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Penn CIT 597 - Language Fundamentals

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JavaScriptAbout JavaScriptUsing JavaScript in a browserDealing with old browsersWhere to put JavaScriptPrimitive data typesVariablesOperators, IOperators, IICommentsStatements, IStatements, IIJavaScript is not JavaException handling, IException handling, IIObject literalsThree ways to create an objectArray literalsFour ways to create an arrayThe length of an arrayArrays and objectsArray functionsThe for…in statementThe with statementFunctionsRegular expressionsWarningsThe EndJavaScriptLanguage FundamentalsAbout JavaScript•JavaScript is not Java, or even related to Java–The original name for JavaScript was “LiveScript”–The name was changed when Java became popular•Statements in JavaScript resemble statements in Java, because both languages borrowed heavily from the C language–JavaScript should be fairly easy for Java programmers to learn–However, JavaScript is a complete, full-featured, complex language•JavaScript is seldom used to write complete “programs”–Instead, small bits of JavaScript are used to add functionality to HTML pages–JavaScript is often used in conjunction with HTML “forms”•JavaScript is reasonably platform-independentUsing JavaScript in a browser•JavaScript code is included within <script> tags:–<script type="text/javascript"> document.write("<h1>Hello World!</h1>") ;</script>•Notes:–The type attribute is to allow you to use other scripting languages (but JavaScript is the default)–This simple code does the same thing as just putting <h1>Hello World!</h1> in the same place in the HTML document–The semicolon at the end of the JavaScript statement is optional•You need semicolons if you put two or more statements on the same line•It’s probably a good idea to keep using semicolonsDealing with old browsers•Some old browsers do not recognize script tags–These browsers will ignore the script tags but will display the included JavaScript–To get old browsers to ignore the whole thing, use: <script type="text/javascript"> <!-- document.write("Hello World!") //--> </script>–The <!-- introduces an HTML comment–To get JavaScript to ignore the HTML close comment, -->, the // starts a JavaScript comment, which extends to the end of the lineWhere to put JavaScript•JavaScript can be put in the <head> or in the <body> of an HTML document–JavaScript functions should be defined in the <head>•This ensures that the function is loaded before it is needed–JavaScript in the <body> will be executed as the page loads•JavaScript can be put in a separate .js file–<script src="myJavaScriptFile.js"></script>–Put this HTML wherever you would put the actual JavaScript code–An external .js file lets you use the same JavaScript on multiple HTML pages–The external .js file cannot itself contain a <script> tag•JavaScript can be put in HTML form object, such as a button–This JavaScript will be executed when the form object is usedPrimitive data types•JavaScript has three “primitive” types: number, string, and boolean–Everything else is an object•Numbers are always stored as floating-point values–Hexadecimal numbers begin with 0x–Some platforms treat 0123 as octal, others treat it as decimal•Strings may be enclosed in single quotes or double quotes–Strings can contains \n (newline), \" (double quote), etc.•Booleans are either true or false–0, "0", empty strings, undefined, null, and NaN are false , other values are trueVariables•Variables are declared with a var statement:–var pi = 3.1416, x, y, name = "Dr. Dave" ;–Variables names must begin with a letter or underscore– Variable names are case-sensitive –Variables are untyped (they can hold values of any type)–The word var is optional (but it’s good style to use it)•Variables declared within a function are local to that function (accessible only within that function)•Variables declared outside a function are global (accessible from anywhere on the page)Operators, I•Because most JavaScript syntax is borrowed from C (and is therefore just like Java), we won’t spend much time on it•Arithmetic operators: + - * / % ++ --•Comparison operators: < <= == != >= >•Logical operators: && || ! (&& and || are short-circuit operators)•Bitwise operators: & | ^ ~ << >> >>>•Assignment operators: += -= *= /= %= <<= >>= >>>= &= ^= |=Operators, II•String operator: +•The conditional operator: condition ? value_if_true : value_if_false•Special equality tests:–== and != try to convert their operands to the same type before performing the test–=== and !== consider their operands unequal if they are of different types •Additional operators (to be discussed):new typeof void deleteComments•Comments are as in C or Java:–Between // and the end of the line–Between /* and */ •Java’s javadoc comments, /** ... */, are treated just the same as /* ... */ comments; they have no special meaning in JavaScriptStatements, I•Most JavaScript statements are also borrowed from C–Assignment: greeting = "Hello, " + name;–Compound statement: { statement; ...; statement }–If statements: if (condition) statement; if (condition) statement; else statement;–Familiar loop statements: while (condition) statement; do statement while (condition); for (initialization; condition; increment) statement;Statements, II•The switch statement: switch (expression){ case label : statement; break; case label : statement; break; ... default : statement; }•Other familiar statements:–break;–continue;–The empty statement, as in ;; or { }JavaScript is not Java•By now you should have realized that you already know a great deal of JavaScript–So far we have talked about things that are the same as in Java•JavaScript has some features that resemble features in Java:–JavaScript has Objects and primitive data types–JavaScript has qualified names; for example, document.write("Hello World");–JavaScript has Events and event handlers–Exception handling in JavaScript is almost the same as in Java•JavaScript has some features unlike anything in Java:–Variable names are untyped: the type of a variable depends on the value it is currently holding–Objects and arrays are defined in quite a different way–JavaScript has with statements and a new kind of for


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Penn CIT 597 - Language Fundamentals

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