DOMContentLoaded, load, beforeunload, unload

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原文连接：https://javascript.info/onload-ondomcontentloaded

The lifecycle of an HTML page has three important events:

DOMContentLoaded – the browser fully loaded HTML, and the DOM tree is built, but external resources like pictures <img> and stylesheets may be not yet loaded.
load – not only HTML is loaded, but also all the external resources: images, styles etc.
beforeunload/unload – the user is leaving the page.

Each event may be useful:

DOMContentLoaded event – DOM is ready, so the handler can lookup DOM nodes, initialize the interface.
load event – external resources are loaded, so styles are applied, image sizes are known etc.
beforeunload event – the user is leaving: we can check if the user saved the changes and ask them whether they really want to leave.
unload – the user almost left, but we still can initiate some operations, such as sending out statistics.

Let’s explore the details of these events.

DOMContentLoaded

The DOMContentLoaded event happens on the document object.

We must use addEventListener to catch it:

document.addEventListener("DOMContentLoaded", ready);
// not "document.onDOMContentLoaded = ..."

For instance:


<script>function ready() {alert('DOM is ready');// image is not yet loaded (unless was cached), so the size is 0x0alert(`Image size: ${img.offsetWidth}x${img.offsetHeight}`);}document.addEventListener("DOMContentLoaded", ready);
</script><img id="img" src="https://en.js.cx/clipart/train.gif?speed=1&cache=0">

In the example the DOMContentLoaded handler runs when the document is loaded, so it can see all the elements, including <img> below.

But it doesn’t wait for the image to load. So alert shows zero sizes.

At the first sight DOMContentLoaded event is very simple. The DOM tree is ready – here’s the event. There are few peculiarities though.

DOMContentLoaded and scripts

When the browser processes an HTML-document and comes across a <script> tag, it needs to execute before continuing building the DOM. That’s a precaution, as scripts may want to modify DOM, and even document.write into it, so DOMContentLoaded has to wait.

So DOMContentLoaded definitely happens after such scripts:

<script>document.addEventListener("DOMContentLoaded", () => {alert("DOM ready!");});
</script><script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.3.0/lodash.js"></script><script>alert("Library loaded, inline script executed");
</script>

In the example above, we first see “Library loaded…”, and then “DOM ready!” (all scripts are executed).

Scripts with async, defer or type="module" don’t block DOMContentLoaded

Script attributes async and defer, that we’ll cover a bit later, don’t block DOMContentLoaded. JavaScript modules behave like defer, they don’t block it too.

So here we’re talking about “regular” scripts, like <script>...</script>, or <script src="..."></script>.

DOMContentLoaded and styles

External style sheets don’t affect DOM, so DOMContentLoaded does not wait for them.

But there’s a pitfall. If we have a script after the style, then that script must wait until the stylesheet loads:

<link type="text/css" rel="stylesheet" href="style.css">
<script>// the script doesn't not execute until the stylesheet is loadedalert(getComputedStyle(document.body).marginTop);
</script>

The reason is that the script may want to get coordinates and other style-dependent properties of elements, like in the example above. Naturally, it has to wait for styles to load.

As DOMContentLoaded waits for scripts, it now waits for styles before them as well.

Built-in browser autofill

Firefox, Chrome and Opera autofill forms on DOMContentLoaded.

For instance, if the page has a form with login and password, and the browser remembered the values, then on DOMContentLoaded it may try to autofill them (if approved by the user).

So if DOMContentLoaded is postponed by long-loading scripts, then autofill also awaits. You probably saw that on some sites (if you use browser autofill) – the login/password fields don’t get autofilled immediately, but there’s a delay till the page fully loads. That’s actually the delay until the DOMContentLoaded event.

window.onload

The load event on the window object triggers when the whole page is loaded including styles, images and other resources.

The example below correctly shows image sizes, because window.onload waits for all images:

<script>window.onload = function() {alert('Page loaded');// image is loaded at this timealert(`Image size: ${img.offsetWidth}x${img.offsetHeight}`);};
</script><img id="img" src="https://en.js.cx/clipart/train.gif?speed=1&cache=0">

window.onunload

When a visitor leaves the page, the unload event triggers on window. We can do something there that doesn’t involve a delay, like closing related popup windows.

The notable exception is sending analytics.

Let’s say we gather data about how the page is used: mouse clicks, scrolls, viewed page areas, and so on.

Naturally, unload event is when the user leaves us, and we’d like to save the data on our server.

There exists a special navigator.sendBeacon(url, data) method for such needs, described in the specification https://w3c.github.io/beacon/.

It sends the data in background. The transition to another page is not delayed: the browser leaves the page, but still performs sendBeacon.

