Homework 2

Due Date: July 5 before midnight;
Max Points: 100

Important Reminder: As per the course Academic Honesty Statement, cheating of any kind will minimally result in your letter grade for the entire course being reduced by one level.

To be turned in via the submission link on brightspace.

Please remember to justify all answers.

You are encouraged to use the web or the library but are required to cite any external sources used in your answers.

It may be the case that some questions cannot be answered as requested.

  1. The solution to Project 1 implements error recovery using an internal undo facility. Discuss how you could extend this internal facility to an externally visible undo-redo facility. 10-points

  2. Many of the functions passed as parameters to functions in the JavaScript standard library provide the invoking object as a parameter. For example, when Array:map() is used as in array.map(f), the third parameter passed to f() will be array. Give an example of a situation where this behavior is useful. 5-points

  3. Assume that you are a evil spammer. Discuss how would you use JavaScript regular expressions within a program to harvest email addresses from a document. Your program should cope with anti-spam workarounds deployed by users like spelling out email addresses using sequences like user at example dot com. Your answer should deal only with the use of regex's to extract the email addresses; you may assume that the contents of the document are available as a string. 10-points

  4. Discuss how you would implement a random() function in JavaScript (without using any library random functions). The details of the random number generation algorithm are unimportant; what is required is the setting up of the function so that subsequent calls usually return different results. 5-points

  5. Assuming no earlier variable declarations, the following JavaScript code will output 4 when run in non-strict mode. 

        x = 1;
    obj1 = { x: 2, f: function() { return this.x; } }
    obj2 = { x: 3, f: function() { return this.x; } }
    f = obj1.f.bind(obj2);
    console.log(obj1.f() - obj2.f() + obj2.f.call(obj1) + f());

    Explain why this is so. 10-points

  6. Novice programmer Ben Novitia asks you for help in identifying the problem in the following nodejs code: 

          
      ...

      const DIR = `${process.env.HOME}/my-stuff`;

      function readFile(filename) {
        fs.readFile(`${DIR}/${filename}`,
	  function(err, result) {
            return result;
          });
      }

      const contents = readFile('test.data');
      console.log(contents);

    What do you tell Ben? 5-points

  7. Multiple objects obj1 and obj2 have a common interface \(I\) (i.e. the same set of methods) but all the methods in obj1 are async whereas the methods in obj2 are synchronous. There is a function f(obj1) which is set up to expect a read-only obj1 argument; so the code for f() assumes that obj1 methods always return promises. Hence calling f(obj2) would not work because even though obj2 has the same methods as obj1, those obj2 methods do not return promises.

    How would you set things up so that f(obj2') can be successfully called, where obj2' is derived from obj2.

    You answer should not assume anything about \(I\), obj1, obj2 or f(); in particular, you should not assume that you have access to the code for obj1, obj2 or f(). 15-points

  8. The following is an attempt to log the connection information for a mongo database: 

          const MONGO_URL = 'mongodb://localhost:27017/data';
      ...
      function getConnection(url=MONGO_URL) {
        const result = mongo.connect(url);
        return result;
      }
	
      const conn = getConnection();
      console.log(conn);

    Assuming that mongo contains a MongoDB client object, what is wrong with the above code? 5-points

  9. A web application needs to make asynchronous calls to independent web services \(s_1\) and \(s_2\), followed by an asynchronous call to a web service \(s_3\) which combines the results from \(s_1\) and \(s_2\) into a single result.

    1. How would you set this up so as to maximize performance.

    2. Assume that web service \(s_1\) is not particularly reliable and that an alternate web service \(s_1'\) is available which is functionally equivalent to \(s_1\). How would you modify your answer to the previous question to make the application more robust by using the results from either \(s_1\) or \(s_1'\) depending on which of \(s_1\) or \(s_1'\) responds? 15-points

  10. Write a function fib(n) which can be used as in 

        for (f of fib(6)) console.log(f);

    to print out successive Fibonacci numbers 1, 1, 2, 3, 5, 8. The return value of fib() is not allowed to be any kind of collection. 5-points

  11. Discuss the validity of the following statements. What is more important than whether you ultimately classify the statement as true or false is your justification for arriving at your conclusion. 15-points

    1. this for a fat-arrow function can be changed using call().

    2. The await keyword can only be used within a function declared async.

    3. A JavaScript event handler will run soon after the event is received.

    4. A function declared async can be called without using an await.

    5. A function called using await must have been declared async.