To Lecture Notes

IT 231 -- Dec 6, 2024

Review Exercises

1. Which method do you use to convert an object literal into a JSON string?
   Answer: JSON.stringify
2. Which method do you use to convert a JSON string into an object literal?
   Answer: JSON.parse
3. Predict the output:
   

   var a = [34, 165, 11649, 4];
   console.log(a.sort( ));
   

   Answer: if we execute this code, the output is
   

   [11649, 165, 34, 4]
   

   To sort these numbers JavaScript first changes them into strings, then sorts them in dictionary order. To sort them as numbers, pass a function to a.sort that tells how to compare two numbers. This function should return a positive number when a > b , it should return a negative number when a < b , and it should return zero when a == b .  The function that does this is defined as (a, b) => a - b;  if a > b , a - b is positive, if a > b, a - b is negative, and if a == b , a - b is 0 . Here are the JavaScript statements:
   

   var a = [34, 165, 11649, 4];
   console.log(a.sort( (s, t) => {
       if (s > t) {
           return 1;
       }
       else if (s < t) {
           return -1;
       }
       else {
           return 0;
       }
   }));
   

4. What is the Node term for a URL entered in the browser? For example:
   

   localhost:3000/home_page
   

   The URL is "/home_page". Answer: route
5. How do you obtain the current URL in a main.js script? Answer:
   

   // req is the request object.
   var route = req.url;
   

Practice Quizzes

• There are no practice quizzes this week because of the D2L midterm exam that opens on Thursday, Feb 8.

Midterm Review Guide

• Look at the Midterm Review Guide.

The File System Module fs

Reference: W3Schools Node.js File System Module
www.w3schools.com/nodejs/nodejs_filesystem.asp

• The File System Module fs contains functions that perform these actions:
  reading from files, creating files, appending to files, writing to files, deleting files, renaming files.
• This table shows the fs functions that perform these actions:
  

  Action	fs Function
  Read from File	readFile
  Create file	open
  Write to File	writeFile
  Append to File	readFile
  Delete File	unlink
  Rename Files	rename

Reading from Files

• ReadFromFile Example
  
  1. Create a data file named data.txt with this content:
     

     This is a test line of data.
     

  2. Create a script named main.js that reads the data line from data.txt using the and writes it out to the node console:
     

     var fs = require("fs");
     fs.readFile("data.txt", (err, data) => {
         if(err) {
             console.log("Error reading file data.txt");
         }
         else {
             console.log(`Data line: ${data}`);
         }
     });
     

  3. Check that the output is
     

     Data line: This is a test line of data.
     

Tutorial 3

• Work on Tutorial 3.
• Tutorial 3 is not to be submitted for a grade. Only submit it if you have trouble getting it to work and would like to submit it for help.

Project 1

• Discuss Project 1.

Writing to Files

• WriteToFile Example
  
  1. Create a main.js file with this script:
     

     var fs = require("fs");
     var output = "Some sample file content.";
     fs.writeFile("output.txt", output, (err) => {
         if(err) {
             console.log("Error writing to file.");
         }
         else {
             console.log(
                 "File contents saved in output.txt file.");
         }
     });
     

• Appending to a file with existing content is accomplished with fs.appendFile:
  

  // Append to file.
  var fs = require("fs");
  var line = "\nSome more sample file content.";
  fs.appendFile("output.txt", line, (err) => {
      if(err) {
          console.log("Error appending to file.");
      }
      else {
          console.log(
              "File contents saved in output.txt file.");
      }
  });