Here’s how to use it:

let analyticsData = { /* object with gathered data */ };window.addEventListener("unload", function() {navigator.sendBeacon("/analytics", JSON.stringify(analyticsData));
};

The request is sent as POST.
We can send not only a string, but also forms and other formats, as described in the chapter Fetch, but usually it’s a stringified object.
The data is limited by 64kb.

When the sendBeacon request is finished, the browser probably has already left the document, so there’s no way to get server response (which is usually empty for analytics).

There’s also a keepalive flag for doing such “after-page-left” requests in fetch method for generic network requests. You can find more information in the chapter Fetch API.

If we want to cancel the transition to another page, we can’t do it here. But we can use another event – onbeforeunload.

window.onbeforeunload

If a visitor initiated navigation away from the page or tries to close the window, the beforeunload handler asks for additional confirmation.

If we cancel the event, the browser may ask the visitor if they are sure.

You can try it by running this code and then reloading the page:

window.onbeforeunload = function() {return false;
};

For historical reasons, returning a non-empty string also counts as canceling the event. Some time ago browsers used show it as a message, but as the modern specification says, they shouldn’t.

Here’s an example:

window.onbeforeunload = function() {return "There are unsaved changes. Leave now?";
};

The behavior was changed, because some webmasters abused this event handler by showing misleading and annoying messages. So right now old browsers still may show it as a message, but aside of that – there’s no way to customize the message shown to the user.

readyState

What happens if we set the DOMContentLoaded handler after the document is loaded?

Naturally, it never runs.

There are cases when we are not sure whether the document is ready or not. We’d like our function to execute when the DOM is loaded, be it now or later.

The document.readyState property tells us about the current loading state.

There are 3 possible values:

"loading" – the document is loading.
"interactive" – the document was fully read.
"complete" – the document was fully read and all resources (like images) are loaded too.

So we can check document.readyState and setup a handler or execute the code immediately if it’s ready.

Like this:

function work() { /*...*/ }if (document.readyState == 'loading') {// loading yet, wait for the eventdocument.addEventListener('DOMContentLoaded', work);
} else {// DOM is ready!work();
}

There’s also readystatechange event that triggers when the state changes, so we can print all these states like this:

// current state
console.log(document.readyState);// print state changes
document.addEventListener('readystatechange', () => console.log(document.readyState));

The readystatechange event is an alternative mechanics of tracking the document loading state, it appeared long ago. Nowadays, it is rarely used.

Let’s see the full events flow for the completeness.

Here’s a document with <iframe>, <img> and handlers that log events:

<script>log('initial readyState:' + document.readyState);document.addEventListener('readystatechange', () => log('readyState:' + document.readyState));document.addEventListener('DOMContentLoaded', () => log('DOMContentLoaded'));window.onload = () => log('window onload');
</script><iframe src="iframe.html" onload="log('iframe onload')"></iframe><img src="http://en.js.cx/clipart/train.gif" id="img">
<script>img.onload = () => log('img onload');
</script>

The working example is in the sandbox.

The typical output:

[1] initial readyState:loading
[2] readyState:interactive
[2] DOMContentLoaded
[3] iframe onload
[4] img onload
[4] readyState:complete
[4] window onload

The numbers in square brackets denote the approximate time of when it happens. Events labeled with the same digit happen approximately at the same time (± a few ms).

document.readyState becomes interactive right before DOMContentLoaded. These two things actually mean the same.
document.readyState becomes complete when all resources (iframe and img) are loaded. Here we can see that it happens in about the same time as img.onload (img is the last resource) and window.onload. Switching to complete state means the same as window.onload. The difference is that window.onloadalways works after all other load handlers.

Summary

Page load events:

DOMContentLoaded event triggers on document when DOM is ready. We can apply JavaScript to elements at this stage.
- Script such as <script>...</script> or <script src="..."></script> block DOMContentLoaded, the browser waits for them to execute.
- Images and other resources may also still continue loading.
load event on window triggers when the page and all resources are loaded. We rarely use it, because there’s usually no need to wait for so long.
beforeunload event on window triggers when the user wants to leave the page. If we cancel the event, browser asks whether the user really wants to leave (e.g we have unsaved changes).
unload event on window triggers when the user is finally leaving, in the handler we can only do simple things that do not involve delays or asking a user. Because of that limitation, it’s rarely used. We can send out a network request with navigator.sendBeacon.
document.readyState is the current state of the document, changes can be tracked in the readystatechange event:
- loading – the document is loading.
- interactive – the document is parsed, happens at about the same time as DOMContentLoaded, but before it.
- complete – the document and resources are loaded, happens at about the same time as window.onload, but before it.