To Lecture Notes

IT 231 -- Jan 18, 2024

Review Exercises

1. List the JavaScript datatypes that you know. Answer:

   string  number  boolean  undefined
   

2. What is the difference between these JavaScript keywords?
   

   var  const
   

   Answer: var is used to create and initialize a variable whose value can be changed later:
   

   var n = 384;
   n = 391;
   

   const is used to create a constant variable that cannot be changed later:
   

   const PI = 3.14159265;
   

3. What is a function?
   Answer: A function is a group of statements that can be tested and called by other JavaScript scripts. Functions are important for modularity. A function can accept parameters, which are input values passed in to the function. This function can also return a value.
4. Write a function named makeGreeting that inputs a name of a person and returns a greeting to the person with that name. For example, if the input is "Larry", the function return value might be "Hello, Larry, how are you?". Test your function. Answer:
   

   function makeGreeting(firstName) {
       var greeting = "Hello, " + fname + ", how are you?";
       return greeting;
   }
   // Test the makeGreeting function:
   var greeting = makeGreeting("Alice");
   var greeting = makeGreeting("Larry");
   
   // Output:
   Hello, Alice, how are you?
   Hello, Larry, how are you?
   

5. Write a JavaScript function that inputs a Fahrenheit temperature as a parameter and returns a descriptor for that temperature. Use the descriptors
   

   frigid  cold  chilly  perfect  warm  hot
   

   Use if..else statements to assign a descriptor to a temperature. Test your function. Answer:
   

   function descriptor(temp) {
       if (temp < 25) {
           return "frigid";
       }
       else if (temp < 50) {
           return "cold"
       }
       else if (temp < 65) {
           return "chilly";
       }
       else if (temp < 75) {
           return "perfect";
       }
       else if (temp < 95) {
           return "warm";
       }
       else {
           return "hot";
       }
   }
   
   console.log(descriptor(-7));
   console.log(descriptor(43));
   console.log(descriptor(62));
   console.log(descriptor(71));
   console.log(descriptor(89));
   console.log(descriptor(102));
   // Output:
   frigid
   cold
   chilly
   perfect
   warm
   hot
   

Practice Quiz 2b

• Take Practice Quiz 2b.

Array Example

• An array is a list of items, delimited by square brackets and separated by commas, for example:
  

  var arr = [67, 43, 90, 48, 71];
  

• Each item in an array has a zero-based index. The indices for the array arr are shown in this table:
  
  

  Index	Value
  0	67
  1	43
  2	90
  3	48
  4	71

• To lookup a value in an array using its index, use the array lookup operator ( [ ] ):
  

  var arr = [67, 43, 90, 48, 71];
  console.log(arr[3]);
  // Output: 48
  

• Practice Problems
  
  1. What is the output?
     

     var b = [5, 3, 2, 9, 4];
     var i = 1;
     console.log(b[i + 1] + " " + b[i] + 1);
     

     Answer: This problem checks if you know the difference between adding one to the index before looking up the array value and adding one to the array value after looking it up.
     

     b[i + 1] = b[1 + 1] = b[2] = 2
     b[i] + 1 = b[1] + 1 = 3 + 1 = 4
     The output: 2 4
     

  2. If n is a positive integer, show how to randomly choose one of the array elements
     

     var items = ["zero", "one", "two", "three", "four"];
     

     with equal probability 1/5. Answer:
     

     var n = 5;
     var randomInt = Math.floor(Math.random( ) * n);
     console.log(items[randomInt]);
     

  3. Place the code for Practice Problem 2 in a function named chooseRandomNumber that returns one of the strings
     

     one  two  three  four  five
     

     with the probability 1/5. Test your function. Answer:
     

     function chooseRandomNumber( ) {
         var items = ["zero", "one", "two", "three", "four"];
         var n = 5;
         var randomInt = Math.floor(Math.random( ) * n);
         console.log(items[randomInt]);
     }
     
     console.log(chooseRandomNumber( ));
     

     You can replace the line
     

     var n = 5;
     

     with

     var n = items.length;
     

     so that the function works with an items array of any size.

Expression Interpolation Example

• Suppose that the variables name and amt are defined like this:
  

  var name = "Larry";
  var amt = 524.67;
  

  Define the variable sentence like this:
  

  var sentence = name + " owes me $" + amt;
  

  The resulting value of sentence is:
  

  "Larry owes me $524.67."
  

• Instead of defining the variable sentence in the traditional way, we can use a format string with expression interpolation:
  

  var sentence = `${name} owes me $${amt}.`;
  

• ${ } contains an expression to insert (interpolate) into the format string.
• The format string must be delimited with backquotes ( ` ` ), not with ordinary single quotes ( ' ' ) or double quotes ( " " ).
• What is the output of these statements?
  

  var x = 2, y = 2;
  console.log(`${x} + ${y} == ${x + y}`);
  

Tutorial 2a

• Work on Tutorial 2a.