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This commit is contained in:
Ilya Kantor 2019-05-26 23:13:13 +03:00
parent 5c6a8ec88c
commit 5f095cc340
2 changed files with 9 additions and 30 deletions
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20
2-ui/5-loading/02-script-async-defer/article.md
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@ -29,11 +29,11 @@ There are some workarounds to that. For instance, we can put a script at the bot
</body>
```
But this solution is far from perfect. For example, the browser actually notices the script (and can start downloading it) only after it downloaded the full HTML document. For long HTML documents, that may be a noticeable delay.
But this solution is far from perfect. For example, the browser notices the script (and can start downloading it) only after it downloaded the full HTML document. For long HTML documents, that may be a noticeable delay.
Such things are invisible for people using very fast connections, but many people in the world still have slower internet speeds and far-from-perfect mobile connectivity.
Such things are invisible for people using very fast connections, but many people in the world still have slower internet speeds and use far-from-perfect mobile internet.
Luckily, there are two `<script>` attributes that solve the problem for us: `defer` and `async`
Luckily, there are two `<script>` attributes that solve the problem for us: `defer` and `async`.
## defer
@ -86,7 +86,7 @@ But the specification requires scripts to execute in the document order, so it w
```
```smart header="The `defer` attribute is only for external scripts"
The `defer` attribute is ignored if the script has no `src`.
The `defer` attribute is ignored if the `<script>` tag has no `src`.
```
@ -120,16 +120,17 @@ So, if we have several `async` scripts, they may execute in any order. Whatever
2. `DOMContentLoaded` may happen both before and after `async`, no guarantees here.
3. Async scripts don't wait for each other. A smaller script `small.js` goes second, but probably loads before `long.js`, so runs first. That's called a "load-first" order.
Async scripts are great when we integrate an independent third-party script into the page: counters, ads and so on.
Async scripts are great when we integrate an independent third-party script into the page: counters, ads and so on, as they don't depend on our scripts, and our scripts shouldn't wait for them:
```html
<!-- Google Analytics is usually added like this -->
<script async src="https://google-analytics.com/analytics.js"></script>
```
## Dynamic scripts
We can also create a script dynamically using JavaScript:
We can also add a script dynamically using JavaScript:
```js run
let script = document.createElement('script');
@ -145,7 +146,7 @@ That is:
- They don't wait for anything, nothing waits for them.
- The script that loads first -- runs first ("load-first" order).
We can change the load-first order into the document order by explicitly setting `async` to `false`:
We can change the load-first order into the document order (just like regular scripts) by explicitly setting `async` property to `false`:
```js run
let script = document.createElement('script');
@ -191,7 +192,4 @@ Please note that if you're using `defer`, then the page is visible before the sc
So, buttons should be disabled by CSS or by other means, to let the user
In practice, `defer` is used for scripts that need DOM and/or their relative execution order is important.
So `async` is used for independent scripts, like counters or ads, that don't need to access page content. And their relative execution order does not matter.
In practice, `defer` is used for scripts that need the whole DOM and/or their relative execution order is important. And `async` is used for independent scripts, like counters or ads. And their relative execution order does not matter.

19
2-ui/5-loading/02-script-async-defer/window-onbeforeunload.view/index.html
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@ -1,19 +0,0 @@
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
</head>
<body>
<script>
function setHandler() {
window.onbeforeunload = function() {
return "There are unsaved changes. Leave now?";
};
}
</script>
<button onclick="setHandler()">Set window.onbeforeunload</button>
<a href="http://example.com">Leave for EXAMPLE.COM</a>
</body>
</html>