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Java script
The document discusses different places where JavaScript code can be included in an HTML file, such as in the <head> or <body> sections, or externally in a .js file. It also covers JavaScript datatypes like numbers, strings, Booleans, and objects. Variables are declared with var and can have either global or local scope. Variable names follow naming conventions like not starting with numbers and avoiding reserved words.
Java script
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- 2. Where to placeJavaScript • There is a flexibility given to include JavaScript code anywhere in an HTML document. • However the most preferred ways to include JavaScript in an HTML file are as follows: – Script in <head>...</head> section. – Script in <body>...</body> section. – Script in <body>...</body> and <head>...</head> sections. – Script in an external file and then include in <head>...</head> section.
- 3. JavaScript in <head>...</head>Section • If you want to have a script run on some event, such as when a user clicks somewhere, then you will place that script in the head as follows: <html> <head> <script type="text/javascript"> <!-- function sayHello() { alert("Hello World") } //--> </script> </head> <body> Click here for the result <input type="button" onclick="sayHello()" value="Say Hello" /> </body> </html>
- 5. JavaScript in <body>...</body>Section • If you need a script to run as the page loads so that the script generates content in the page, then the script goes in the <body> portion of the document. • In this case, you would not have any function defined using JavaScript. • Take a look at the following code.
- 6. Example <html> <head> </head> <body> <script type="text/javascript"> <!-- document.write("Hello World") //--> </script> <p>Thisis web page body </p> </body> </html>
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- 8. JavaScript in <body>and <head> Sections • You can put your JavaScript code in <head> and <body> section altogether as follows: <html> <head> <script type="text/javascript"> <!-- function sayHello() { alert("Hello World") } //--> </script> </head> <body> <script type="text/javascript"> <!-- document.write("Hello World") //--> </script> <input type="button" onclick="sayHello()" value="Say Hello" /> </body> </html>
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- 10. JavaScript in ExternalFile • As you begin to work more extensively with JavaScript, you will be likely to find that there are cases where you are reusing identical JavaScript code on multiple pages of a site. • You are not restricted to be maintaining identical code in multiple HTML files. • The script tag provides a mechanism to allow you to store JavaScript in an external file and then include it into your HTML files.
- 11. Example <html> <head> <script type="text/javascript" src="filename.js"></script> </head> <body> <input type="button" onclick="sayHello()" value="Say Hello" /> </body> </html> • To use JavaScript from an external file source, you need to write all your JavaScript source code in a simple text file with the extension ".js" • Then include that file as shown above.
- 12. External Javascript • filename.js functionsayHello() { alert("Hello World") }
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- 14. JavaScript Datatypes • Oneof the most fundamental characteristics of a programming language is the set of data types it supports. • These are the type of values that can be represented and manipulated in a programming language. • JavaScript allows you to work with three primitive data types: – Numbers, e.g., 123, 120.50 etc. – Strings of text, e.g. "This text string" etc. – Boolean, e.g. true or false. • JavaScript also defines two trivial data types, null and undefined, each of which defines only a single value. • In addition to these primitive data types, JavaScript supports a composite data type known as object.
- 15. JavaScript Variables • Likemany other programming languages, JavaScript has variables. • Variables can be thought of as named containers. • You can place data into these containers and then refer to the data simply by naming the container. • Before you use a variable in a JavaScript program, you must declare it. • Variables are declared with the var keyword as follows.
- 16. Example <script type="text/javascript"> <!-- var money; varname; //--> </script> You can also declare multiple variables with the same var keyword as follows: <script type="text/javascript"> <!-- var money, name; //--> </script>
- 17. Initialization • Storing avalue in a variable is called variable initialization. • You can do variable initialization at the time of variable creation or at a later point in time when you need that variable. • For instance, you might create a variable named money and assign the value 2000.50 to it later. • For another variable, you can assign a value at the time of initialization as follows
- 18. Example <script type="text/javascript"> <!-- var name= "Ali"; var money; money = 2000.50; //--> </script>
- 19. JavaScript Variable Scope •The scope of a variable is the region of your program in which it is defined. JavaScript variables have only two scopes. – Global Variables: A global variable has global scope which means it can be defined anywhere in your JavaScript code. – Local Variables: A local variable will be visible only within a function where it is defined. Function parameters are always local to that function. – Within the body of a function, a local variable takes precedence over a global variable with the same name. If you declare a local variable or function parameter with the same name as a global variable, you effectively hide the global variable.
- 20. Example <script type="text/javascript"> <!-- var myVar= "global"; // Declare a global variable function checkscope( ) { var myVar = "local"; // Declare a local variable document.write(myVar); } //--> </script> It will produce the following result:
- 21. JavaScript Variable Names •While naming your variables in JavaScript, keep the following rules in mind: • You should not use any of the JavaScript reserved keywords as a variable name. • For example: – break or boolean variable names are not valid. • JavaScript variable names should not start with a numeral (0-9). • They must begin with a letter or an underscore character. • For example, 123test is an invalid variable name but _123test is a valid one. • JavaScript variable names are case-sensitive. • For example, Name and name are two different variables.
- 22. JavaScript Reserved Words •A list of all the reserved words in JavaScript are given in the following table. • They cannot be used as JavaScript variables, functions, methods, loop labels, or any object names.