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When should you add the defer attribute to the script element? (on Go Make Things)

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For many years I’ve placed script elements just before the closing body tag rather than in the <head>. Since a standard <script> element is render-blocking, the theory is that by putting it at the end of the document – after the main content of the page has loaded – it’s no longer blocking anything, and there’s no need to wrap it in a DOMContentLoaded event listener.

It turns out that my time-honoured default is OK, but there is a better approach.

Chris has done the research for us and ascertained that placing the <script> in the <head> and adding the defer attribute has the same effect as putting that <script> just before the closing body tag but offers improved performance.

This treads fairly complex territory but my general understanding is this:

Using defer on a <script> in the <head> allows the browser to download the script earlier, in parallel, so that it is ready to be used as soon as the DOM is ready rather than having to be downloaded and parsed at that point.

Some additional points worth noting:

  • Only use the defer attribute when the src attribute is present. Don’t use it on inline scripts because it will have no effect.
  • The defer attribute has no effect on module scripts (script type="module"). They defer by default.
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