15-Nov-15 JavaScript Language Fundamentals. 2 About JavaScript JavaScript is not Java, or even...
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Transcript of 15-Nov-15 JavaScript Language Fundamentals. 2 About JavaScript JavaScript is not Java, or even...
Apr 20, 2023
JavaScript
Language Fundamentals
2
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 reasonably platform-independent
3
Using 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 semicolons
4
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:
<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 line
Some users turn off JavaScript Use the <noscript>message</noscript> to display a message in
place of whatever the JavaScript would put there
5
Where 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 an HTML form object, such as a button This JavaScript will be executed when the form object is used
6
Primitive 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
Since you can’t be sure, avoid octal altogether!
Strings may be enclosed in single quotes or double quotes Strings can contain \n (newline), \" (double quote), etc.
Booleans are either true or false 0, "0", empty strings, undefined, null, and NaN are false ,
other values are true
7
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 local to that function (accessible only within that function) local variables must be declared using var
Variables declared outside a function are global (accessible from anywhere on the page)
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Operators, I
Most JavaScript syntax is borrowed from C Arithmetic operators (all numbers are floating-point):
+ - * / % ++ -- Comparison operators:
< <= == != >= > Logical operators:
&& || ! (&& and || are short-circuit operators) Bitwise operators:
& | ^ ~ << >> >>> Assignment operators:
+= -= *= /= %= <<= >>= >>>= &= ^= |=
9
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
new typeof void delete
10
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
11
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); for (initialization; condition; increment) statement;
12
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 { }
13
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
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Exception handling, II
try { statements to try} catch (e if test1) { exception handling for the case that test1 is true} catch (e if test2) { exception handling for when test1 is false and test2 is true} catch (e) { exception handling for when both test1and test2 are false} finally { // optional, as usual code that is always executed}
Typically, the test would be something like e == "InvalidNameException"
15
JavaScript is not Java
By now you should have realized that you already know a great deal of JavaScript
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
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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);
Really just Key-Value pairs
17
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 <head>
this.number = n; // keyword "this" is required, not optional this.teacher = t;}
var course = new Course("CIT597", "Dr. Dave"); We will talk more about objects later
18
Array literals
You don’t declare the types of variables in JavaScript JavaScript has array literals, written with brackets and
commas Example: color = ["red", "yellow", "green", "blue"]; Arrays are zero-based: color[0] is "red"
If you put two commas in a row, the array has an “empty” element in that location Example: color = ["red", , , "green", "blue"];
color has 5 elements However, a single comma at the end is ignored
Example: color = ["red", , , "green", "blue”,]; still has 5 elements
19
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");
20
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 232-1
As in C and Java, there are no two-dimensional arrays; but you can have an array of arrays: myArray[5][3]
21
Arrays and objects
Arrays are objects – both are just key-value pairs car = { myCar: "Saturn", 7: "Mazda" }
car[7] is the same as car.7 car.myCar is the same as car["myCar"]
If you know the name of a property, you can use dot notation: car.myCar
If you don’t know the name of a property, but you have it in a variable (or can compute it), you must use array notation: car["my" + "Car"]
22
Array functions
If myArray is an array, myArray.sort() sorts the array alphabetically myArray.sort(function(a, b) { return a - b; }) sorts
numerically myArray.reverse() reverses the array elements myArray.push(…) adds any number of new elements to
the end of the array, and increases the array’s length myArray.pop() removes and returns the last element of the
array, and decrements the array’s length myArray.toString() returns a string containing the values
of the array elements, separated by commas
23
The for…in statement
You can loop through all the properties of an object with for (variable in object) statement;
Example: for (var prop in course) { document.write(prop + ": " + course[prop]); }
Possible output: teacher: Dr. Dave number: CIT597
The properties are accessed in an undefined order If you add or delete properties of the object within the loop, it is
undefined whether the loop will visit those properties Arrays are objects; applied to an array, for…in will visit the
“properties” 0, 1, 2, … Notice that course["teacher"] is equivalent to course.teacher
You must use brackets if the property name is in a variable
24
Functions
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(arg1, …, argN) { statements } 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(arg1, …, argN)
Simple parameters are passed by value, objects are passed by reference
Functions
Remember no parameter types or return types If you want to return something, return it
Remember to declare local variables so they are local Undeclared variables are global
No function overloading
25
function myFunction(a,b){
var ans = a*b;return ans;
}
x = myFunction(y,z);
Functions
26
function sayHi() { for(var i=0; i<arguments.length; i++) { alert("Hi, " + arguments[i]) }} sayHi("Cat", "Alice") // 'Hi, Cat', then 'Hi, Alice'
Arguments object is available to every function
Functions as arguments
In JavaScript, functions are first-class objects. Functions can be passed as arguments
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// returns the current hourfunction GetCurrentHour(){ // obtaining the current hour var date = new Date(); var hour = date.getHours(); // return the hour return hour;}
// display greetingfunction DisplayGreeting(hourFunc){ // retrieve the hour using the function received as parameter hour = hourFunc();
// display greeting if (hour >= 22 || hour <= 5) document.write("Goodnight, world!"); else document.write("Hello, world!");}
// call DisplayGreetingDisplayGreeting(GetCurrentHour);
Anonymous Functions
Can create functions at runtime using the function operator
Often used to pass unamed functions as a parameter
28
var sayHello= function() { alert(”Hello World!");}sayHello();
function messageToTheWorld(message, callback){
alert(message);callback();
}messageToTheWorld(“Howdy”, function() {
alert(“Doody”);});
Closures
An “inner” function has access to an enclosing functions variables
29
function sayHello2(name) { var text = 'Hello ' + name; // local variable var sayAlert = function() {
alert(text); }
return sayAlert;}
Returns a function that still has access to the local var text
JavaScript Closure
30
var createGreeting = function(greeting) { return function(name) { document.write(greeting + ', ' + name + '.'); };};
helloGreeting = createGreeting("Hello");howdyGreeting = createGreeting("Howdy");
helloGreeting("John"); // Hello, John.helloGreeting("Sally"); // Hello, Sally.howdyGreeting("John"); // Howdy, John.howdyGreeting("Sally"); // Howdy, Sally.
