CIS 461 Web Development I

1. CIS 461 Web Development I Javascript Jongwook Woo, PhD jwoo5@calstatela.edu California State University, Los Angeles Computer Information System Department

2. Content • Statements • Comments • Variables • Operators • Functions • Loops

3. JavaScript • THE scripting language of the Web. • add interactivity to HTML pages • a lightweight programming language • usually embedded directly into HTML pages • an interpreted language • means that scripts execute without preliminary compilation • That runs on client site, that is, web browser • Everyone can use JavaScript without purchasing a license • used in billions of Web pages • to add functionality, validate forms, communicate with the server, and much more

4. What can JavaScript do? • JavaScript gives HTML designers a programming tool • Almost anyone can put small "snippets" of code into their HTML pages • JavaScript can react to events – • execute when something happens, • like when a user clicks on an HTML element • JavaScript can read and write HTML elements • can read and change the content of an HTML element • JavaScript can be used to validate data • used to validate form data before it is submitted to a server

5. What can JavaScript do? (Cont’d) • JavaScript can be used to detect the visitor's browser • load another page specifically designed for that browser • JavaScript can be used to create cookies • used to store and retrieve information on the visitor's computer

6. Statements • JavaScript is Case Sensitive • Unlike HTML, watch your capitalization closely • A JavaScript statement • is a command to a browser. • to tell the browser what to do. document.write("Hello Dolly"); • tells the browser to write "Hello Dolly" to the web page: • The semicolon is optional (according to the JavaScript standard), • the end of the statement. • a good programming practice. • Note: Using semicolons makes it possible to write multiple statements on one line.

7. JavaScript Codes • is a sequence of JavaScript statements. • Example <script type="text/javascript">document.write("<h1>This is a heading</h1>");document.write("<p>This is a paragraph.</p>");document.write("<p>This is another paragraph.</p>");</script>

8. JavaScript Blocks • JavaScript statements can be grouped together in blocks. • to make the sequence of statements execute together. • Blocks start with a left curly bracket {, • and end with a right curly bracket } • Example <script type="text/javascript">{document.write("<h1>This is a heading</h1>");document.write("<p>This is a paragraph.</p>");document.write("<p>This is another paragraph.</p>");}</script>

9. JavaScript Comments • make the code more readable • Single line comments start with // <script type="text/javascript">// Write a headingdocument.write("<h1>This is a heading</h1>"); // heading</script> • JavaScript Multi-Line Comments • Multi line comments start with /* and end with */. • The following example uses a multi line comment to explain the code: <script type="text/javascript">/*The code below will writeone heading and two paragraphs*/document.write("<h1>This is a heading</h1>");</script>

10. JavaScript Variables • used to hold values or expressions • As with algebra • declare JavaScript variables with the var keyword: varx; varcarname; • the variables are empty (they have no values yet). • you can also assign values to the variables varx=5; varcarname="Volvo"; • After the execution of the statements above, the variable x will hold the value 5, • andcarname will hold the value Volvo. • Note: When you assign a text value to a variable, use quotes around the value.

11. Rules for JavaScript variable names • Variable names are case sensitive • (y and Y are two different variables) • Note: Because JavaScript is case-sensitive, variable names are case-sensitive. • Variable names must begin • with a letter or the underscore character • Local JavaScript Variables • A variable declared within a JavaScript function becomes LOCAL  • and can only be accessed within that function. • the variable has local scope • Local variables are destroyed when you exit the function.

12. JavaScript variables • Global JavaScript Variables • Variables declared outside a function become GLOBAL, • and all scripts and functions on the web page can access it. • Global variables are destroyed when you close the page. • If you declare a variable, without using "var", the variable always becomes GLOBAL. • Assigning Values to Undeclared JavaScript Variables • These statements: x=5; carname="Volvo"; • will declare the variables x and carname as global variables (if they don't already exist).

13. Arithmetic Operators • = is used to assign values. • + is used to add values. y=5; z=2; x=y+z; • The value of x, after the execution of the statements above, is 7. • The + Operator Used on Strings • used to add string variables or text values together. • To add two or more string variables together, use the + operator. txt1="What a very"; txt2="nice day"; txt3=txt1+txt2; • After the execution of the statements above, the variable txt3 contains "What a verynice day".

14. Arithmetic Operators (Given that y=5)

15. ComparisonOperators (Given that x=5)

16. LogicalOperators • Logical operators are used to determine the logic between variables or values. • &&: both operands should be true to have the result true • ||: at least one operand should be true to have the result true

17. Functions • In Math: • Function declaration: • f(x) = x + 1 • Function call: y = f(x) when x = 2 • What is y? • Function Syntax in JavaScript function functionname(var1,var2,...,varX){some code} • The parameters var1, var2, etc. are variables or values passed into the function. The { and the } defines the start and end of the function.

18. Functions (Example) <html><head> <script type="text/javascript"> function displaymessage() { alert("Hello World!"); } </script></head><body> <form><input type="button" value="Click me!" onclick="displaymessage()" /> </form></body> </html>

19. Functions with Return Statement • specify the value that is returned from the function. <html><head> <script type="text/javascript"> function product(a,b) { return a*b; } </script></head><body> <script type="text/javascript">document.write(product(4,3)); </script></body></html>

20. Loop • want the same block of code to run over and over again in a row. • In JavaScript, there are two different kind of loops: • for - loops through a block of code a specified number of times • while - loops through a block of code while a specified condition is true • The for Loop • The for loop is used when you know in advance how many times the script should run. for (variable=startvalue; variable<=endvalue; variable=variable+increment){code to be executed}

21. For Loop <html><body> <script type="text/javascript">vari=0; for (i=0;i<=5;i++) {document.write("The number is " + i);document.write("<br />"); } </script></body></html> The number is 0 The number is 1 The number is 2 … The number is 5