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Unformatted text preview: Iauc.ac.ir [email protected] JavaScript Tutorial JavaScript is the programming language of HTML and the Web. Programming makes computers do what you want them to do. JavaScript is easy to learn. This tutorial will teach you JavaScript from basic to advanced. Examples in Each Chapter With our "Try it Yourself" editor, you can change all examples and view the results. Example My First JavaScript Click me to display Date and Time Try it Yourself » <!DOCTYPE html> <body> <h1> My First JavaScript </h1> <button type="button" onclick="document.getElementById('demo').innerHTML = Date()"> Click me to display Date and Time.</button> <p id="demo"> </p> </body> </html> We recommend reading this tutorial, in the sequence listed in the left menu. 1 Learn by Examples Examples are better than 1000 words. Examples are often easier to understand than text explanations. This tutorial supplements all explanations with clarifying "Try it Yourself" examples. If you try all the examples, you will learn a lot about JavaScript, in a very short time! Why Study JavaScript? JavaScript is one of the 3 languages all web developers must learn: 1. HTML to define the content of web pages 2. CSS to specify the layout of web pages 3. JavaScript to program the behavior of web pages This tutorial is about JavaScript, and how JavaScript works with HTML and CSS. Learning Speed In this tutorial, the learning speed is your choice. Everything is up to you. If you are struggling, take a break, or reread the material. Always make sure you understand the "Try-it-Yourself" examples and exercises. JavaScript References W3Schools maintains a complete JavaScript reference, including all HTML DOM, and browser objects. The reference contains examples for all objects, properties, and methods, and is continuously updated according to the latest web standards. 2 JavaScript Quiz Test Test your JavaScript skills at W3Schools! Start JavaScript Quiz! Did You Know? JavaScript and Java are completely different languages, both in concept and design. JavaScript was invented by Brendan Eich in 1995, and became an ECMA standard in 1997. ECMA-262 is the official name. ECMAScript 6 (released in June 2015) is the latest JavaScript version. JavaScript Introduction This page contains some examples of what JavaScript can do. JavaScript Can Change HTML Content One of many JavaScript HTML methods is getElementById(). This example uses the method to "find" an HTML element (with id="demo") and changes the element content (innerHTML) to "Hello JavaScript": Example document.getElementById("demo").innerHTML = "Hello JavaScript"; Try it Yourself » <p id="demo">JavaScript can change HTML content.</p> <button type="button" onclick="document.getElementById('demo').innerHTML = 'Hello JavaScript!'">Click Me!</button> JavaScript Can Change HTML Attributes This example changes an HTML image by changing the src (source) attribute of an <img> tag: 3 The Light Bulb Turn on the light Turn off the light Try it Yourself » <button onclick="document.getElementById('myImage').src='pic_bulbon.gif'">Turn on the light</button> <img id="myImage" src="pic_bulboff.gif" style="width:100px"> <button onclick="document.getElementById('myImage').src='pic_bulboff.gif'">Turn off the light</button> JavaScript Can Change HTML Styles (CSS) Changing the style of an HTML element, is a variant of changing an HTML attribute: Example document.getElementById("demo").style.fontSize = "25px"; Try it Yourself » <p id="demo">JavaScript can change the style of an HTML element.</p> <button type="button" onclick="document.getElementById('demo').style.fontSize='35px'">Click Me!</button> JavaScript Can Hide HTML Elements Hiding HTML elements can be done by changing the display style: Example document.getElementById("demo").style.display="none"; Try it Yourself » 4 <p id="demo">JavaScript can hide HTML elements.</p> <button type="button" onclick="document.getElementById('demo').style.display='none'">Click Me!</button> JavaScript Can Show HTML Elements Showing hidden HTML elements can also be done by changing the display style: Example document.getElementById("demo").style.display="block"; Try it Yourself » <p id="demo" style="display:none">Hello JavaScript!</p> <button type="button" onclick="document.getElementById('demo').style.display='block'">Click Me!