Scripts: async, defer - Browser: Document, Events, Interfaces - Document and resource loading

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6 Apr 2024

Scripts: async, defer

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 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:

Scripts can’t see DOM elements below them, so they can’t add handlers etc.
If there’s a bulky script at the top of the page, it “blocks the page”. Users can’t see the page content till it downloads and runs:

<p>...content before script...</p>

<script src="https://javascript.info/article/script-async-defer/long.js?speed=1"></script>

<!-- This isn't visible until the script loads -->
<p>...content after script...</p>

There are some workarounds to that. For instance, we can put a script at the bottom of the page. Then it can see elements above it, and it doesn’t block the page content from showing:

<body>
  ...all content is above the script...

  <script src="https://javascript.info/article/script-async-defer/long.js?speed=1"></script>
</body>

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 slow internet speeds and use a far-from-perfect mobile internet connection.
Luckily, there are two <script> attributes that solve the problem for us: defer and async.

defer

The defer attribute tells the browser not to wait for the script. Instead, the browser will continue to process the HTML, build DOM. The script loads “in the background”, and then runs when the DOM is fully built.

Here’s the same example as above, but with defer:

<p>...content before script...</p>

<script defer src="https://javascript.info/article/script-async-defer/long.js?speed=1"></script>

<!-- visible immediately -->
<p>...content after script...</p>

In other words:

Scripts with defer never block the page.
Scripts with defer always execute when the DOM is ready (but before DOMContentLoaded event).

The following example demonstrates the second part:

<p>...content before scripts...</p>

<script>
  document.addEventListener('DOMContentLoaded', () => alert("DOM ready after defer!"));
</script>

<script defer src="https://javascript.info/article/script-async-defer/long.js?speed=1"></script>

<p>...content after scripts...</p>

The page content shows up immediately.
DOMContentLoaded event handler waits for the deferred script. It only triggers when the script is downloaded and executed.

Deferred scripts keep their relative order, just like regular scripts.

Let’s say, we have two deferred scripts: the long.js and then small.js:

<script defer src="https://javascript.info/article/script-async-defer/long.js"></script>
<script defer src="https://javascript.info/article/script-async-defer/small.js"></script>

Browsers scan the page for scripts and download them in parallel, to improve performance. So in the example above both scripts download in parallel. The small.js probably finishes first.

…But the defer attribute, besides telling the browser “not to block”, ensures that the relative order is kept. So even though small.js loads first, it still waits and runs after long.js executes.
That may be important for cases when we need to load a JavaScript library and then a script that depends on it.

The defer attribute is only for external scripts

The defer attribute is ignored if the <script> tag has no src.

async

The async attribute is somewhat like defer. It also makes the script non-blocking. But it has important differences in the behavior.

The async attribute means that a script is completely independent:

The browser doesn’t block on async scripts (like defer).
Other scripts don’t wait for async scripts, and async scripts don’t wait for them.
DOMContentLoaded and async scripts don’t wait for each other:
- DOMContentLoaded may happen both before an async script (if an async script finishes loading after the page is complete)
- …or after an async script (if an async script is short or was in HTTP-cache)

In other words, async scripts load in the background and run when ready. The DOM and other scripts don’t wait for them, and they don’t wait for anything. A fully independent script that runs when loaded. As simple, as it can get, right?
Here’s an example similar to what we’ve seen with defer: two scripts long.js and small.js, but now with async instead of defer.

They don’t wait for each other. Whatever loads first (probably small.js) – runs first:

<p>...content before scripts...</p>

<script>
  document.addEventListener('DOMContentLoaded', () => alert("DOM ready!"));
</script>

<script async src="https://javascript.info/article/script-async-defer/long.js"></script>
<script async src="https://javascript.info/article/script-async-defer/small.js"></script>

<p>...content after scripts...</p>

The page content shows up immediately: async doesn’t block it.
DOMContentLoaded may happen both before and after async, no guarantees here.
A smaller script small.js goes second, but probably loads before long.js, so small.js runs first. Although, it might be that long.js loads first, if cached, then it runs first. In other words, async scripts run in the “load-first” order.

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:

<!-- Google Analytics is usually added like this -->
<script async src="https://google-analytics.com/analytics.js"></script>

The async attribute is only for external scripts

Just like defer, the async attribute is ignored if the <script> tag has no src.

Dynamic scripts

There’s one more important way of adding a script to the page.

We can create a script and append it to the document dynamically using JavaScript:

let script = document.createElement('script');
script.src = "/article/script-async-defer/long.js";
document.body.append(script); // (*)

The script starts loading as soon as it’s appended to the document (*).

Dynamic scripts behave as “async” by default.

That is:

They don’t wait for anything, nothing waits for them.
The script that loads first – runs first (“load-first” order).

This can be changed if we explicitly set script.async=false. Then scripts will be executed in the document order, just like defer.
In this example, loadScript(src) function adds a script and also sets async to false.

So long.js always runs first (as it’s added first):

function loadScript(src) {let script = document.createElement('script');
  script.src = src;
  script.async = false;
  document.body.append(script);
}

// long.js runs first because of async=false
loadScript("/article/script-async-defer/long.js");
loadScript("/article/script-async-defer/small.js");

Without script.async=false, scripts would execute in default, load-first order (the small.js probably first).
Again, as with the defer, the order matters if we’d like to load a library and then another script that depends on it.

Summary

Both async and defer have one common thing: downloading of such scripts doesn’t block page rendering. So the user can read page content and get acquainted with the page immediately.

But there are also essential differences between them:
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.

Page without scripts should be usable

Please note: if you’re using defer or async, then user will see the page before the script loads.
In such case, some graphical components are probably not initialized yet.
Don’t forget to put “loading” indication and disable buttons that aren’t functional yet. Let the user clearly see what he can do on the page, and what’s still getting ready.

Original Content at: https://javascript.info/script-async-defer

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