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Introduction to JavaScript A First JavaScript

Introduction to JavaScript A First JavaScript. Topics. A First JavaScript Program About the <script> tag How to create a JavaScript source file How to add comments to a JavaScript Program How to hide JavaScript from incompatible browsers

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Introduction to JavaScript A First JavaScript

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  1. Introduction to JavaScriptA First JavaScript

  2. Topics • A First JavaScript Program • About the <script> tag • How to create a JavaScript source file • How to add comments to a JavaScript Program • How to hide JavaScript from incompatible browsers • About placing JavaScript in <head> or <body> sections of HTML documents

  3. A First JavaScript Program • The <script> Tag • JavaScript programs are run from within an HTML document • <script> and </script> • Used to notify the browser that JavaScript statements are contained within

  4. A First JavaScript Program • The <script> Tag • JavaScript • Object-based language • Object • Programming code and data that can be treated as an individual unit or component • Statements • Individual lines in a programming language • Methods • Groups of statements related to a particular object

  5. A First JavaScript Program • Creating a JavaScript Source File • JavaScript programs can be used in two ways: • Incorporated directly into an HTML file • Using <script> tag • Placed in an external (source) file • Has file extension .js • Contains only JavaScript statements

  6. A First JavaScript Program • Creating a JavaScript Source File • JavaScript source files • Use src attribute of <script> tag to denote source of JavaScript statements • Browser will ignore any JavaScript statements inside <script> and </script> if src attribute is used • Cannot include HTML tags in source file <script language=“JavaScript” src=“c:\source.js”> </script>

  7. A First JavaScript Program • Creating a JavaScript Source File • Advantages of JavaScript source files • Makes HTML document neater (less confusing) • JavaScript can be shared among multiple HTML files • Hides JavaScript code from incompatible browsers

  8. A First JavaScript Program • Adding comments to a JavaScript Program • Comments • Non-printing lines that are placed in the code to contain various remarks about the code • Makes it easier to understand the code operation • Two types • Line comments • // ignore all text to the end of the line • Block comments • /* ignore all text between these symbols */

  9. JavaScript Tutorial 1 - Introduction to JavaScript

  10. A First JavaScript Program • Hiding JavaScript from Incompatible Browsers • Two methods • Place JavaScript in external source file • Enclose the code within HTML comments <!-- beginning of HTML block comment end of HTML block comments -->

  11. A First JavaScript Program • Hiding JavaScript from Incompatible Browsers • Alternate message display • If browser doesn’t support JavaScript • Use <noscript> & </noscript> to place alternate message to users of back-level browsers

  12. A Simple Program: Printing a Line of Text in a Web Page • Methods in window object • Callon-screen windows • window.alert( “argument” ); • Method calls alert window with window text "argument" • Outputs button with text and ‘OK’ button

  13. Another JavaScript Program: Adding Integers • Variables • Variable name can be any combination of: • Letters, digits, underscores (‘_’) and dollar signs (‘$’) • Cannot begin with a digit • variables are case-sensitive

  14. Another JavaScript Program: Adding Integers • Variable name convention • Begin with lowercase first letter • Every following word has first letter capitalized • goRedSox, bostonUniversityRules • Declarations • var name1, name2 • Indicate that name1 and name2 are program variables

  15. Another JavaScript Program: Adding Integers • Method window.prompt( “arg1”, “arg2” ) • Calls window that allows user to enter value to use in the script • arg1 :text that will appear in window • arg2 :text that will initially appear in input line • firstNumber = window.prompt(); • Assigns value entered by the user in prompt window to variable first • "=" a binary operator • Assigns value of right operand to left operand

  16. Another JavaScript Program: Adding Integers • parseInt(); • Function accepts a string and returns an integer value • Not a method because we do not refer to an object name number1 = parseInt( firstNumber ); • Operates right-to-left (due to the "=" sign)

  17. Arithmetic • Binary Operators • Used in arithmetic operations • Modulus operator(%) • Yields remainder after division • Examples: 43 % 5 = 3 8.7 % 3.4 = 1.9 24 % 6 = 0

  18. Programmer-Defined Functions • Functions allow program modularization • Variables declared in function are local variables • Only known inside function in which defined • Most functions have list of parameters • Means for communicating info between functions & function calls • Local variables • When function called, arguments assigned to parameters in function definition

  19. Programmer-Defined Functions • Motives for modularizing a program with functions • Makes program development more manageable • Allows software reusability • Avoid repeating code in program • Do not re-invent the wheel • Save time

  20. Function Definitions • Function-definition format functionfunction-name(parameter-list) { Declarations and Statements }

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