Using an External JavaScript File

JavaScript is an essential part of web development, responsible for adding interactivity and functionality to websites. However, as your project grows, so does the complexity and size of your JavaScript code. To maintain cleaner and more efficient code, using external JavaScript files is a best practice that every developer should adopt. In this article, we'll explore the benefits of external JavaScript files, how to create them, and how to include them in your HTML files.

External JavaScript Files

The Advantages of External JavaScript Files

  • Code Reusability: External JavaScript files allow you to write code once and use it across multiple HTML pages. This reusability not only saves time but also keeps your code consistent across your project.
  • Better Organization: Separating your JavaScript code from your HTML code enables a more structured and organized project. This separation makes it easier to find, maintain, and debug your code.
  • Improved Performance: Caching external JavaScript files in the browser reduces the amount of data that needs to be downloaded, resulting in faster page load times.

Creating an External JavaScript File

Creating an external JavaScript file is as simple as creating a new file with a ".js" extension. You can use any text editor or integrated development environment (IDE) of your choice.

  1. Open your text editor or IDE and create a new file.
  2. Save the file with a ".js" extension, such as "myscript.js".
  3. Write your JavaScript code inside the file.

Note: Make sure not to include the <script> tags in your external JavaScript file, as they are only needed when embedding JavaScript directly in your HTML file.

Including the External JavaScript File in Your HTML

To include an external JavaScript file in your HTML, use the <script> tag with the src attribute. The src attribute should be set to the relative or absolute path of your external JavaScript file.


<!DOCTYPE html>
<html lang="en">
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>My Web Page</title>
  <h1>Welcome to My Web Page</h1>
  <p>Click the button below to display an alert:</p>
  <button onclick="displayAlert()">Click me</button>

  <!-- Include the external JavaScript file -->
  <script src="myscript.js" type=”text/javascript” [defer|async|charset|crossorigin|integrity]></script>

Notice I placed the script tag at the bottom of the document. This is the proper place for scripts. The reason is when the browser encounters JavaScript it stops rendering, waits till the file is loaded, interpreted, and executed, before resuming the rendering cycle. Often this means the browser must start the rendering cycle over from the beginning, so it can be a big performance hit.

Script Element Attributes

In HTML, the script element is used to define client-side scripts, such as JavaScript. In the previous example, I included many of the attributes as optional. Here are the different options that can be used with the script element:

  • src: Specifies the URL of the external script file to be included.
  • type: Defines the MIME type of the script. The default value is "text/javascript".
  • async: Specifies that the script should be executed asynchronously. This means that the script will continue to load in the background while the rest of the page is being parsed.
  • defer: Specifies that the script should be executed after the page has finished parsing.
  • charset: Defines the character encoding used in the external script file.
  • crossorigin: Specifies how the browser should handle requests for resources from different origins.
  • integrity: Provides a subresource integrity (SRI) hash for the external script file, which can be used to ensure that the file has not been tampered with.
  • language: Specifies the scripting language used in the script. This attribute is deprecated and should not be used.
  • text: Defines the script as inline text within the element. This option is rarely used.

These are the options that can be used with the script element in HTML.

Load Order and Multiple External JavaScript Files

In some cases, you may need to include multiple external JavaScript files in your project. The order in which you include these files matters, as some scripts may depend on others.

When including multiple external JavaScript files, ensure that they are loaded in the correct order to prevent errors due to dependencies. Place the files with no dependencies first, followed by files that depend on them.


<!-- Include external JavaScript files -->
<script src="library.js"></script>
<script src="plugin.js"></script>
<script src="custom.js"></script>

In this example, "library.js" is loaded first, followed by "plugin.js", which depends on the library, and finally "custom.js", which depends on both the library and the plugin.


Using external JavaScript files is a best practice that leads to cleaner, more efficient, and better-organized web development projects. By understanding the benefits of creating external JavaScript files, and including them in your HTML, you can streamline your development process, improve code maintainability, and enhance your web app's performance.

Furthermore, be mindful of the load order when including multiple external JavaScript files in your project to avoid errors due to dependencies. With this knowledge in hand, you're now better equipped to create powerful and efficient web applications.