What are some of the best practices for loading and running JavaScript in HTML?

1 Where to place your scripts

One of the first decisions you need to make is where to put your JavaScript code in your HTML document. Generally, you have three options: in the head, in the body, or in an external file. Each option has its pros and cons, depending on your goals and preferences. For example, placing your scripts in the head can ensure that they are loaded and executed before the rest of the page, but it can also delay the rendering of the content and cause a blank screen for the user. Placing your scripts in the body can improve the perceived loading speed and allow you to access the DOM elements, but it can also interfere with the layout and style of the page and create dependencies between scripts. Placing your scripts in an external file can reduce the size and complexity of your HTML document and enable caching and reuse, but it can also introduce additional HTTP requests and dependencies.

2 How to defer or async your scripts

Another factor that affects the loading and running of JavaScript in HTML is the way you specify when and how your scripts should be executed. By default, the browser will block the rendering of the page until it downloads and executes the script, which can result in a poor user experience. However, you can use the defer or async attributes to modify this behavior and improve the performance and responsiveness of your page. The defer attribute tells the browser to download the script in the background and execute it after the page has finished parsing. The async attribute tells the browser to download the script in the background and execute it as soon as it is available, without waiting for the page to finish parsing. Both attributes can help you avoid blocking the rendering of the page, but they also have some differences and limitations. For example, the defer attribute preserves the order of execution of the scripts, while the async attribute does not. The async attribute is more suitable for independent scripts that do not rely on other scripts or DOM elements, while the defer attribute is more suitable for dependent scripts that need to run after the page is ready.

3 How to use modules and bundlers

A third best practice for loading and running JavaScript in HTML is to use modules and bundlers to organize and optimize your code. Modules are a way of dividing your code into separate files that can export and import variables, functions, classes, and other features. Modules can help you avoid global variables, namespace conflicts, and code duplication, and make your code more modular, maintainable, and testable. Bundlers are tools that can take your modules and combine them into one or more files that can be easily loaded and run in the browser. Bundlers can also perform other tasks, such as minifying, transpiling, and compressing your code, to reduce the size and improve the compatibility of your scripts.

4 How to handle errors and exceptions

A fourth best practice for loading and running JavaScript in HTML is to handle errors and exceptions that may occur during the execution of your code. Errors and exceptions are situations that prevent your code from running normally or produce unexpected results. They can be caused by syntax errors, logic errors, runtime errors, or external factors, such as network failures, user input, or browser compatibility. If you do not handle errors and exceptions properly, they can cause your page to crash, display incorrect information, or expose sensitive data. Therefore, you should use techniques such as try-catch-finally blocks, error events, console methods, and debugging tools to catch, report, and fix errors and exceptions in your code.

5 How to test and debug your scripts

A fifth best practice for loading and running JavaScript in HTML is to test and debug your scripts to ensure that they work as intended and do not contain any bugs or errors. Testing and debugging are processes of checking and correcting your code for any problems or defects that may affect its functionality, performance, or quality. Testing and debugging can involve different methods and tools, such as unit testing, integration testing, end-to-end testing, code analysis, breakpoints, watch expressions, call stacks, and source maps. Testing and debugging can help you identify and resolve issues in your code before they affect your users or clients.

6 How to follow coding standards and conventions

A sixth best practice for loading and running JavaScript in HTML is to follow coding standards and conventions that can improve the readability, consistency, and quality of your code. Coding standards and conventions are guidelines and rules that define how you should write, format, structure, and document your code. They can cover aspects such as indentation, spacing, naming, comments, syntax, style, best practices, and common patterns. Coding standards and conventions can help you avoid errors, confusion, and conflicts in your code, and make it easier for yourself and others to understand, maintain, and modify your code.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?