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Introduction to JavaScript

Introduction to JavaScript 15
Introduction to JavaScript Pat Morin COMP2405Outline What is JavaScript • – History – Uses • Adding JavaScript to HTML • JavaScript syntax JavaScript events • JavaScript classes • The HTML Document Object Model • 2What is JavaScript JavaScript is a programming language for use in • HTML pages • Invented in 1995 at Netscape Corporation (LiveScript) • JavaScript has nothing to do with Java • JavaScript programs are run by an interpreter built into the user's web browser (not on the server) 3Where does JavaScript Fit In Recall • 1. client opens connection to server 2. client sends request to server 3. server sends response to client 4. client and server close connection What about Step 5 • 5. Client renders (displays) the response received from server • Step 5 involves displaying HTML • And running any JavaScript code within the HTML 4What can JavaScript Do JavaScript can dynamically modify an HTML page • • JavaScript can react to user input • JavaScript can validate user input • JavaScript can be used to create cookies (yum) • JavaScript is a fullfeatured programming language • JavaScript user interaction does not require any communication with the server 5Pros and Cons of JavaScript Pros: • – Allows more dynamic HTML pages, even complete web applications Cons: • – Requires a JavaScriptenabled browser – Requires a client who trusts the server enough to run the code the server provides • JavaScript has some protection in place but can still cause security problems for clients Remember JavaScript was invented in 1995 and web – browsers have changed a lot since then 6Using JavaScript in your HTML JavaScript can be inserted into documents by • using the SCRIPT tag html head titleHello World in JavaScript/title /head body script type="text/javascript" document.write("Hello World"); /script /body /html 7Where to Put your Scripts You can have any number of scripts • • Scripts can be placed in the HEAD or in the BODY – In the HEAD, scripts are run before the page is displayed – In the BODY, scripts are run as the page is displayed • In the HEAD is the right place to define functions and variables that are used by scripts within the BODY 8Using JavaScript in your HTML html head titleHello World in JavaScript/title script type="text/javascript" function helloWorld() document.write("Hello World"); /script /head body script type="text/javascript" helloWorld(); /script /body /html 9External Scripts Scripts can also be loaded from an external file • • This is useful if you have a complicated script or set of subroutines that are used in several different documents script src="myscript.js"/script 10JavaScript Variables JavaScript has variables that you can declare with • the optional var keyword • Variables declared within a function are local to that function • Variables declared outside of any function are global variables var myname = "Pat Morin"; 11JavaScript Operators and Constructs JavaScript has most of the operators we're used to • from C/Java – Arithmetic (+, , , /, ) Assignment (=, +=, =, = /=, =, ++, ) – Logical (, , ) – – Comparison (, , =, =, ==) • Note: + also does string concatentation • Constructs: – if, else, while, for, switch, case 12Simple User Interaction There are three builtin methods of doing simple • user interaction – alert(msg) alerts the user that something has happened confirm(msg) asks the user to confirm (or cancel) – something prompt(msg, default) asks the user to enter some text – alert("There's a monster on the wing"); confirm("Are you sure you want to do that"); prompt("Enter you name", "Adam"); 13JavaScript Functions JavaScript lets you define functions using the • function keyword • Functions can return values using the return keyword function showConfirm() confirm("Are you sure you want to do that"); 14JavaScript Arrays JavaScript has arrays that are indexed starting at 0 • • Special version of for works with arrays script type="text/javascript" var colors = new Array(); colors0 = "red"; colors1 = "green"; colors2 = "blue"; colors3 = "orange"; colors4 = "magenta"; colors5 = "cyan"; for (var i in colors) document.write("div style=\"backgroundcolor:" + colorsi + ";\"" + colorsi + "/div\n"); /script 15JavaScript Events JavaScript can be made to respond to user events • • Common Events: – onload and onunload : when a page is first visited or left – onfocus, onblur, onchange : events pertaining to form elements – onsubmit : when a form is submitted – onmouseover, onmouseout : for "menu effects" A complete list of event types is available here • – 16Exception Handling JavaScript also has try, catch, and throw • keywords for handling JavaScript errors try runSomeCode(); catch(err) var txt="There was an error on this page.