WebFund 2016W Lecture 10

Video

The video from the lecture given on February 9, 2016 is now available.

Student Notes

Assignment 3 Solutions

• Solutions to the assignment questions have been posted on the [assignment 3 page http://homeostasis.scs.carleton.ca/wiki/index.php/WebFund_2016W:_Assignment_3]
• Below is a review of these solutions

Q1

• What is the type of:
  • line: lines may be classified as an object but more specifically it is an array
  • rawContents: rawContents is of type string
  • split(): is a function. It is also acceptable to say that it is a method

Q2

• Line 24 should be printed first
• Since JavaScript is single threaded there is a high guarantee it is printed first since the program runs asynchronously
• In general, you can consider the program to do all of its main sequential work first and to then check for callback functions from asynchronous calls that must be run

Q3

• A one line change would be adding a line at line 42 by setting status = 404;
• We can also change the docroot but it's an unsafe way

Q4

• Just before line 92 we can add another if statement
• We check to see if the user-agent headers exists and whether it contains iPhone as a substring
  • If the condition is met, we respond with 403

Q5

A

• By commenting out this line, app will no longer be defined
• Anything referencing app will produce a reference error so the program crashes on the first line that uses app

Important Point

• What is the difference between JavaScript's require and C's include?
  • In C include looks similar to a require but in reality it’s a very different mechanism.
  • The include in C pre-processes the included file to put all of its code into the file that included it
    • Symbols are thus defined for us but there is no visible code! We cant tell what kind of symbols include has brought in
  • In JavaScript the file we require is pure JavaScript code!
    • The required script will run and put some value into its module.exports which is then returned via the require call
    • We know what symbol we are working with because we assign the return value of the require call ourselves

B

• Setting var port = 3000;
• It works perfectly fine but we can no longer change the port using the PORT environment variable

C

• Since we change the directory, it can no longer find “views”
• This means that the server will not be able to find any templates to render

D

• Commenting out line 24
• We are no longer exporting anything from the file
• Any calls to require the file will not get an expected return value from the require call and so the server will not work properly

E

• This line acts as a placeholder for child templates to put content into the body of this template
• By removing it, the child templates have nowhere to put the content in the body

F

• When we indent the code in this way, the submit button is no longer a part of the form
• The button will therefore not cause the form to be submitted
• Essentially, the button no longer does anything

Possible questions for the Midterm

• What is the purpose of app.post? What if app.post(‘/add’, ...) were be changed to app.get('/add', ...)?
  • Answer: The server will no longer know how to deal with any incoming POST requests for /add so it will not properly handle the form submission
    • Instead, the server could handle an incoming GET request for the form submission (but there are some other problems that would need to be fixed for parsing the incoming data)
• How can we keep it a get and still make it work?
  • Answer: Change the method in the form from method “post” to “get”
    • POST requests have the form data in their body whereas a GET request stores it in a query strings
    • So we can change req.body to req.query in the routing function to access the data properly

• What is app.use()?
  • What happens when we comment out the line app.use( ‘logger'..)?
    • The server no longer logs incoming request to the console
  • What if we comment out app.use(body.parser...)?
    • The program would crash because it wouldn’t be able to find req.body since the body of the request has no been parsed
  • What is the purpose of app.use(express.static...)?
    • Its tells the server to use a static web server to serve basic webpages and other static resources
• So app.use() is telling the server to use a given module to do something with the incoming requests