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Where to Append JavaScript in HTML.

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Where to Append JavaScript in HTML.

Anubhav Kumar Gupta's photo
Anubhav Kumar Gupta
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3 min read

Table of contents

When working with HTML and JavaScript, deciding where to place your JavaScript code is crucial for ensuring smooth page performance and a great user experience.

Let’s explore the various options and determine the best practices for including JavaScript in your HTML files.

Recommendation

  • It is generally recommended to add JavaScript files at the end of the <body> tag.

  • This ensures that your page's content loads first, improving the overall performance and user experience.

JavaScript File Placement Cases

1. JavaScript in <head> Tag

  • How it works: Adding JavaScript files using <script src="file.js"></script> inside the <head> tag.

  • Issue:

    • The browser pauses the HTML parsing to download and execute the JavaScript file.

    • This blocking behavior can significantly slow down the rendering of the page.

  • Effect: Lower HTML elements won’t load until the script is fully executed, causing a poor user experience.

2. JavaScript at the Top of <body> Tag

  • Process:

    1. The browser starts parsing HTML from the top.

    2. When it encounters the <script> tag, it downloads and executes the JavaScript.

    3. Only after execution is complete does HTML parsing resume.

  • Effect:

    • Similar blocking behavior to placing the script in the <head> tag.

    • Slightly better as some HTML content above the <script> tag is already parsed.

3. Using async Attribute

  • How it works: Place <script src="file.js" async></script> in your HTML.

  • Purpose: The async attribute allows the browser to download the JavaScript file asynchronously (without stopping HTML parsing).

  • Effect:

    • Both the JavaScript and HTML parsing happen simultaneously.

    • The script executes as soon as it is downloaded, which may cause issues if the script depends on elements that haven’t been loaded yet.

    • Best for: Non-blocking scripts that don’t rely on fully loaded HTML.

4. Using defer Attribute

  • How it works: Use <script src="file.js" defer></script> in your HTML.

  • Purpose: The defer attribute also downloads JavaScript asynchronously but ensures the script executes only after the entire HTML is parsed.

  • Effect:

    • HTML parsing isn’t blocked, and JavaScript execution occurs only when the DOM is fully loaded.

    • Best for: Scripts that interact with HTML elements or depend on fully loaded content.

Best Practice

  • Use async or defer for external JavaScript files to optimize page load performance.

    • async: Best for scripts that are independent of the HTML structure.

    • defer: Best for scripts that interact with or depend on HTML elements.

  • Place your JavaScript files at the end of the <body> tag when not using async or defer to ensure that HTML content loads first.

Summary

For the best page performance:

  1. Avoid placing JavaScript in the <head> tag.

  2. Use async for independent scripts and defer for scripts requiring the DOM.

  3. When in doubt, place your <script> tags at the end of the <body> tag.

Following these practices ensures faster page loads and a seamless user experience.

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