line

Brackets and Theseus

Note: Theseus and Brackets are purely optional. They are both still in active development. THEY HAVE BUGS. But when they work, they are pretty amazing!

To get Brackets running in the class VM

• Download Brackets: visit http://download.brackets.io/ and download the DEB file. It should go in your Downloads folder.
• Install it: sudo dpkg -i ~/Downloads/brackets-sprint-31-LINUX32.deb
• Make an alias for google-chrome: sudo ln -s /usr/bin/chromium-browser /usr/bin/google-chrome
• Run it: brackets &
• Select "Live Preview" from the File Menu.
• Edit the source and watch the getting started page change as you type!

To get Theseus working in Brackets

• Select File->Extension Manager in Brackets.
• Under the Available tab. search for Theseus. Click the "Install" button.
• Install node-theseus globally: sudo npm install -g node-theseus
• Select File->Open Folder with the form-demo folder.
• Run the app: node-theseus app.js
• Note how you now see information on how each file is called.

Tasks

• Observe the request and response for the app's home page (http://localhost:3010). Look at both the network panel (load the page after selecting the network panel) and the HTML DOM view (Inspector/Elements)
• Observe the contents of the form submit POST request: how much data is sent to the server? Observe it both from the browser side (to see what is sent) and inside of node, particularly where the POST results are returned.
• Look at other web pages!