JavaScript: A Beginner's Guide to Understanding the Differences with Java and Fundamentals, Slides of Computer Science

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JavaScript

Language Fundamentals

About 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
  • Now that Microsoft no longer likes Java, its name for their JavaScript dialect is “Active Script”
• 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-independent

JavaScript isn’t always available

• 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:

  • The , the // starts a JavaScript comment, which extends to the end of the line

• Some users turn off JavaScript
  • Use the message to display a message in place of whatever the JavaScript would put there

Where to put JavaScript

• JavaScript can be put in the or in the of an HTML document - JavaScript functions should be defined in the - This ensures that the function is loaded before it is needed - JavaScript in the will be executed as the page loads

• JavaScript can be put in a separate .js file

  • 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

Variables

• 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

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 (all numbers are floating-point):
  • • • / % ++ --
• Comparison operators: < <= == != >= >
• Logical operators: && ||! (&& and || are short-circuit operators)
• Bitwise operators: & | ^ ~ << >> >>>
• Assignment operators: += -= *= /= %= <<= >>= >>>= &= ^= |=

Comments

• 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 JavaScript

Statements, 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 ); Docsity.com

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 statement

Exception handling, I

• Exception handling in JavaScript is almost the same as in Java
• throw expression creates and throws an exception
  • The expression is the value of the exception, and can be of any type (often, it's a literal String)
• try {

statements to try } catch ( e ) { // Notice: no type declaration for e exception handling statements } finally { // optional, as usual code that is always executed }

• With this form, there is only one catch clause

Object literals

• You don’t declare the types of variables in JavaScript
• JavaScript has object literals, written with this syntax:
  • { name1 : value1 , ... , nameN : valueN }
• Example (from Netscape’s documentation):
  • car = {myCar: "Saturn", 7: "Mazda", getCar: CarTypes("Honda"), special: Sales}
• The fields are myCar, getCar, 7 (this is a legal field name) , and special
• "Saturn" and "Mazda" are Strings
• CarTypes is a function call
• Sales is a variable you defined earlier
  • Example use: document.write("I own a " + car.myCar); Docsity.com

Three ways to create an object

• You can use an object literal:
  • var course = { number: "CIT597", teacher: "Dr. Dave" }
• You can use new to create a “blank” object, and add fields to

it later:

• var course = new Object(); course.number = "CIT597"; course.teacher = "Dr. Dave";
• You can write and use a constructor:
• function Course(n, t) { // best placed in this.number = n; // keyword "this" is required, not optional this.teacher = t; }
• var course = new Course("CIT597", "Dr. Dave");

Four ways to create an array

• You can use an array literal: var colors = ["red", "green", "blue"];
• You can use new Array() to create an empty array:
  • var colors = new Array();
  • You can add elements to the array later: colors[0] = "red"; colors[2] = "blue"; colors[1]="green";
• You can use new Array( n ) with a single numeric

argument to create an array of that size

• var colors = new Array(3);
• You can use new Array(…) with two or more arguments

to create an array containing those values:

• var colors = new Array("red","green", "blue");

The length of an array

• If myArray is an array, its length is given by

myArray.length

• Array length can be changed by assignment beyond the

current length

• Example: var myArray = new Array(5); myArray[10] = 3;
• Arrays are sparse, that is, space is only allocated for elements that have been assigned a value
• Example: myArray[50000] = 3; is perfectly OK
• But indices must be between 0 and 2 32 -
• As in C and Java, there are no two-dimensional arrays; but you can have an array of arrays: myArray[5][3]