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JavaScript. An Introduction. What is JavaScript?. JavaScript adds “dynamic” behavior to HTML pages, adding programming capabilities. JavaScript is a predominantly "client side" scripting language used for a large variety of tasks, such as form validation, image rollovers and fly-out menus.
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JavaScriptAn IntroductionWhat is JavaScript?
  • JavaScript adds “dynamic” behavior to HTML pages, adding programming capabilities.
  • JavaScript is a predominantly "client side" scripting language used for a large variety of tasks, such as form validation, image rollovers and fly-out menus.
  • JavaScript was created by Netscape, and is now the “official” client side language.
  • Where Does JavaScript Go?
  • JavaScript code belongs inside HTML <script> tags.
  • Inside the <script> tags, the rules of JavaScript apply. No HTML can go directly inside the script tag.
  • JavaScript is embedded directly into the HTML page, usually in the <head> tag.
  • What Does JavaScript Look Like?The format of a typical JavaScript looks like this: <script type=“text/javascript”> //Code goes here</script> Notice the comment tag (//Code goes here) inserted in the text. The double slashes indicate no code will be executed beyond that point on the line. Client Side vs. Server Side
  • Server side scripting is used for interaction with the user, and is frequently used to create dynamic pages by interacting with a database.
  • Client side scripting is designed to run on the user's machine, and as such is cut off from the server while it is running.
  • Any work that can be done on the client is best done there, as any processing there takes a load off a busy server.
  • JavaScript Statements
  • JavaScript expects us to speak to it in lines of code it understands, called “statements”
  • Each JavaScript statement should go on the same “line” of text, and most end with a semi-colon:
  • var myAnswer = 2 + 2;Browser Differences
  • There are differences in how browsers are designed, and therefore differences in how JavaScript is processed.
  • One such difference appears in how the document object model (DOM) is handled.
  • The DOM allows us to navigate an HTML document, and make changes to the page, without reloading the page.
  • No Errors Shown, By Design
  • Because JavaScript errors are so common and disruptive to a user’s experience, most browsers elect NOT to display any JavaScript errors, by default
  • In order to troubleshoot scripting errors, we use a browser like Firefox, which gives feedback on errors by selecting:
  • Tools >> Error ConsoleOverly Sensitive
  • JavaScript is case sensitive, some say “overly” so.
  • If you use a JavaScript function or variable and get even one letter in the wrong case, JavaScript will either show you an error, or will give you unexpected results
  • Data Types
  • Numerical data is entered in JavaScript without quotations around it, while alphanumeric data (hereafter called string data) needs to be inserted with quotes.
  • There are 3 basic types of data in JavaScript, strings, numbers and booleans.
  • While the JavaScript engine is processing, it will give it's best guess as to the data type involved. In general, any number that can be used in a calculation is considered numerical. An address number, like a zip code, would be a string.
  • Numerical Data
  • While strings must be quoted, numbers must not be, if they are to be calculated.
  • JavaScript treats most numbers automatically, meaning there is one numerical data type
  • Strings Of Data
  • In JavaScript, user data, like names or HTML look like “strings” of data that is NOT specifically meaningful to JavaScript.
  • This data type is called a “string” because it is alphanumeric data that is “strung” together like beads on a chain
  • Strings are required to be in quotes, either single or double quotes, but not both
  • Variables
  • Variables are programming constructs that have contents inside them that ‘vary’
  • Variables allow us to tailor our code to the circumstances:
  • var myAnswer = 2 + 2;
  • In the above code, a variable named ‘myAnswer’ is declared, and it is assigned a value, the sum of 2 and 2.
  • Concatenation
  • Concatenation is a programmer’s word for ‘connect’.
  • When we use variables, we ‘concatenate’ data into meaningful statements:
  • myVar = ‘The Year is: ‘;myVar += myDate.getFullYear();Operators
  • JavaScript uses “operators” which allow us to make mathematical calculations, and also to concatenate strings of data:
  • var myVar = 2 + 2;
  • In the above example the “plus” sign is the operator that allows us to add 2 and 2. The same operator is used to concatenate strings:
  • var myName = “Bill ” + “Newman”;
  • Note the “space” left after the first name. This is there so that the two words have a space between them, once they are concatenated.
  • Keywords
  • Keywords are words that have special meaning in JavaScript. These words must be used in accordance to JavaScript rules and can't be used as variable or function names.
  • Some common keywords are break, do, else, for, this, true, false, if, var, void, null, return, while, function, in and switch.
  • If we are tempted to use a word that logically is very close to a keyword, the suggestion is to attach the prefix "my" to it, ie: myFunction.
  • Read Right To Left
  • Isn’t it odd that the statements read right to left?
