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Short answer: You do not need to move every script to the end of the page. For an external classic script that needs the page’s DOM, placing it in the <head> with defer is often a good default: the browser can keep parsing HTML, then runs deferred scripts in document order after parsing. A script immediately before </body> is also valid, but it is not automatically faster or better for SEO.
What changes when a script is in the head?
An ordinary external classic script in the <head>, without async or defer, can pause HTML parsing while the browser fetches and executes it. That can delay the browser from reaching and processing the page’s visible markup. The browser’s behavior depends on script type and attributes; “in the head” alone does not tell you whether parsing will be blocked.
With defer, an external classic script can download while HTML parsing continues. It executes after the document has been parsed, and multiple deferred classic scripts run in the order they appear. MDN documents this ordering in its script element reference.
Choose the loading behavior that matches the script
| Script situation | Suitable approach | What to expect |
|---|---|---|
| External classic script needs the DOM or depends on earlier scripts | Usually use defer, often in the head |
HTML parsing continues during the fetch; execution follows parsing and deferred scripts retain document order. MDN |
| Independent external script whose execution order does not matter | Use async when appropriate |
It executes as soon as it is available, so its order relative to other scripts is not guaranteed. Do not use it for code that depends on another script running first. MDN |
| Inline classic JavaScript | Use a suitable placement or move it to an external script | defer has no effect on an inline classic script without src. MDN |
| JavaScript module | Use a module script; add async only if its timing is intended |
Modules are deferred by default; async changes their execution timing. MDN |
Code placed immediately before </body> |
Keep this placement if it suits the page | Markup above the script has been parsed before it runs, but the placement alone does not establish a speed advantage. |
Should a header image slider go at the end?
Not automatically. First identify what the slider script needs. If it reads or modifies the slider’s markup, it must run when that markup is available; a deferred external script in the head can do that after parsing. A script before </body> also runs after the preceding slider markup has been parsed. If the slider relies on a library or other application code, preserve the required execution order rather than marking dependent scripts async.
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Then consider whether the slider is part of the page’s initially visible content. Moving its code later may change when the slider initializes, but it does not by itself establish that the page will feel faster or that the visible header will appear sooner. The right choice depends on the page’s actual dependencies and loading behavior.
How script placement relates to page speed and SEO
Putting JavaScript at the end of the body can avoid one specific problem: a parser-blocking script in the head delaying HTML parsing. But defer addresses that problem for external classic scripts while allowing them to stay in the head. Neither placement is a universal performance fix; the result depends on the scripts, page, and browser loading behavior.
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The historical SitePoint discussion that prompted this question contains competing claims about perceived rendering and total load time, not controlled measurements that settle the issue for current websites. It does not establish a universal SEO ranking benefit for placing scripts before </body>. If speed is the concern, evaluate the actual page and its loading behavior rather than treating a placement convention as a measured result.
Account for DOMContentLoaded
The DOMContentLoaded event waits for deferred and module scripts to download and execute. Async scripts and dynamically inserted scripts may execute after that event has fired, so code that relies on them should not assume they always run first. See MDN’s DOMContentLoaded event reference.
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