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JavaScript: Understanding the Benefits, Disadvantages, and Basics, Exercises of Web Design and Development
Front-end DevelopmentHTML and CSSWeb DesignJavaScript Frameworks
An overview of JavaScript, its advantages and disadvantages, and basic syntax. JavaScript simplifies coding tasks, adds interactivity, and reduces server-side processing. However, it has cross-browser compatibility issues and limited authoring and debugging tools. examples of JavaScript syntax, such as variables, conditional statements, and operators.
What you will learn
- What are the benefits of using JavaScript?
- How do you initialize a JavaScript variable?
- What is the difference between == and === in JavaScript?
- What is the syntax for an if statement in JavaScript?
- What are the disadvantages of using JavaScript?
Typology: Exercises
2019/2020
1 / 17
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Partial preview of the text
Download JavaScript: Understanding the Benefits, Disadvantages, and Basics and more Exercises Web Design and Development in PDF only on Docsity! Webpage Design and Development Chapter three: Client Side Scripts 1 | P a g e Chapter Three Client Side Scripting Languages – Java Script Scripting languages are used to embed programming instructions to the browser in a web page. The most widely used client-side scripting language is JavaScript. JavaScript offers several benefits including: o Simplifying some repetitive coding tasks for HTML pages o Providing functionality and interactivity not possible in HTML o Providing client functions that would otherwise require requests for additional pages and/or server-side processing JavaScript in the HTTP Transaction The following diagram shows how the browser and its components interact with the web server and server-side resources and processing components: Advantages of JavaScript JavaScript adds a great deal to a static web site. Here are some of the main advantages of using it: o Lightweight for fast downloading o Performs well o Minimal security risks o Works across browsers and platforms (Netscape 2.0+ and IE 3.0+) Disadvantages JavaScript does have some weaknesses as well: o implemented differently in different browsers, and not always with backward compatibility o scripts need to be tested on all target browsers o Authoring and debugging tools are limited JavaScript Syntax Basics JavaScript is plain ASCII text (ECMA allows for Unicode), so you can write it with any text editor (including HomeSite and ColdFusion Studio, which include a JavaScript Wizard and JavaScript Object Tree to simplify coding) o Statements terminated by semi-colon (;) or line-break (the semi-colon is optional if you have a line-break, but recommended as good coding practice) o JavaScript is case-sensitive o Comments: // Comment to end of line /* Multi-line comment */ Sample: /* Set global variables */ var browserName = navigator.appName; // browser name var browserVersion = navigator.appVersion; // version Webpage Design and Development Chapter three: Client Side Scripts 2 | P a g e Note: a JavaScript keyword var is used to initialize variables The <SCRIPT> Tag Embed scripts in HTML pages using the <SCRIPT> tag, which is often found within the <HEAD> tag or even in <BODY> tag: <script language="JavaScript" type="text/javascript"> <!-- {script code goes here} // --> </script> The HTML comment prevents very old browsers that do not support the SCRIPT tag from displaying the JavaScript code as plain text. The JavaScript comment before the closing bracket for the HTML comment prevents the JavaScript interpreter from trying to interpret the end of the HTML comment as JavaScript code. The language attribute of the <SCRIPT> tag may be set to a particular version, such as "Javascript 1.2". This will prevent older browsers that do not support this version of JavaScript from trying to interpret the script. However, this should be a last resort for preventing JavaScript errors; whenever possible you should ensure that your scripts run on all browsers that may be used to view the site. Your First Script <html> <head><title>My First Script</title></head> <body> <h1>Let’s Script...</h1> <hr> <script language="JavaScript" type="text/javascript"> <!-- document.write ("Hello Ladies and Gentlemen!"); // --> </script> </body> </html> Example: <html> <body> <script type="text/javascript"> document.write("Hello World!") </script> </body> </html> The code above will produce this output on an HTML page: Hello World! The word document.write is a standard JavaScript command for writing output to a page. By entering the document.write command between the <script type="text/javascript"> and </script> tags, the browser will recognize it as a JavaScript command and execute the code line. In this case the browser will write Hello World! to the page: Webpage Design and Development Chapter three: Client Side Scripts 5 | P a g e strname = "Hege" The variable name is on the left side of the expression and the value you want to assign to the variable is on the right. Now the variable "strname" has the value "Hege". Lifetime of