INFO1003_Foundamental of IT_2012 Semester 2_tutorial week 5 Solution_Week 5 - JavaScript Answers

Now let’s log our output to the console as a means of debugging: console.log("The subtotal is: " + subtotal); console.log('The total cost is $' + total); As you can see in the console, the subtotal variable has a value of 500500. Now, the problem is clearer. JavaScript is appending 500 with 500 giving us 500500. Looking back at our code, we notice that the line product2='500'. The '500' in single quotes indicates a String, thus, when we perform the operation product1 + product2, it appends (or concatenates) the two Strings together. To fix this problem, remove the single quotes from 500 and run the program again. Next, log the total and subtotal variable into the console. Your program should now produce the correct result.
Exercise 2: Understanding Pop Up Boxes
a. Let’s now play with a few pop up boxes, one line at a time alert("Thank you for visiting"); confirm("Do you accept the terms and conditions?"); var name = prompt("What's your name"); Now after you enter a name for the prompt box, let’s see what value is stored in the name variable, by typing in the variable name: name b. This time, we want our prompt to take in a number. var age = prompt("How old are you?");
Question 4: Understanding console.log
Displaying alerts can be quite annoying. Let's now suppress the alert, and just print things out to the console (instead of to a pop up box via console.log)
Exercise 3: Understanding Arrays
Let’s familiarise ourselves with arrays in order to store more than one value per variable. There are three ways to create arrays as described in page 75 of the reference book. Let’s try one technique: // Declaring the array var planets = new Array(); // Initialising the array planets[0] = "Mercury"; planets[1] = "Venus"; planets[2] = "Earth"; a. Retrieve the first element by calling the variable and the index position: planets[0] b. Overwrite the third element by assigning it a different value: planets[2] = "Mars"; c. Print out what is stored in the planet variable by calling the variable name: planets
JavaScript Exercises
1. JavaScript Concept Questions
1. Given the following JavaScript statement (or line of code), which part is the declaration, and which part is the initialisation? var chicken = 12; Declaration: var chicken Initialisation: chicken = 12; 2. Initially, var x = 10. What is the final value of x = x + 5? Or is this statement even valid? It is valid whereby anything on the right hand side is equated to the variable on the left hand side. Thus, x = 15 3. Does all JavaScript statements need to end with a semi-colon? No, but it is good practice to do so. 4. What is the final value for each variable? var x var y var z x = x x++; a = x z = x = = = + 6; "INFO1003"; 30; z;
+ y; % z;
x 6 36 37

y INFO1003
z 30 7
a 37INFO1003
2. JavaScript Practical Exercise
Exercise 1: Understanding Variables and Operators
Before you start this exercise, you need to open up the JavaScript console (either the Chrome console or Firefox’s Firebug) a) From page 72 of the reference book, to differentiate the difference between Numbers and Strings, notice what is the console output when you type: 40 + 5; // Compared with "40" + "5"; Notice the "+" operator joins the two strings together (called concatenation) b) To prove we’re not lying to you, try out the following statements: var x = 10; x = x + 5;
INFO1003 | FOUNDATIONS OF INFORMATION TECHNOLOGY
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JavaScript Exercises
var var var var var
product1 = 500; product2 = '500'; subtotal = product1 + product2; tax = .1 * subtotal; total = subtotal + tax;
INFO1003 | FOUNDATIONS OF INFORMATION TECHNOLOGY
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JavaScript Exercises
However, notice what happens when we try to add an integer with our variable: age + 10; The reason you will see this concatenation is because by default, the prompt saves any input from the user as a String. We therefore want to extract out the integer value from the prompt variable. We achieve this with a parseInt() function: age = parseInt(age); Now let’s try our addition again: age + 10; c. Let’s concatenate this Number with a String, to say, "You are [INSERT AGE] years old", and define this as a new variable, ageText var ageText = "You are " + age + " years old" Remember, the "+" operator acts as a concatenation operator to join all Strings. Also notice, the space before and after the double quotation mark so that the age number is not printed directly after the text with no space. age + 10;