31
Regular expressions A regular expression can be written in either of two ways:
Within slashes, such as re = /ab+c/ With a constructor, such as re = new RegExp("ab+c")
Regular expressions are almost the same as in Perl or Java (only a few unusual features are missing)
string.match(regexp) searches string for an occurrence of regexp
It returns null if nothing is found If regexp has the g (global search) flag set, match returns an array of
matched substrings If g is not set, match returns an array whose 0th element is the matched
text, extra elements are the parenthesized subexpressions, and the index property is the start position of the matched substring
32
Warnings
JavaScript is a big, complex language We’ve only scratched the surface It’s easy to get started in JavaScript, 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
JavaScript is not totally platform independent Expect different browsers to behave differently Write and test your programs a little bit at a time
Browsers aren’t designed to report errors Don’t expect to get any helpful error messages Write and test your programs a little bit at a time
33
Example
<html><head><title> JavaScript 1 </title><script language="JavaScript">document.write("<h1>This is it. </h1>"); document.write(Date());</script></head><body>This is the rest of the page.</body></html>
34
Modified example<html> <head> <title> JavaScript 1 </title> <script language="JavaScript"><!-- var rawDate = Date(); var mon = rawDate.substr(4,3);
document.write("The date is"); document.write(rawDate, "<br>") document.write("The month is ", mon); //--></script> </head><body><br>This is the rest of the page.</body> </html>
35
Using built-in functions, variables
<html> <head> <title> JavaScript 1 </title>
<script language="JavaScript">
<!--
var rawDate = Date();
var mon = rawDate.substr(4,3);
document.write("The month is ");
document.write(mon);
//-->
</script> </head>
<body>
<br>
This is the rest of the page.
</body> </html>
Variable set to what Date() returns.
Extract the month.
36
<body><h1> Address Information </h1> <br><form method=post enctype="text/plain" action="mailto:[email protected]"onSubmit="return verify(this);">
First Name: <input type="text" name="fname"> <br>Last Name: <input type="text" name="lname"> <br>Street Address: <input type="text" name="address" size=30> <br>Town/City: <input type="text" name="city"> <br>State: <select name="state" size=1> <br><option value="NY" selected> Utah<option value="NY" selected> Idaho<option value="NY" selected> New York<option value="NJ"> New Jersey<option value="CT"> Connecticut<option value="PA"> Pennsylvania</select> <br>Status: <input type="radio" name="status" value="R"> Returning client<input type="radio" name="status" value="N"> New client<hr> Thank you <p><input type="submit" value="Send information">
</form> </body> </html>
JavascriptEvent
Form Verification
37
<html><head><title>Form example </title><script language="JavaScript">function verify(f){
if (f.lname.value == null || f.address.value == null){ alert("Form needs a last name and an address"); return false;}if (f.lname.value == "" || f.address.value == "") { alert("Form needs a last name and an address"); return false;}return true;
}</script> </head>
Form Verification
38
Javscript Events
Events are actions that can be detected by Javascript Every element on a web page has certain events which
can trigger Javascript functions Often placed within the HTML tag
<tag attribute1 attribute2 onEventName="javascript code;"> <a href="" onMouseOver="popupFunc();">
The set of all events which may occur and the page elements on which they can occur is part of the Document Object Model(DOM) not Javascript Browsers don’t necessarily share the same set of events
39
Common Javascript Events
Event Occurs when... Event Handler
click User clicks on form element or link onClick change User changes value of text, textarea, or select element onChange focus User gives form element input focus onFocus blur User removes input focus from form element onBlur mouseover User moves mouse pointer over a link or anchor
onMouseOver mouseout User moves mouse pointer off of link or anchor
onMouseOut select User selects form element's input field onSelect submit User submits a form onSubmit resize User resizes the browser window onResize load User loads the page in the Navigator onLoad unload User exits the page onUnload
The Document Object Model When a document is loaded in the web browser, a number of
objects are created. Most commonly used are window and document
Window open(), close(), alert(), confirm(), prompt()
Document Contains arrays which store all the components of your page You can access and call methods on the components using the
arrays An object may also be accessed by its name
document.myform.address.value = “123 Main” document.myform.reset()
Can also search for element by name or id document.getElementById(“myelementid”) document.getElementsByName(“myelementname”)
W3C DOM with Javascript Each xml document also has a DOM
Using Ajax you can get the returned value in xml XMLfile = request.responseXML
Parse using the DOM methods from the documentElement
var rootElement = XMLfile.documentElement;
document.write("The root node of the XML file is: ");
document.writeln("<b>" + rootElement.nodeName +"</b>");
//traverse through each child of the root element//and print out its namefor (i=0; i<rootElement.childNodes.length; i++) {
var node = rootElement.childNodes.item(i);document.write("The name of the node is ");document.write("<b>" + node.nodeName + "</b>");
}
The Document Object Properties
childNodes - returns a NodeList of child nodes documentElement - returns the root node documentURI - sets or returns the location of the document firstChild, lastChild, nodeName, nodeType, nodeValue
Methods createAttribute, createComment, createElement getElementById getElementsByTagName
Return a NodeList of all elements with a specified name
The Document Object
Go to: http://www.w3schools.com/dom/default.asp