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How to Import an External JavaScript File

Posted in Javascript Tutorial

Importing an external file is relatively painless.

  • First, the file you are importing must be valid JavaScript, and only JavaScript.
  • Second, the file must have the file extension “.js”.
  • Lastly, you must know the location of the file.

You can import an external JavaScript file into your HTML document using the <script> element. There are a few ways to do this, and I’ll explain two common methods:

1. Using the <script> Tag in the HTML Document:

You can include an external JavaScript file by adding a <script> tag in the HTML document’s <head> or at the end of the <body> section.

Here’s how to do it:

<!DOCTYPE html>
<html>
<head>
    <title>My Web Page</title>
    <!-- Include an external JavaScript file using the 'src' attribute -->
    <script src="path/to/your-script.js"></script>
</head>
<body>
    <!-- Your HTML content goes here -->
</body>
</html>

In this method, the src attribute of the <script> tag is set to the file path of your external JavaScript file. When the browser encounters this <script> tag, it fetches and executes the JavaScript code from the specified file.

2. Using the defer or async Attribute:

You can also use the defer or async attribute with the <script> tag to control how the external script is loaded and executed.

  • defer: The script is loaded in the background and executed after the HTML content is parsed. It preserves the order of execution.
<script src="path/to/your-script.js" defer></script>
  • async: The script is loaded and executed as soon as it’s available, without blocking HTML parsing. The order of execution may not be preserved.
<script src="path/to/your-script.js" async></script>

When using defer or async, it’s important to consider how your JavaScript interacts with the HTML content to ensure that it works as intended.

Make sure to replace "path/to/your-script.js" with the actual file path or URL of your external JavaScript file. The file path should be relative to your HTML document or an absolute URL.

Including external JavaScript files allows you to separate your code into reusable modules and keep your HTML files cleaner and more maintainable.

Example

Let us assume we have a file “myjs.js” that contains a one line Hello World alert function. Also, let us assume that the file is the same directory as the HTML file we are going to code up. To import the file you would do the following in your HTML document.

File myjs.js Contents:

function popup() {
alert("Hello World")
}

HTML & JavaScript Code:

<html>
<head>
<script src="myjs.js">
</script>
</head>
<body>
<input type="button" onclick="popup()" value="Click Me!">
</body>
</html>

When you click “click me!” button, it will pop up a box :

 

Benefits of External JavaScript Files

  1. Modularity: External files allow you to modularize your code, making it more organized and easier to maintain.
  2. Reusability: You can reuse the same script across multiple HTML pages, reducing code duplication.
  3. Separation of Concerns: It separates the structure (HTML), presentation (CSS), and behavior (JavaScript) of your web application, following the principles of web development.
  4. Caching: Browsers can cache external files, leading to faster load times for returning visitors.

Tips for Using External JavaScript Files:

  1. Create a Separate .js File: Save your JavaScript code in a separate .js file. For example, you can create a file named myscript.js.
  2. Linking to External Files: Use the <script> element to link to your external JavaScript file. You can include the <script> tag in the <head> or at the end of the <body>. 
    <!-- Inside the <head> (best practice) -->
<script src="myscript.js"></script>

<!-- Or at the end of the <body> -->
<script src="myscript.js"></script>
  3. Relative and Absolute Paths: Specify the file path in the src attribute of the <script> tag. It can be a relative path from the HTML file or an absolute URL.
  4. Loading Order: Be aware of the loading order. External scripts are loaded and executed sequentially in the order they appear in the HTML.
  5. Using async or defer: Consider using the async or defer attributes to control script loading and execution. defer ensures the script executes after the HTML content is parsed, while async allows non-blocking loading and execution. 
    <script src="myscript.js" defer></script>
<script src="myscript.js" async></script>
  6. Separation of Concerns: Keep JavaScript separate from HTML and CSS. Avoid inline scripts in your HTML document.
  7. Code Organization: Organize your JavaScript code into functions, classes, or modules for better maintainability.
  8. Error Handling: Use the browser’s console for debugging and error handling. Utilize try…catch blocks for error management.
  9. Optimization: Minify and compress your JavaScript files for production to reduce file size and improve loading times.

Recap

  • Create a separate .js file for your JavaScript code.
  • Link to the external JavaScript file using the <script> tag with the src attribute.
  • Specify the file path or URL in the src attribute.
  • Consider using defer or async for script loading control.
  • Keep your JavaScript, HTML, and CSS separate for clean code organization.
  • Organize your JavaScript code for modularity and maintainability.
  • Use the browser console for debugging and error handling.
  • Optimize your JavaScript files for production.

By following these best practices and tips, you can effectively work with external JavaScript files and create well-structured and maintainable web applications.

 

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