\n\n" + "Error description: " + err.description + "\n\n" alert(txt) 17Comments in JavaScript Comments in JavaScript are delimited with // and / • / as in Java and C++ 18JavaScript Objects JavaScript is objectoriented and uses the same • methodcalling syntax as Java • We have already seen this with the document object document.write("Hello World"); 19BuiltIn JavaScript Objects Some basic objects are builtin to JavaScript • – String – Date – Array Boolean – Math – 20JavaScript Strings A String object is created every time you use a • string literal (just like in Java) • Have many of the same methods as in Java charAt, concat, indexOf, lastIndexOf, match, replace, search, – slice, split, substr, substring, toLowerCase, toUpperCase, valueOf • There are also some HTML specific methods – big, blink, bold, fixed, fontcolor, fontsize, italics, link, small, strike, sub, sup • Don't use the HTML methods (use CSS instead) – This is the worst kind of visual formatting 21JavaScript Dates The Date class makes working with dates easier • • A new date is initialized with the current date • Dates can be compared and incremented var myDate = new Date(); myDate.setFullYear(2007,2,14); var today = new Date(); var nextWeek = today + 7; if (nextWeek today) alert("You have less than one week left"); 22JavaScript Arrays and Booleans We have already seen the Array class • • The Boolean class encapsulates a boolean value 23The JavaScript Math Class The Math class encapsulates many commonly • used mathematical entities and formulas • These are all class methods abs, acos, asin, atan, atan2, ceil, cos, exp, floor, log, max, – min, pow, random, round, sin, sqrt, tan • These are all class methods E, LN2, LN10, LOG2E, LOG10E, PI, SQRT12, SQRT2 – if (Math.cos(Math.PI) = 0) alert("Something is wrong with Math.cos"); 24JavaScript and the DOM The Document Object Model (DOM) is a • specification that determines a mapping between programming language objects and the elements of an HTML document • Not specific to JavaScript 25HTML DOM Objects Environment objects • – Window, Navigator, Screen, History, Location, Document • HTML objects – Anchor, Area, Base, Body, Button, Event, Form, Frame, Frameset, Iframe, Image, Checkbox, FileUpload, Hidden, Password, Radio, Reset, Submit, Text, Link, Meta, Object, Option, Select, Style, Table, TableCell, TableRow, TextArea 26HTML DOM: Document The Document object represents an HTML • document and can be used to access all documents in a page A Document contains several collections • – anchors, forms, images, links • Has several properties – body, cookie, domain, lastModified, referrer, title, URL • Has several useful methods getElementById, getElementsByName, – getElementsByTagName, write, writeln, open, close 27HTML DOM: Document An instance of Document is already created for • you, called document function changeF() var cText = document.getElementById("c"); var fText = document.getElementById("f"); ... ... input type="text" id="c" onchange="changeC()"C input type="text" id="f" onchange="changeF()"F 28HTML DOM: Form Elements One of the most common uses of JavaScript is for • form validation • Several HTML DOM classes encapsulate form elements – Form, Button, Checkbox, Hidden, Password, Text, Radio, Reset, Submit, Textarea • Warning: Using JavaScript is not a substitute for validating form data in CGI scripts 29HTML DOM: Text A text entry field (input type="text") is • encapsulated by a Text object • Variables value, maxLength, id, size, name, tabindex, readOnly – Changing these variables has an immediate effect • on the displayed data 30HTML DOM: The Document Tree Accessing elements and changing their properties • lets us do simple things like form validation, data transfer etc HTML DOM lets us do much more • • We can create, delete, and modify parts of the HTML document • For this we need to understand the Document Tree 31HTML DOM: The Document Tree 32Navigating the Document Tree With JavaScript we can navigate the document • tree • document.getElementById(), getElementsByName(), and getElementsByTagName() return nodes in the document tree Information can be obtained from • – nodeName – The tag name – nodeValue – The the text of a text node – nodeType – The kind of node 3334
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