Tonight’s JavaScript Topics
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Transcript of Tonight’s JavaScript Topics
Tonight’s JavaScript Topics
• Conditional Statements: if and switch• The Array Object• Looping Statements: for, while and do-while
Conditional Statements in JavaScript• A conditional statement is a statement that
runs a command or command block only when certain conditions are met.
The if Statement in JavaScript
• To test a single condition, use: if (condition) {
commands }
• To test between two conditions, use: if (condition) {
commands if condition is true } else {
commands if otherwise }
The if Statement in JavaScript (cont.)• To test multiple conditions, use: if (condition 1) {
first command block } else if (condition 2) {
second command block } else if (condition 3) {
third command block } else {
default command block }
The Switch Statement in JavaScript• To test for different values of an expression, use:
switch (expression) { case label1: commands1 break; case label2: commands2 break; case label3: commands3 break;
... default: default commands
}• The break statement terminates any program loop or conditional.
A Complete Example
• Let’s modify last week’s dice program to play a simplified version of craps:– if the dice roll is 2, 3 or 12, the user loses (craps)– if the dice roll is 7 or 11, the user wins– any other number and the user continues rolling
• We’ll use if statements to do the job. As an outside-class exercise, try changing the program to use a switch statement.
The Array Object in JavaScript• An array is a collection of data values organized under a
single name– Each individual data value is identified by an index
• To create an array: var array = new Array(length);
• To populate an element of the array: array[i] = value;
• To create and populate an array: var array = new Array(values);
• To get the size of the array, use the property array.length
Some Useful Array Methods
A Program Example Using an Array
• Let’s write a JavaScript program that does the following:– displays a button on the web page– whenever the user presses the button, a random
fortune will be displayed to the user.– the list of “random” fortunes will be stored in an
array
Looping in JavaScript: The for Loop
• A program loop is a set of commands executed repeatedly until a stopping condition has been met
• for loops use a counter variable to track the number of times a block of commands is run
• To create a for loop, use:for (start at; continue until; update by){
commands to repeat}
A for Loop Example
Another for Loop Example
• for loops are often used to cycle through the different values contained within an array:
Loops in JavaScript: The while Loop• A while loop runs as long as a specific condition
is true• To create a while loop, use: while(continue until condition){
commands to repeat }
• Example: var sum = 0; while(sum < 10) {
sum = sum + Math.random(); }
Loops in JavaScript: The do-while Loop
• A do-while loop runs as long as a specific condition is true but the condition is at the end
• To create a do-while loop, use: do {
commands to repeat while(continue until condition); • A do-while loop is always executed at least one
time which isn’t necessarily true of while loops.• Watch out for infinite loops!
A Final Example: Rock, Paper, Scissors
1. For player 1, generate a random # between 1 and 3. 1= rock, 2=paper, 3-scissors. 2. For player 2, generate a random # between 1 and 3. 1= rock, 2=paper, 3-scissors. 3. Figure out which player wins by the rules:
- paper covers rock (paper wins) - scissors cuts paper (scissors win)
- rock breaks scissors (rock wins) - both players have the same item (tie)
4. Display an alert message like: “Player 1 has rock. Player 2 has paper. Player 2 wins.” 5. Keep playing if they tie until one player wins.