External JavaScript refers to a JavaScript code that is stored in a separate file with a .js extension, rather than directly embedded within an HTML document. This external JavaScript file can be reused across multiple web pages, providing a way to separate the JavaScript logic from the HTML structure and promoting code organization and maintainability. It is linked to an HTML document using the script> tag with a src attribute, which specifies the file path of the external JavaScript file.
There are several advantages of using external JavaScript:
1. Code Reusability: By storing JavaScript code in external files, it can be reused across multiple web pages, resulting in more efficient and organized code. This reduces the need for duplicating code and makes it easier to make changes or updates to the code.
2. Separation of Concerns: External JavaScript allows for a clear separation of concerns between the HTML structure and the JavaScript logic. This makes the code more modular and easier to understand, maintain, and debug.
3. Improved Performance: By linking to an external JavaScript file, the browser can cache the file, which means that subsequent requests to the same file do not require additional downloads. This can result in faster page load times and improved performance.
4. Ease of Maintenance: External JavaScript files can be easily edited and updated independently from the HTML documents. This makes it easier to make changes to the JavaScript code without having to modify every HTML page that uses it.
5. Collaboration: External JavaScript files can be shared and collaborated on by multiple developers, making it easier to work on different parts of a project concurrently. This promotes teamwork and allows for better version control.
6. Scalability: External JavaScript files can be easily managed and organized, even in larger projects. This helps to ensure a more scalable and maintainable codebase as the project grows.
While there are several advantages to using external JavaScript, there are also a few potential disadvantages to consider:
1. Increased File Requests: Using external JavaScript files requires additional HTTP requests to fetch those files from the server. This can increase the number of file requests and potentially slow down the page load time, especially if there are many external JavaScript files being loaded.
2. Dependency on External Resources: If the external server hosting the JavaScript file goes down or if there are network issues, it can prevent the JavaScript from being loaded, leading to broken functionality on the web page.
3. Security Risks: External JavaScript files can pose a security risk if they are hosted on unreliable or potentially malicious websites. Including malicious JavaScript code can lead to vulnerabilities, such as cross-site scripting (XSS) attacks.
4. Lack of Control: When using external JavaScript files, developers may have limited control over the performance, reliability, and availability of the external server hosting the files. This dependence on external resources can result in unexpected issues or difficulties in maintaining or updating the codebase.
5. Versioning and Compatibility: External JavaScript files may require versioning and compatibility considerations. If a newer version of a library or framework is released, it may require updating the external file references, which can result in compatibility issues with existing code and require additional testing and debugging.
6. Code Readability: While separation of concerns can be an advantage, it can also make the code more challenging to read and understand, particularly for developers who are unfamiliar with the project or who do not have access to the external JavaScript files.
To add an external JavaScript file to an HTML document, follow these steps:
1. Create or obtain the JavaScript file that you want to include externally. Save it with a `.js` file extension. For example, you could create a file named `script.js`.
2. Place the JavaScript file in a location accessible to your web project, such as the same directory as your HTML file or a folder specifically designated for scripts.
3. In your HTML document, locate the 'head' or `body` section where you want to include the external JavaScript file. It is best practice to place the JavaScript file just before the closing 'body` tag, as this ensures that the HTML content is fully loaded before the JavaScript runs.
4. Insert the following code inside the chosen HTML section:
<script src="path/script.js"></script>
Replace `path/script.js` with the relative or absolute path to your JavaScript file. For example, if the JavaScript file is in the same directory as your HTML file, you can simply use the file name like this:
If the JavaScript file is located in a different directory, specify the path accordingly.
5. Save the changes to your HTML document.
Now, when the HTML page is loaded by a browser, it will also load and execute the external JavaScript file specified in the `script` tag. Make sure to check the browser's console for any JavaScript errors and verify that the file is correctly included in your project structure.
Let's create an external JavaScript file that prints Hello World in a JavaScript alert box.
Creating the external JavaScript File
Filename: myscript.js
function greet(){
alert("Hello World");
}
Now include the JavaScript file you just created into html page and call the JavaScript function on button click.
Filename: index.html
<html>
<head>
<script type="text/javascript" src="myscript.js"></script>
</head>
<body>
<p>Use of External JavaScript File</p>
<form>
<input type="button" value="click" onclick="greet()"/>
</form>
</body>
</html>