</button> JavaScript Where To JavaScript can be placed in the <body> and the <head> sections of an HTML page. The <script> Tag In HTML, JavaScript code must be inserted between <script> and </script> tags. Example 5 <script> document.getElementById("demo").innerHTML = "My First JavaScript"; </script> Older examples may use a type attribute: <script type="text/javascript">. This type attribute is not required. JavaScript is the default scripting language in HTML JavaScript Functions and Events A JavaScript function is a block of JavaScript code, that can be executed when "asked" for. For example, a function can be executed when an event occurs, like when the user clicks a button. You will learn much more about functions and events in later chapters. JavaScript in <head> or <body> You can place any number of scripts in an HTML document. Scripts can be placed in the <body>, or in the <head> section of an HTML page, or in both. Keeping all code in one place, is always a good habit. JavaScript in <head> In this example, a JavaScript function is placed in the <head> section of an HTML page. The function is invoked (called) when a button is clicked: Example 6 <!DOCTYPE html> <html> <head> <script> function myFunction() { document.getElementById("demo").innerHTML = "Paragraph changed."; } </script> </head> <body> <h1>My Web Page</h1> <p id="demo">A Paragraph</p> <button type="button" onclick="myFunction()">Try it</button> </body> </html> Try it Yourself » JavaScript in <body> In this example, a JavaScript function is placed in the <body> section of an HTML page. The function is invoked (called) when a button is clicked: Example 7 <!DOCTYPE html> <html> <body> <h1>My Web Page</h1> <p id="demo">A Paragraph</p> <button type="button" onclick="myFunction()">Try it</button> <script> function myFunction() { document.getElementById("demo").innerHTML = "Paragraph changed."; } </script> </body> </html> Try it Yourself » It is a good idea to place scripts at the bottom of the <body> element. This can improve page load, because script compilation can slow down the display. External JavaScript Scripts can also be placed in external files: myScript.js function myFunction() { document.getElementById("demo").innerHTML = "Paragraph changed."; } External scripts are practical when the same code is used in many different web pages. JavaScript files have the file extension .js. To use an external script, put the name of the script file in the src (source) attribute of a <script> tag: Example 8 <!DOCTYPE html> <html> <body> <script src="myScript.js"></script> </body> </html> Try it Yourself » <p id="demo">A Paragraph.</p> <button type="button" onclick="myFunction()">Try it</button> <p><strong>Note:</strong> myFunction is stored in an external file called "myScript.js".</p> <script src="myScript.js"></script> You can place an external script reference in <head> or <body> as you like. The script will behave as if it was located exactly where the <script> tag is located. External scripts cannot contain <script> tags. External JavaScript Advantages Placing JavaScripts in external files has some advantages: It separates HTML and code It makes HTML and JavaScript easier to read and maintain Cached JavaScript files can speed up page loads JavaScript Output 9 JavaScript does NOT have any built-in print or display functions. JavaScript Display Possibilities JavaScript can "display" data in different ways: Writing Writing Writing Writing into into into into an alert box, using window.alert(). the HTML output using document.write(). an HTML element, using innerHTML. the browser console, using console.log(). Using window.alert() You can use an alert box to display data: Example <!DOCTYPE html> <html> <body> <h1>My First Web Page</h1> <p>My first paragraph.</p> <script> window.alert(5 + 6); </script> </body> </html> Try it Yourself » Using document.write() For testing purposes, it is convenient to use document.write(): Example <!DOCTYPE html> <html> <body> 10 <h1>My First Web Page</h1> <p>My first paragraph.</p> <script> document.write(5 + 6); </script> </body> </html> Try it Yourself » Using document.write() after an HTML document is fully loaded, will delete all existing HTML: Example <!DOCTYPE html> <html> <body> <h1>My First Web Page</h1> <p>My first paragraph.</p> <button onclick="document.write(5 + 6)">Try it</button> </body> </html> Try it Yourself » The document.write() method should only be used for testing. Using innerHTML To access an HTML element, JavaScript can use the document.getElementById(id) method. The id attribute defines the HTML element. The innerHTML property defines the HTML content: Example <!DOCTYPE html> <html> 11 <body> <h1>My First Web Page</h1> <p>My First Paragraph</p> <p id="demo"></p> <script> document.getElementById("demo").innerHTML = 5 + 6; </script> </body> </html> Try it Yourself » To "display data" in HTML, (in most cases) you will set the value of an innerHTML property. Using console.log() In your browser, you can use the console.log() method to display data. Activate the browser console with F12, and select "Console" in the menu. Example <!DOCTYPE html> <html> <body> <h1>My First Web Page</h1> <p>My first paragraph.