Java Script Language Fundamentals I About Java

Java. Script Language Fundamentals I

About Java. Script n Java. Script is not Java, or even related to Java n n Statements in Java. Script resemble statements in Java, because both languages borrowed heavily from the C language n n n Java. Script should be fairly easy for Java programmers to learn However, Java. Script is a complete, full-featured, complex language Java. Script is seldom used to write complete “programs” n n n The original name for Java. Script was “Live. Script” The name was changed when Java became popular Now that Microsoft no longer likes Java, its name for their Java. Script dialect is “Active Script” Instead, small bits of Java. Script are used to add functionality to HTML pages Java. Script is often used in conjunction with HTML “forms” Java. Script is reasonably platform-independent 2

Using Java. Script in a browser n Java. Script code is included within <script> tags: n n <script type="text/javascript"> document. write("<h 1>Hello World!</h 1>") ; </script> Notes: n n n The type attribute is to allow you to use other scripting languages (but Java. Script is the default) This simple code does the same thing as just putting <h 1>Hello World!</h 1> in the same place in the HTML document The semicolon at the end of the Java. Script statement is optional n n You need semicolons if you put two or more statements on the same line It’s probably a good idea to keep using semicolons 3

Java. Script isn’t always available n Some old browsers do not recognize script tags n n n These browsers will ignore the script tags but will display the included Java. Script To get old browsers to ignore the whole thing, use: <script type="text/javascript"> <!-document. write("Hello World!") //--> </script> The , the // starts a Java. Script comment, which extends to the end of the line Some users turn off Java. Script n Use the <noscript>message</noscript> to display a message in place of whatever the Java. Script would put there 4

Where to put Java. Script n Java. Script can be put in the <head> or in the <body> of an HTML document n n n Java. Script functions can be put in a separate. js file n n n Java. Script functions should be defined in the <head> n This ensures that the function is loaded before it is needed Java. Script in the <body> will be executed as the page loads <script src="my. Java. Script. File. js"></script> Put this in the <head> An external. js file lets you use the same Java. Script on multiple HTML pages The external. js file cannot itself contain a <script> tag Java. Script can be put in an HTML form object, such as a button n This Java. Script will be executed when the form object is used 5

Primitive data types n Java. Script has three “primitive” types: number, string, and boolean n n Everything else is an object Numbers are always stored as floating-point values n n Hexadecimal numbers begin with 0 x Some platforms treat 0123 as octal, others treat it as decimal n n Strings may be enclosed in single quotes or double quotes n n Since you can’t be sure, avoid octal altogether! Strings can contains n (newline), " (double quote), etc. Booleans are either true or false n 0, "0", empty strings, undefined, null, and Na. N are false , other values are true 6

Variables n Variables are declared with a var statement: n n n n 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) 7

Operators, I n n n Because most Java. Script syntax is borrowed from C (and is therefore just like Java), we won’t spend much time on it Arithmetic operators (all numbers are floating-point): + * / % ++ -Comparison operators: < <= == != >= > Logical operators: && || ! (&& and || are short-circuit operators) Bitwise operators: & | ^ ~ << >> >>> Assignment operators: += -= *= /= %= <<= >>>= &= ^= |= 8

Operators, II n n n String operator: + The conditional operator: condition ? value_if_true : value_if_false Special equality tests: n n n == 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 delete 9

Comments n Comments are as in C or Java: n n n 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 Java. Script 10

Statements, I n Most Java. Script statements are also borrowed from C n n Assignment: greeting = "Hello, " + name; Compound statement: { statement; . . . ; statement } If statements: if (condition) statement; else statement; Familiar loop statements: while (condition) statement; do statement while (condition); for (initialization; condition; increment) statement; 11

Statements, II n The switch statement: switch (expression) { case label : statement; break; . . . default : statement; } n Other familiar statements: n n n break; continue; The empty statement, as in ; ; or { } 12

Java. Script is not Java n By now you should have realized that you already know a great deal of Java. Script n n Java. Script has some features that resemble features in Java: n n n So far we have talked about things that are the same as in Java. Script has Objects and primitive data types Java. Script has qualified names; for example, document. write("Hello World"); Java. Script has Events and event handlers Exception handling in Java. Script is almost the same as in Java. Script has some features unlike anything in Java: n n n 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 Java. Script has with statements and a new kind of for statement 13

Object literals n n n You don’t declare the types of variables in Java. Script has object literals, written with this syntax: n { name 1 : value 1 , . . . , name. N : value. N } Example (from Netscape’s documentation): n n car = {my. Car: "Saturn", 7: "Mazda", get. Car: Car. Types("Honda"), special: Sales} n The fields are my. Car, get. Car, 7 (this is a legal field name) , and special n "Saturn" and "Mazda" are Strings n Car. Types is a function call n Sales is a variable you defined earlier Example use: document. write("I own a " + car. my. Car); 14

Three ways to create an object n You can use an object literal: n n You can use new to create a “blank” object, and add fields to it later: n n var course = { number: "CIT 597", teacher: "Dr. Dave" } var course = new Object(); course. number = "CIT 597"; course. teacher = "Dr. Dave"; You can write and use a constructor: n n function Course(n, t) { // best placed in <head> this. number = n; // keyword "this" is required, not optional this. teacher = t; } var course = new Course("CIT 597", "Dr. Dave"); 15

Functions n n Functions should be defined in the <head> of an HTML page, to ensure that they are loaded first The syntax for defining a function is: function name(arg 1, …, arg. N) { statements } n n The function may contain return value; statements Any variables declared within the function are local to it The syntax for calling a function is just name(arg 1, …, arg. N) Simple parameters are passed by value, objects are passed by reference 16

Warnings n Java. Script is a big, complex language n n Java. Script is not totally platform independent n n n We’ve only scratched the surface It’s easy to get started in Java. Script, but if you need to use it heavily, plan to invest time in learning it well Write and test your programs a little bit at a time Expect different browsers to behave differently Write and test your programs a little bit at a time Browsers aren’t designed to report errors n n Don’t expect to get any helpful error messages Write and test your programs a little bit at a time 17

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