Variables When you declare a variable within a function, the variable can only be accessed within that function. When you exit the function, the variable is destroyed. These variables are called local variables. You can have local variables with the same name in different functions, because each is recognized only by the function in which it is declared. If you declare a variable outside a function, all the functions on your page can access it. The lifetime of these variables starts when they are declared, and ends when the page is closed. Conditional Statements Conditional statements in JavaScript are used to perform different actions based on different conditions. Very often when you write code, you want to perform different actions for different decisions. You can use conditional statements in your code to do this. In JavaScript we have the following conditional statements: if statement - use this statement if you want to execute some code only if a specified condition is true if...else statement - use this statement if you want to execute some code if the condition is true and another code if the condition is false if...else if....else statement - use this statement if you want to select one of many blocks of code to be executed switch statement - use this statement if you want to select one of many blocks of code to be executed If Statement You should use the if statement if you want to execute some code only if a specified condition is true. Syntax: if (condition) { code to be executed if condition is true } Note that if is written in lowercase letters. Using uppercase letters (IF) will generate a JavaScript error! Example 1 <script type="text/javascript"> //Write a "Good morning" greeting if //the time is less than 10 var d=new Date() var time=d.getHours() if (time<10) { document.write("<b>Good morning</b>") } </script> Example 2 <script type="text/javascript"> //Write "Lunch-time!" if the time is 11 var d=new Date() var time=d.getHours() if (time==11) { document.write("<b>Lunch-time!</b>") } Webpage Design and Development Chapter three: Client Side Scripts 6 | P a g e </script> Note: Apply same operators used in other programming languages. Notice that there is no “else” in this syntax. You just tell the code to execute some code only if the specified condition is true. If...else Statement If you want to execute some code if a condition is true and another code if the condition is not true, use the if....else statement. Syntax: if (condition) { code to be executed if condition is true } else { code to be executed if condition is not true } Example <script type="text/javascript"> //If the time is less than 10, //you will get a "Good morning" greeting. //Otherwise you will get a "Good day" greeting. var d = new Date() var time = d.getHours() if (time < 10) { document.write("Good morning!") } else { document.write("Good day!") } </script> If...else if...else Statement You should use the if....else if...else statement if you want to select one of many sets of lines to execute. Syntax if (condition1) { code to be executed if condition1 is true } else if (condition2) { code to be executed if condition2 is true } else { code to be executed if condition1 and condition2 are not true } Webpage Design and Development Chapter three: Client Side Scripts 7 | P a g e Example <script type="text/javascript"> var d = new Date() var time = d.getHours() if (time<10) { document.write("<b>Good morning</b>") } else if (time>10 && time<16) { document.write("<b>Good day</b>") } else { document.write("<b>Hello World!</b>") } </script> The JavaScript Switch Statement Conditional statements in JavaScript are used to perform different actions based on different conditions. You should use the switch statement if you want to select one of many blocks of code to be executed. Syntax switch(n) { case 1: execute code block 1 break case 2: execute code block 2 break default: code to be executed if n is different from case 1 and 2 } This is how it works: o First we have a single expression n (most often a variable), that is evaluated once. o The value of the expression is then compared with the values for each case in the structure. o If there is a match, the block of code associated with that case is executed. o Use break to prevent the code from running into the next case automatically. Example <script type="text/javascript"> //You will receive a different greeting based //on what day it is. Note that Sunday=0, Monday=1, Tuesday=2, etc. var d=new Date() theDay=d.getDay() switch (theDay) { case 5: document.write("Finally Friday") break case 6: document.write("Super Saturday") break case 0: Webpage Design and Development Chapter three: Client Side Scripts 10 | P a g e Prompt Box: A prompt box is often used if you want the user to input a value before entering a page. When a prompt box pops up, the user will have to click either "OK" or "Cancel" to proceed after entering an input value. If the user clicks "OK" the box returns the input value. If the user clicks "Cancel" the box returns null. Syntax: prompt("sometext","defaultvalue") JavaScript Functions A function is a reusable code-block that will be