IT 452 - Tips for Debugging

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JavaScript / DOM manipulation

• Error Console: The Chrome browser has made things easier for you. Right-click anywhere on a page and select "Inspect Element". This brings up a view of the DOM tree, and a list of icons. Click the Console to see JavaScript errors! Inspect the DOM tree to see if it is what you expect.
• Firefox has an Error Console too. Go to Tools, and then "Error Console". It also has a DOM inspector like Chrome
• Make sure to validate your HTML -- bad HTML will break your code
• Add calls to window.alert() to verify that parts of your code are executing, and to check what the values of things are.
• A particular useful command is:

                   window.alert(curNode.nodeName);
              

  This tells you whether the 'curNode' is really a "TD" or a "TR" or a "H1" element or what.
• If you try to manipulate the DOM tree (e.g. to move something) and it doesn't work, the problem might not be your code to manipulate -- it could be that your HMTL was malformed to begin with, or that you previously added some new DOM elements (like adding a new row) that didn't work. (see below)
• Use the DOM inspector to check what your DOM tree looks like (and see if you added things correctly to it). To install it in Firefox, select Tools->Add-ons, then click "Get Add-ons" and search for "DOM inspector"

File permissions / "Permission denied"

• If you map a drive to intranet.cs.usna.edu, all the default permissions should be fine. (it should by default set "read" and "execute" permissions for all your files). Note that it *is* possible to find your file via other network paths (such as mara.cs.usna.edu) -- but using this will NOT lead to the correct permissions.
• If you have trouble, ssh into intranet.cs.usna.edu (use putty or similar tool). Find your files inside your publc_html file and run "ls -l". Here is an example of a file with a problem:

  #[54] [~/]> ls -l ~mXXXXXX/public_html/Lab02/info.xml
  -rwx------   1 mXXXXXX  mids         886 Jan 29 17:51 /home/mXXXXXX/public_html/Lab02/info.xml*
  

  This could be fixed with "chmod a+rx ~mXXXXXX/public_html/Lab02/info.xml"

Perl: line endings and debugging

• VERY IMPORTANT: You are likely editing your Perl files on a Windows machine. On Windows, files are usually save in "DOS" mode, which means each line ending is actually two characters (CR/LF). On Unix, only one character is used, and our Unix-based web server will fail on any files saved in Windows mode. For this reason:
  • Do not use Notepad! Use Crimson Editor or Notepad++
  • If you use Crimson Editor, do this: Tools->Preferences->File. Check "Save files in Unix format" and "Save remote files in Unix format."
  • Notepad++: Settings->Preferences->New Document/Deefault Directory. Select "Unix" for format. Note that you must do this BEFORE you create your Perl file, but then it should remember the setting on that computer. If you already have a file in Dos\Windows mode (look on the status bar at the bottom of the screen), then do this: Edit->EOL Conversion->UNIX format.
  • Whatever tool you use: do this change on your Bancroft machine as well.
• Test your Perl by typing the URL into the browser -- be sure to include the Carp module in your Perl file so that errors will show up here. You can include CGI arguments in the URL. For instance, you might use this URL:

  http://intranet.cs.usna.edu/~mXXXXXX/Lab21/query.cgi?user=joe&page=20
  

• If you really get no output from Perl, ssh into the server and check the file permissions (see above). If permissions seem okay, you could try to run it directly from the server. Here are some examples:
  • Note: do NOT do the following:

    # DON'T DO THIS!!!
    perl -c -w myquery.cgi  # DON'T DO THIS -- calls wrong version of Perl!!!!
    

  • To run Perl, just execute the program directly (it will tell you if there are any syntax errors):

    ./myquery.cgi
    

  • If needed, you can provide CGI arguments; below is a simple example. Notice the syntax is a little different for the arguments then it is with a URL:

    #[80] [~/public_html/webtest/]> ./test.cgi "user=joe&page=27"
    Content-type: text/plain;
    
    hello27
    user: joe
    page: 27
    

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