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Merge pull request #2151 from haond10adp/patch-1

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Ilya Kantor 2020-10-07 19:45:43 +03:00 • committed by GitHub
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2-ui/5-loading/02-script-async-defer/article.md
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In modern websites, scripts are often "heavier" than HTML: their download size is larger, and processing time is also longer.
When the browser loads HTML and comes across a `<script>...</script>` tag, it can't continue building the DOM. It must execute the script right now. The same happens for external scripts `<script src="..."></script>`: the browser must wait until the script downloads, execute it, and only after process the rest of the page.
When the browser loads HTML and comes across a `<script>...</script>` tag, it can't continue building the DOM. It must execute the script right now. The same happens for external scripts `<script src="..."></script>`: the browser must wait for the script to download, execute the downloaded script, and only then can it process the rest of the page.
That leads to two important issues:
@ -185,7 +185,7 @@ But there are also essential differences between them:
| | Order | `DOMContentLoaded` |
|---------|---------|---------|
| `async` | *Load-first order*. Their document order doesn't matter -- which loads first | Irrelevant. May load and execute while the document has not yet been fully downloaded. That happens if scripts are small or cached, and the document is long enough. |
| `async` | *Load-first order*. Their document order doesn't matter -- which loads first runs first | Irrelevant. May load and execute while the document has not yet been fully downloaded. That happens if scripts are small or cached, and the document is long enough. |
| `defer` | *Document order* (as they go in the document). | Execute after the document is loaded and parsed (they wait if needed), right before `DOMContentLoaded`. |
In practice, `defer` is used for scripts that need the whole DOM and/or their relative execution order is important.