executed by an event, or when the function is called. To keep the browser from executing a script as soon as the page is loaded, you can write your script as a function. A function contains some code that will be executed only by an event or by a call to that function. You may call a function from anywhere within the page (or even from other pages if the function is embedded in an external .js file). Functions are defined at the beginning of a page, in the <head> section. How to Define a Function The syntax for creating a function is: function functionname(var1,var2,...,varX) { some code } o var1, var2, etc are variables or values passed into the function. o The { and the } defines the start and end of the function. Note: A function with no parameters must include the parentheses () after the function name: function functionname() { some code } Example <html> <head> <script type="text/javascript"> function displaymessage() { alert("Hello World!") } </script> </head> <body> <form> <input type="button" value="Click me!" onclick="displaymessage()" > </form> </body> </html> o If the line: alert("Hello world!!"), in the example above had not been written within a function, it would have been executed as soon as the line was loaded. o Now, the script is not executed before the user hits the button. o We have added an onClick event to the button that will execute the function displaymessage() when the button is clicked. Webpage Design and Development Chapter three: Client Side Scripts 11 | P a g e Note: Do not forget about the importance of capitals in JavaScript! The word function must be written in lowercase letters, otherwise a JavaScript error occurs! Also note that you must call a function with the exact same capitals as in the function name. The return Statement The return statement is used to specify the value that is returned from the function. So, functions that are going to return a value must use the return statement. Example The function below should return the product of two numbers (a and b): function prod(a,b) { x=a*b return x } When you call the function above, you must pass along two parameters: product=prod(2,3) The returned value from the prod() function is 6, and it will be stored in the variable called product. JavaScript Loops Loops in JavaScript are used to execute the same block of code a specified number of times or while a specified condition is true. Very often when you write code, you want the same block of code to run over and over again in a row. Instead of adding several almost equal lines in a script we can use loops to perform a task like this. In JavaScript there are two different kind of loops: for - loops through a block of code a specified number of times while - loops through a block of code while a specified condition is true do...while – executes at least once and loops through a block of code while a specified condition is true The for Loop The for loop is used when you know in advance how many times the script should run. Syntax for (var=startvalue;var<=endvalue;var=var+increment) { code to be executed } Example The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 10. i will increase by 1 each time the loop runs. Note: The increment parameter could also be negative, and the <= could be any comparing statement. <html> <body> <script type="text/javascript"> var i=0 for (i=0;i<=10;i++) { document.write("The number is " + i) document.write("<br />") } </script> </body> </html> Webpage Design and Development Chapter three: Client Side Scripts 12 | P a g e Result The number is 0 The number is 1 The number is 2 The number is 3 The number is 4 The number is 5 The number is 6 The number is 7 The number is 8 The number is 9 The number is 10 The while loop The while loop is used when you want the loop to execute and continue executing while the specified condition is true. Syntax: while (var<=endvalue) { code to be executed } Note: The <= could be any comparing statement. Example The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 10. i will increase by 1 each time the loop runs. <html> <body> <script type="text/javascript"> var i=0 while (i<=10) { document.write("The number is " + i) document.write("<br />") i=i+1 } </script> </body> </html> Result The number is 0 The number is 1 The number is 2 The number is 3 The number is 4 The number is 5 The number is 6 The number is 7 The number is 8 The number is 9 The number is 10 The do...while Loop Webpage Design and Development Chapter three: Client Side Scripts 15 | P a g e Events are actions that can be detected by JavaScript. Every element on a web page has certain events which can trigger JavaScript functions. For example, we can use the onClick event of a button element to indicate that a function will run when a user clicks on the button. We define the events in the HTML tags. Examples of events: o A mouse click o A web page or an image loading o Mousing over a hot spot on the web page o Selecting an input box in an HTML form o Submitting an HTML form o A keystroke Events are normally used in combination with functions, and the function will not be executed before the event occurs! onload