No, JavaScript does not load before HTML by default. In a typical web page, the browser processes and loads HTML first, which defines the structure of the page. JavaScript is then loaded and executed afterward, depending on where the script is placed and how it’s configured.
Here’s how the loading order typically works:
- HTML Parsing:
- The browser starts parsing the HTML from top to bottom.
- When the browser encounters a
<script>tag, it pauses the HTML parsing to download and execute the JavaScript, unless specified otherwise.
- JavaScript Execution:
- By default, if a
<script>tag is placed in the<head>or middle of the document, the browser will stop loading and parsing the rest of the HTML until the JavaScript file is fully loaded and executed. - This can block rendering of the page if the script takes time to load, leading to a “render-blocking” issue.
- By default, if a
Ways to Control When JavaScript Loads:
<script>at the End of Body:- A common practice is to place the
<script>tag just before the closing</body>tag. This ensures that the HTML is fully parsed before the JavaScript runs. - Example:
- A common practice is to place the
<body>
<!-- Page content -->
<script src="script.js"></script>
</body>
2. Using defer Attribute:
- If you place your script in the
<head>, you can add thedeferattribute to the<script>tag. This ensures that the script is downloaded during HTML parsing but is only executed after the HTML document has been fully loaded. - Example:
<head>
<script src="script.js" defer></script>
</head>3. Using async Attribute:
- The
asyncattribute tells the browser to download the JavaScript file while the HTML is being parsed, and execute it as soon as it finishes downloading (without waiting for the rest of the HTML to load). However, this can lead to execution before the full page is loaded. - Example:
<head>
<script src="script.js" async></script>
</head>Key Differences Between async and defer:
async: Script is downloaded and executed as soon as possible, potentially before HTML parsing is complete.defer: Script is downloaded during HTML parsing but only executed once the entire document has been parsed.
My Opinion:
- Default Behavior: JavaScript does not load before HTML unless you explicitly configure it to do so.
- Controlling Script Loading: Use the
deferorasyncattributes to load JavaScript without blocking HTML parsing or place scripts at the bottom of the body for non-blocking execution.
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