</p> <script> console.log(5 + 6); </script> </body> </html> Try it Yourself » JavaScript Syntax 12 JavaScript syntax is the set of rules, how JavaScript programs are constructed. JavaScript Programs A computer program is a list of "instructions" to be "executed" by the computer. In a programming language, these program instructions are called statements. JavaScript is a programming language. JavaScript statements are separated by semicolons. Example <p id="demo"></p> <script> var x = 5; var y = 6; var z = x + y; document.getElementById("demo").innerHTML = z; </script> Try it Yourself » In HTML, JavaScript programs can be executed by the web browser. JavaScript Statements JavaScript statements are composed of: Values, Operators, Expressions, Keywords, and Comments. JavaScript Values 13 The JavaScript syntax defines two types of values: Fixed values and variable values. Fixed values are called literals. Variable values are called variables. JavaScript Literals The most important rules for writing fixed values are: Numbers are written with or without decimals: 10.50 1001 Try it Yourself » Strings are text, written within double or single quotes: "John Doe" 'John Doe' Try it Yourself » JavaScript Variables In a programming language, variables are used to store data values. JavaScript uses the var keyword to declare variables. An equal sign is used to assign values to variables. In this example, x is defined as a variable. Then, x is assigned (given) the value 6: var x; x = 6; Try it Yourself » JavaScript Operators 14 JavaScript uses an assignment operator ( = ) to assign values to variables: var x = 5; var y = 6; Try it Yourself » JavaScript uses arithmetic operators ( + - * / ) to compute values: (5 + 6) * 10 Try it Yourself » JavaScript Expressions An expression is a combination of values, variables, and operators, which computes to a value. The computation is called an evaluation. For example, 5 * 10 evaluates to 50: 5 * 10 Try it Yourself » Expressions can also contain variable values: x * 10 Try it Yourself » The values can be of various types, such as numbers and strings. For example, "John" + " " + "Doe", evaluates to "John Doe": "John" + " " + "Doe" <p id="demo"></p> <script> document.getElementById("demo").innerHTML = "John" + " " </script> Try it Yourself » 15 + "Doe"; JavaScript Keywords JavaScript keywords are used to identify actions to be performed. The var keyword tells the browser to create a new variable: <p id="demo"></p> <script> var x = 5 + 6; var y = x * 10; document.getElementById("demo").innerHTML = y; </script> Try it Yourself » JavaScript Comments Not all JavaScript statements are "executed". Code after double slashes // or between /* and */ is treated as a comment. Comments are ignored, and will not be executed: var x = 5; // I will be executed // var x = 6; I will NOT be executed Try it Yourself » JavaScript Identifiers Identifiers are names. In JavaScript, identifiers are used to name variables (and keywords, and functions, and labels). The rules for legal names are much the same in most programming languages. In JavaScript, the first character must be a letter, an underscore (_), or a dollar sign ($). 16 Subsequent characters may be letters, digits, underscores, or dollar signs. Numbers are not allowed as the first character. This way JavaScript can easily distinguish identifiers from numbers. JavaScript is Case Sensitive All JavaScript identifiers are case sensitive. The variables lastName and lastname, are two different variables. lastName = "Doe"; lastname = "Peterson"; Try it Yourself » JavaScript does not interpret VAR or Var as the keyword var. JavaScript and Camel Case Historically, programmers have used three ways of joining multiple words into one variable name: Hyphens: first-name, last-name, master-card, inter-city. Underscore: first_name, last_name, master_card, inter_city. Camel Case: FirstName, LastName, MasterCard, InterCity. 17 In programming languages, especially in JavaScript, camel case often starts with a lowercase letter: firstName, lastName, masterCard, interCity. Hyphens are not allowed in JavaScript. It is reserved for subtractions. JavaScript Character Set JavaScript uses the Unicode character set. Unicode covers (almost) all the characters, punctuations, and symbols in the world. For a closer look, please study our Complete Unicode Reference. JavaScript Statements ❮ Previous Next ❯ In HTML, JavaScript statements are "instructions" to be "executed" by the web browser. JavaScript Statements 18 This statement tells the browser to write "Hello Dolly." inside an HTML element with id="demo": Example document.getElementById("demo").innerHTML = "Hello Dolly."; Try it Yourself » JavaScript Programs Most JavaScript programs contain many JavaScript statements. The statements are executed, one by one, in the same order as they are written. In this example x, y, and z are given values, and finally z is displayed: Example var x = 5; var y = 6; var z = x + y; document.getElementById("demo").innerHTML = z; Try it Yourself » JavaScript programs (and JavaScript statements) are often called JavaScript code. Semicolons ; Semicolons separate JavaScript statements. Add a semicolon at the end of each executable statement: a = 5; b = 6; c = a + b; Try it Yourself » When separated by semicolons, multiple statements on one line are allowed: 19 a = 5; b = 6; c = a + b; Try it Yourself » On the web, you might see examples without