There are several common ways to add JavaScript to HTML pages, which can be selected according to your needs. First, inline scripts are suitable for small functions or tests, and they write code directly in HTML, but are not convenient for maintenance; second, external JS files are introduced, with clear structure, conducive to collaboration, and suitable for project development; third, use defer or async attributes to control the loading order to avoid blocking HTML parsing; fourth, dynamically load scripts, loading on demand to improve performance. It is recommended to choose the appropriate method according to the scene, pay attention to the script execution order and loading timing.
It is actually quite straightforward to add JavaScript to HTML pages. The key is to figure out how you want to use it - whether to write a simple script to make the web page move, or to refer to external files more convenient to manage. The following methods are the most commonly used and practical.
Inline scripts: the most direct way
If you just want to add a small piece of code, such as clicking a button and popping a prompt box, you can write the <script></script> tag directly in HTML.
<script>
alert("Page loading is completed!");
</script>This method is suitable for quick testing or small functions, but it is not recommended to be used on large projects because it is difficult to maintain when mixed together.
Note: This code is usually placed before the end of <body> , so that it will wait until the DOM is loaded before executing, which is not easy to make mistakes.
Introduce external JS files: clear structure and better manipulation
If your script is longer, or you want multiple pages to share a piece of code, you should use external files.
First create a script.js file and write your code:
console.log("This script was introduced from the outside");Then quote it in HTML like this:
<script src="script.js"></script>
The advantage of this approach is that HTML and JS are separated, easy to collaborate and debug. Remember to have the right path, otherwise the console will report an error "Resource not found".
Put it in <head> or <body> ? The order is important
By default, when the browser reads <script> it will pause the parsing of HTML, download and execute the script first. If you put <script> in <head> , the page content must wait until the script is executed before it appears.
So the general advice:
- Put the script at the end of
<body>(before closing the tag), so that the HTML content is loaded first. - If you have to put
<head>, you can add adeferorasyncattribute to<script>:-
defer: wait until the HTML is parsed before executing, keeping the order -
async: HTML is not blocked when downloading, but the execution order is uncertain
-
These two properties are only valid for external scripts.
Dynamic loading scripts: more efficient loading on demand
Sometimes you don't want to load all scripts at the beginning, but wait for an action to trigger before loading, such as clicking a button or scrolling to the bottom.
You can use JS to create <script> tags dynamically:
const script = document.createElement('script'); script.src = 'lazy-script.js'; document.body.appendChild(script);
This can reduce the initial loading time and improve performance, especially suitable for large projects or third-party plug-ins.
Basically these are the methods. Which scenario to choose depends on: simple tests are used for inline, project development is used for external links, and loading optimization is based on defer/async or dynamic loading. The uncomplexing but easy to ignore is the script execution order and loading time. Don't just throw scripts into the head as soon as you come up.
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