  • var myVar = 2 + 2;
  • This is because the most important part of the statement (the sum, if it’s math) is easier to scan this way in a long program.
  • JavaScript Functions
  • JavaScript has several built in “functions” which allow us to perform actions automatically by calling the name of the function, and supplying some data. Here is a very common function, called “alert()”.
  • alert(‘Here is a message that pops up!’);
  • Note the function includes a “string” of data, that is a message to a user
  • Anatomy Of A Function
  • For a function to “function” certain rules must be followed:
  • Functions all require parenthesis:
  • alert(“message here”);
  • Sometimes they require “data” inserted inside the parenthesis. In this case, the “string” of text saying, “message here”.
  • Booleans
  • Boolean data gets its name from the Mathematician George Boole
  • Boolean data consists of logic types. For us, that would be the values true and false
  • Boolean data, like numerical data, does NOT use quotation marks
  • Booleans (true or false) are always lower case
  • If(x == true){alert(“It’s true!!”);}Equal? NOT!
  • An important operator, often associated with Booleans, is the NOT operator, symbolized by the exclamation mark “!”
  • The “not” operator is used with the equal sign to create “not equal”, which is the same as false:
  • If(x != true){alert(“That would be false!”);}Late Binding
  • JavaScript reads and processes statements one at a time.
  • JavaScript makes assumptions about the code it sees, since it processes data on the fly.
  • If it sees numbers that are not quoted, it tries to calculate them.
  • If it sees numbers that are quoted, it tries to connect, or concatenate them
  • Curly Braces
  • Curly braces (the “{“ and the “}” signs) are required to let JavaScript know where to start and stop.
  • These are required when you build your own functions, and for if statements, for example:
  • if(x == y){alert(“x equals y!”);}Conditionals
  • JavaScript allows us to do different things according to different circumstances
  • We do this as people, when we say:
  • If it rains, bring an umbrella. Otherwise, bring shorts.
  • A ‘conditional’ is a statement that allows us to determine what happens “if” something occurs
  • If Statement
  • Here is a sample ‘If’ statement:
  • If(x == y){ alert(“I guess x equals y!”);}The “double equal” is the operator that checks for equality. If the variables x and y are equivalent, then the alert displaysAssignment vs. Comparison
  • The “equals” sign carries special significance in JavaScript
  • To “compare” data, we need to use the “double” equal:
  • if(x == y){alert(“x equals y!”);}
  • To “assign” data, we use a single equal sign:
  • var myVar = 2 + 2;var myName = “Bill “ + “Newman”;
  • Both of these examples “assign” data to the variable on the left.
  • Custom Message Based On DateThe sample below accesses the date object on the user’s machine, and displays an “alert()” message, if it’s noon, or later:var myDate = new Date();If(myDate.getHour() > 12){ alert(‘It’s after Noon!’);}JavaScript Objects
  • JavaScript uses “objects” to allow us to access resources like the “date” on the system clock of the user’s machine:
  • var myDate = new Date();
  • We need to create an ‘instance’ (example) of a date object in order to use it further. This allows us to work with different date objects at once.
  • The Date Object
  • Users come to our website from different parts of the world
  • Accessing the user’s system clock is useful for tailoring a user’s experience to their time of day
  • What if you wanted to show a custom image or message based on the time of day of the user? JavaScript does this.
  • Document Object Model (DOM)
  • Document Object Model: The elements that make up an HTML document can be put to various purposes by JavaScript. 
  • The way that an HTML document is navigated is by a structure called the Document Object Model (DOM). 
  • The DOM allows us to read down to a part of an HTML document and either read or change data. 
  • You can call out the object by it's exact name, or by climbing the DOM "tree" elements sequentially.
  • The Navigator Object
  • JavaScript can determine information about what browser is being used. This is useful for tailoring HTML for a specific browser:
  • var myBrowser = navigator.appName();alert(“Your browser is: “ + myBrowser);Event Handlers
  • Events are occurrences on an HTML page, like a “page load” or the actions by the user, like mouse clicks and mouse hovers, etc.
  • Events can be “captured” and used as a trigger for actions taken by JavaScript.
  • The mouse clicks and mouse hovers, etc. are called "events", and the JavaScript "handles" or reacts to the actions of the user, hence the phrase "event handler".
  • Rollovers
  • The most common kind of event handler is a “rollover", where an image is "swapped out" (changed) when a user hovers the mouse over an image.
  • The simple "mouse over" example code below simply (although inefficiently) handles a mouse over event:
  • <a href="mypage.htm" onMouseOver="document.myImage.src='images/on.gif'" onMouseOut="document.myImage.src='images/off.gif'"><img src="images/on.gif" name="myImage"><a/>
  • The above example uses the “document” object model to identify an object by name, in this case an image named “myImage”
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