semicolons. Ending statements with semicolon is not required, but highly recommended. JavaScript White Space JavaScript ignores multiple spaces. You can add white space to your script to make it more readable. The following lines are equivalent: var person = "Hege"; var person="Hege"; A good practice is to put spaces around operators ( = + - * / ): var x = y + z; JavaScript Line Length and Line Breaks For best readability, programmers often like to avoid code lines longer than 80 characters. If a JavaScript statement does not fit on one line, the best place to break it, is after an operator: Example document.getElementById("demo").innerHTML = "Hello Dolly."; Try it Yourself » JavaScript Code Blocks JavaScript statements can be grouped together in code blocks, inside curly brackets {...}. The purpose of code blocks is to define statements to be executed together. 20 One place you will find statements grouped together in blocks, are in JavaScript functions: Example function myFunction() { document.getElementById("demo").innerHTML = "Hello Dolly."; document.getElementById("myDIV").innerHTML = "How are you?"; } Try it Yourself » In this tutorial we use 4 spaces of indentation for code blocks. You will learn more about functions later in this tutorial. JavaScript Keywords JavaScript statements often start with a keyword to identify the JavaScript action to be performed. Here is a list of some of the keywords you will learn about in this tutorial: Keyword Description break Terminates a switch or a loop continue Jumps out of a loop and starts at the top debugger Stops the execution of JavaScript, and calls (if available) the debugging function do ... while Executes a block of statements, and repeats the block, while a condition is true for Marks a block of statements to be executed, as long as a condition is true 21 function Declares a function if ... else Marks a block of statements to be executed, depending on a condition return Exits a function switch Marks a block of statements to be executed, depending on different cases try ... catch Implements error handling to a block of statements var Declares a variable JavaScript keywords are reserved words. Reserved words cannot be used as names for variables. JavaScript Comments JavaScript comments can be used to explain JavaScript code, and to make it more readable. JavaScript comments can also be used to prevent execution, when testing alternative code. Single Line Comments Single line comments start with //. Any text between // and the end of the line will be ignored by JavaScript (will not be executed). This example uses a single-line comment before each code line: 22 Example <h1 id="myH"></h1> <p id="myP"></p> <script> // Change heading: document.getElementById("myH").innerHTML = "My First Page"; // Change paragraph: document.getElementById("myP").innerHTML = "My first paragraph."; </script> Try it Yourself » This example uses a single line comment at the end of each line to explain the code: Example var x = 5; var y = x + 2; // Declare x, give it the value of 5 // Declare y, give it the value of x + 2 Try it Yourself » Multi-line Comments Multi-line comments start with /* and end with */. Any text between /* and */ will be ignored by JavaScript. This example uses a multi-line comment (a comment block) to explain the code: Example /* The code below will change the heading with id = "myH" and the paragraph with id = "myP" in my web page: */ document.getElementById("myH").innerHTML = "My First Page"; document.getElementById("myP").innerHTML = "My first paragraph."; Try it Yourself » 23 It is most common to use single line comments. Block comments are often used for formal documentation. Using Comments to Prevent Execution Using comments to prevent execution of code is suitable for code testing. Adding // in front of a code line changes the code lines from an executable line to a comment. This example uses // to prevent execution of one of the code lines: Example //document.getElementById("myH").innerHTML = "My First Page"; document.getElementById("myP").innerHTML = "My first paragraph."; Try it Yourself » This example uses a comment block to prevent execution of multiple lines: Example /* document.getElementById("myH").innerHTML = "My First Page"; document.getElementById("myP").innerHTML = "My first paragraph."; */ Try it Yourself » JavaScript Variables JavaScript Variables JavaScript variables are containers for storing data values. In this example, x, y, and z, are variables: Example var x = 5; var y = 6; var z = x + y; Try it Yourself » 24 From the example above, you can expect: x stores the value 5 y stores the value 6 z stores the value 11 Much Like Algebra In this example, price1, price2, and total, are variables: Example <script> var price1 = 5; var price2 = 6; var total = price1 + price2; document.getElementById("demo").innerHTML ="The total is: " + total; </script> Try it Yourself » In programming, just like in algebra, w...
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