Inline vs external javascript

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In this video we will discuss
1. Different places where JavaScript can be present
2. Advantages of external JavaScript over inline JavaScript

JavaScript can be stored either inline on the page or in an external .js file. Let's look at an example of both the approaches.

Inline JavaScript example : In this example, IsEven() JavaScript function is present inline on the page.
[html]
[head]
[script type="text/javascript"]
function IsEven()
{
var number = document.getElementById("TextBox1").value;
if (number % 2 == 0)
{
alert(number + " is even number");
}
else
{
alert(number + " is odd number");
}
}
[/script]
[/head]
[body]
[form id="form1" runat="server"]
Number :
[asp:TextBox ID="TextBox1" runat="server"][/asp:TextBox]
[input type="button" value="Check Number" onclick="IsEven()" /]
[/form]
[/body]
[/html]

External JavaScript example : Steps to store JavaScript in an external .js file
1. In Visual Studio, right click on the project name in Solution Explorer and select add select Add =] New Item
2. From the "Add New Item" dialog box select "JScript File". Name the file "ExternalJavaScript.js" and click Add.
3. Copy and paste the following JavaScript function in the "ExternalJavaScript.js" file

function IsEven()
{
var number = document.getElementById("TextBox1").value;
if (number % 2 == 0)
{
alert(number + " is even number");
}
else
{
alert(number + " is odd number");
}
}

4. On your webform in the head section include a reference to the external JavaScript file using script element as shown below
[script type="text/javascript" src="ExternalJavaScript.js"][/script]

5. At this point the HTML on your webform should be as shown below.
[html]
[head]
[script type="text/javascript" src="ExternalJavaScript.js"][/script]
[/head]
[body]
[form id="form1" runat="server"]
Number :
[asp:TextBox ID="TextBox1" runat="server"][/asp:TextBox]
[input type="button" value="Check Number" onclick="IsEven()" /]
[/form]
[/body]
[/html]

Advantages of external JavaScript over inline JavaScript : Here are some of the general advantages of external JavaScript over inline JavaScript

Maintainability : JavaScript in external files can be referenced on multiple pages without having to duplicate the code inline on every page. If something has to change, you only have one place to change. So external JavaScript code can be reused and maintenance will be much easier.

Separation of Concerns : Storing JavaScript in a separate external .js file adheres to Separation of concerns design principle. In general it is a good practice to separate HTML, CSS and JavaScript as it makes it easier working with them. Also allows multiple developers to work simultaneously.

Performance : An external JavaScript file can be cached by the browser, where as an inline JavaScript on the page is loaded every time the page loads.