and onUnload The onload and onUnload events are triggered when the user enters or leaves the page. The onload event is often used to check the visitor's browser type and browser version, and load the proper version of the web page based on the information. Both the onload and onUnload events are also often used to deal with cookies that should be set when a user enters or leaves a page. For example, you could have a popup asking for the user's name upon his first arrival to your page. The name is then stored in a cookie. Next time the visitor arrives at your page, you could have another popup saying something like: "Welcome John Doe!". onFocus, onBlur and onChange The onFocus, onBlur and onChange events are often used in combination with validation of form fields. Below is an example of how to use the onChange event. The checkEmail() function will be called whenever the user changes the content of the field: <input type="text" size="30" id="email" onchange="checkEmail()">; onSubmit The onSubmit event is used to validate ALL form fields before submitting it. Below is an example of how to use the onSubmit event. The checkForm() function will be called when the user clicks the submit button in the form. If the field values are not accepted, the submit should be cancelled. The function checkForm() returns either true or false. If it returns true the form will be submitted, otherwise the submit will be cancelled: <form method="post" action="xxx.htm" onsubmit="return checkForm()"> onMouseOver and onMouseOut onMouseOver and onMouseOut are often used to create "animated" buttons. Below is an example of an onMouseOver event. An alert box appears when an onMouseOver event is detected: <a href="http://www.w3schools.com" onmouseover="alert('An onMouseOver event'); return false"> <img src="w3schools.gif" width="100" height="30"> </a> JavaScript Try...Catch Statement The try...catch statement allows you to test a block of code for errors. Webpage Design and Development Chapter three: Client Side Scripts 16 | P a g e JavaScript - Catching Errors When browsing Web pages on the internet, I guess we have all seen a JavaScript alert box, telling you there is a runtime error, and asking: "Do you wish to debug?" on some pages. Error message like that may be useful for developers, but not for the users. When users see errors, they often leave the Web page. There are two ways of catching errors in a Web page: By using the try...catch statement (available in IE5+, Mozilla 1.0, and Netscape 6) By using the onerror event. This is the old standard solution to catch errors (available since Netscape 3) Try...Catch Statement The try...catch statement allows you to test a block of code for errors. The try block contains the code to be run, and the catch block contains the code to be executed if an error occurs. Syntax: try { //Run some code here } catch(err) { //Handle errors here } Note that try...catch is written in lowercase letters. Using uppercase letters will generate a JavaScript error! Example 1 The example below contains a script that is supposed to display the message "Welcome guest!" when you click on a button. However, there's a typo in the message() function. alert() is misspelled as adddlert(). A JavaScript error occurs: <html> <head> <script type="text/javascript"> function message() { adddlert("Welcome guest!") } </script> </head> <body> <input type="button" value="View message" onclick="message()" /> </body> </html> To take more appropriate action when an error occurs, you can add a try...catch statement. The example below contains the "Welcome guest!" example rewritten to use the try...catch statement. Since alert() is misspelled, a JavaScript error occurs. However, this time, the catch block catches the error and executes a custom code to handle it. The code displays a custom error message informing the user what happened: <html> <head> <script type="text/javascript"> Webpage Design and Development Chapter three: Client Side Scripts 17 | P a g e var txt="" function message() { try { adddlert("Welcome guest!") } catch(err) { txt="There was an error on this page.\n\n" txt+="Error description: " + err.description + "\n\n" txt+="Click OK to continue.\n\n" alert(txt) } } </script> </head> <body> <input type="button" value="View message" onclick="message()" /> </body> </html> Example 2 The next example uses a confirm box to display a custom message telling users they can click OK to continue viewing the page or click Cancel to go to the homepage. If the confirm method returns false, the user clicked Cancel, and the code redirects the user. If the confirm method returns true, the code does nothing: <html><head> <script type="text/javascript"> var txt="" function message(){ try { adddlert("Welcome guest!") } catch(err) { txt="There was an error on this page.\n\n" txt+="Click OK to continue viewing this page,\n" txt+="or Cancel to return to the home page.\n\n" if(!confirm(txt)) { document.location.href="http://www.w3schools.com/" } } } </script></head> <body> <input type="button" value="View message" onclick="message()" /> </body></html>
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