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JavaScript

JavaScript. CMPT 281. Outline. Introduction to JavaScript Resources What is JavaScript? JavaScript in w eb pages. Announcements. Assignment 3. Layout with tables. JavaScript Resources. Resources. Why JavaScript?. HTML + CSS + JavaScript. Separation of Concerns.

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JavaScript

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  1. JavaScript CMPT 281

  2. Outline • Introduction to JavaScript • Resources • What is JavaScript? • JavaScript in web pages

  3. Announcements

  4. Assignment 3 • Layout with tables

  5. JavaScript Resources

  6. Resources

  7. Why JavaScript?

  8. HTML + CSS + JavaScript

  9. Separation of Concerns

  10. Separation of Concerns

  11. What is Javascript?

  12. What is Javascript? • A scripting programming language • Cannot be run without a browser • Embedded in most web browsers • A way to create web pages that respond dynamically to user action • A tool for server-side development

  13. Types of computer languages • High-Level vs. Low Level • Compiled vs. Interpreted • Structured vs. Object Oriented • Scripting vs. Programming

  14. JavaScript • High-Level vs. Low Level • Compiled vs. Interpreted • Structured vs. Object Oriented • Scripting vs. Programming

  15. History • Need for ‘client side’ interaction • Code executes on the local machine • No need for network connection once script runs • Developed by Netscape in 1995 • (Not related to the Java Language) • Actually ECMAScript(ECMA-262) • en.wikipedia.org/wiki/ECMAScript • Several variants (JScript, Action Script…)

  16. What can JavaScript do? • It is a full programming language • API (application programming interface) is specific to working with browsers • Restrictions: • Security-based limitations • No networking • No access to local file system • Limited UI toolkit and graphics • (This is changing with HTML5)

  17. What can JavaScript do? • Benefits: • Close integration with the browser • Access the webpage and all elements within • Adjust or create HTML • Open and resize browser windows • Run animations, play sounds

  18. Example: writing to the page

  19. Example: loading a random image

  20. Where do scripts go? • In the HMTL page • Like styles, can be external, internal, or inline • Use these for different situations

  21. Where do scripts go? • For scripts that respond to user events, and for functions: • Either external or internal • In <head>…</head> • For scripts that write document content: • In <body>…</body>

  22. Body example <html> <head> </head> <body> <script type="text/javascript"> document.write("This message written by JavaScript"); </script> </body> </html>

  23. Internal example <html> <head> <script type="text/javascript"> function message() { alert("This alert was called with the onload event"); } </script> </head> <body onload="message()"> </body> </html

  24. External example <html> <head> <script type="text/javascript" src="xyz.js"></script> </head> <body> </body> </html>

  25. How does JS access the page? • DOM: Document Object Model • A framework to describe a web page (document) in a tree-like structure

  26. DOM

  27. DOM

  28. Using the DOM to get at the page <html><body><p id="intro">Hello World!</p><script type="text/javascript">x=document.getElementById("intro");document.write(x.firstChild.nodeValue);</script></body></html>

  29. Using the DOM to get at the page <html><body><p id="intro">Hello World!</p><script type="text/javascript">x=document.getElementById("intro");document.write(x.firstChild.nodeValue);</script></body></html>

  30. JavaScript Basics

  31. JavaScript Basics • Statements • Variables • Events • Functions • Dialogs

  32. Statements <script type="text/javascript"> var a = 10; var b = 11; var c; c = a + b; alert(‘The answer is’ + c); </script>

  33. JavaScript Variables • Variables are used to store data. • Rules for variable names: • Variable names are case sensitive • They must begin with a letter or the underscore character • strname – STRNAME (not same)

  34. JavaScript Variables • Variables: containers for data • All variables have • Name • Type – JavaScript is loosely typed • Value or “null” • To declare a variable • varvariableName • Beware of reserved words: • E.g., ‘if’, ‘Document’, ‘Math’, etc.

  35. JavaScript Operators • Arithmetic Operators

  36. JavaScript Operators • Assignment Operators

  37. JavaScript Operators • Comparison Operators

  38. JavaScript Operators • Logical Operators

  39. User and Browser Events

  40. Functions <script type=“text/javascript”> function dontClick() { alert("I told you not to click!"); } </script> ... <input type="button" value="Don't Click Me!" onClick="dontClick()"/>

  41. Dialogs • alert("Hello!"); • confirm("Delete files?"); • var name=prompt("Your name?", "Tim Berners-Lee");

  42. Coding Tips • Check your statements are on one line • Check your " and ' quotes match • Take care with capitalisation • Lay it out neatly - use tabs • Use the Chrome developer tools • Or Firebug for Firefox

  43. A Word About Comments • JavaScript Comments • These comments must be within a script • // begins a single line comment • These can start anywhere on the line, but the remainder of the line becomes a comment • /* All of your comments here… */ • Everything between the start and end comment markers are comments • Can be on a single line or span many… • HTML Comments • <!-- All of your comments here… -->

  44. Ending Statements With a Semicolon? • With traditional programming languages, like C++ and Java, each code statement has to end with a semicolon (;). • Many programmers continue this habit when writing JavaScript, but in general, semicolons are optional! • Semicolons are required if you want to put more than one statement on a single line.

  45. Example

  46. JavaScript Libraries

  47. Reminders for this week • Do the JavaScript tutorials on w3schools: • 'JS Basic‘: www.w3schools.com/js/ • Assignment • Reading: • Pattern group I: Designing Effective Page Layouts • Pages 631-657

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