Javascript when script is loaded

JavaScript onload

Summary: in this tutorial, you will learn how to handle the load event that fires on the document, image, and script elements in JavaScript.

The window’s load event

For the window object, the load event is fired when the whole webpage (HTML) has loaded fully, including all dependent resources, including JavaScript files, CSS files, and images.

To handle the load event, you register an event listener using the addEventListener() method:

window.addEventListener('load', (event) => < console.log('The page has fully loaded'); >);Code language: JavaScript (javascript)

Or use the onload property of the window object:

window.onload = (event) => < console.log('The page has fully loaded'); >;Code language: JavaScript (javascript)

If you maintain a legacy system, you may find that the load event handler is registered in of the body element of the HTML document, like this:

html> html> head> title>JS load Event Demo title> head> body onload="console.log('Loaded!')"> body> html>Code language: HTML, XML (xml)

It’s a good practice to use the addEventListener() method to assign the onload event handler whenever possible.

The image’s load event

The load event also fires on images. To handle the load event on images, you use the addEventListener() method of the image elements.

The following example uses the load event handler to determine if an image, which exists in the DOM tree, has been completely loaded:

html> html> head> title>Image load Event Demo title> head> body> img id="logo"> script> let logo = document.querySelector('#logo'); logo.addEventListener('load', (event) => < console.log('Logo has been loaded!'); >); logo.src = "logo.png"; script> body> html> Code language: HTML, XML (xml)

You can assign an onload event handler directly using the onload attribute of the element, like this:

img id="logo" src="logo.png" onload="console.log('Logo loaded!')"> Code language: HTML, XML (xml)

If you create an image element dynamically, you can assign an onload event handler before setting the src property as follows:

window.addEventListener('load' () => < let logo = document.createElement('img'); // assign and onload event handler logo.addEventListener('load', (event) => < console.log('The logo has been loaded'); >); // add logo to the document document.body.appendChild(logo); logo.src = 'logo.png'; >); Code language: JavaScript (javascript)

• First, create an image element after the document has been fully loaded by placing the code inside the event handler of the window’s load event.
• Second, assign the onload event handler to the image.
• Third, add the image to the document.
• Finally, assign an image URL to the src attribute. The image will be downloaded to the element as soon as the src property is set.

The script’s load event

The element also supports the load event slightly different from the standard ways. The script’s load event allows you to check if a JavaScript file has been completely loaded.

Unlike images, the web browser starts downloading JavaScript files only after the src property has been assigned and the element has been added to the document.

The following code loads the app.js file after the page has been completely loaded. It assigns an onload event handler to check if the app.js has been fully loaded.

window.addEventListener('load', checkJSLoaded) function checkJSLoaded( ) < // create the script element let script = document.createElement('script'); // assign an onload event handler script.addEventListener('load', (event) => < console.log('app.js file has been loaded'); >); // load the script file script.src = 'app.js'; document.body.appendChild(script); >Code language: JavaScript (javascript)

Summary

• The load event occurs when the document has been completely loaded, including dependent resources like JavaScript files, CSS files, and images.
• The and elements also support the load event.
• Use the addEventListener() method to register an onload event handler.

Источник

Resource loading: onload and onerror

The browser allows us to track the loading of external resources – scripts, iframes, pictures and so on.

There are two events for it:

Loading a script

Let’s say we need to load a third-party script and call a function that resides there.

We can load it dynamically, like this:

let script = document.createElement('script'); script.src = "my.js"; document.head.append(script);

…But how to run the function that is declared inside that script? We need to wait until the script loads, and only then we can call it.

For our own scripts we could use JavaScript modules here, but they are not widely adopted by third-party libraries.

script.onload

The main helper is the load event. It triggers after the script was loaded and executed.

let script = document.createElement('script'); // can load any script, from any domain script.src = "https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.3.0/lodash.js" document.head.append(script); script.onload = function() < // the script creates a variable "_" alert( _.VERSION ); // shows library version >;

So in onload we can use script variables, run functions etc.

…And what if the loading failed? For instance, there’s no such script (error 404) or the server is down (unavailable).

script.onerror

Errors that occur during the loading of the script can be tracked in an error event.

For instance, let’s request a script that doesn’t exist:

let script = document.createElement('script'); script.src = "https://example.com/404.js"; // no such script document.head.append(script); script.onerror = function() < alert("Error loading " + this.src); // Error loading https://example.com/404.js >;

Please note that we can’t get HTTP error details here. We don’t know if it was an error 404 or 500 or something else. Just that the loading failed.

Events onload / onerror track only the loading itself.

Errors that may occur during script processing and execution are out of scope for these events. That is: if a script loaded successfully, then onload triggers, even if it has programming errors in it. To track script errors, one can use window.onerror global handler.

Other resources

The load and error events also work for other resources, basically for any resource that has an external src .

let img = document.createElement('img'); img.src = "https://js.cx/clipart/train.gif"; // (*) img.onload = function() < alert(`Image loaded, size $x$`); >; img.onerror = function() < alert("Error occurred while loading image"); >;

There are some notes though:

• Most resources start loading when they are added to the document. But is an exception. It starts loading when it gets a src (*) .
• For , the iframe.onload event triggers when the iframe loading finished, both for successful load and in case of an error.

That’s for historical reasons.

Crossorigin policy

There’s a rule: scripts from one site can’t access contents of the other site. So, e.g. a script at https://facebook.com can’t read the user’s mailbox at https://gmail.com .

Or, to be more precise, one origin (domain/port/protocol triplet) can’t access the content from another one. So even if we have a subdomain, or just another port, these are different origins with no access to each other.

This rule also affects resources from other domains.

If we’re using a script from another domain, and there’s an error in it, we can’t get error details.

For example, let’s take a script error.js that consists of a single (bad) function call:

Now load it from the same site where it’s located:

We can see a good error report, like this:

Uncaught ReferenceError: noSuchFunction is not defined https://javascript.info/article/onload-onerror/crossorigin/error.js, 1:1

Now let’s load the same script from another domain:

The report is different, like this:

Details may vary depending on the browser, but the idea is the same: any information about the internals of a script, including error stack traces, is hidden. Exactly because it’s from another domain.

Why do we need error details?

There are many services (and we can build our own) that listen for global errors using window.onerror , save errors and provide an interface to access and analyze them. That’s great, as we can see real errors, triggered by our users. But if a script comes from another origin, then there’s not much information about errors in it, as we’ve just seen.

Similar cross-origin policy (CORS) is enforced for other types of resources as well.

To allow cross-origin access, the tag needs to have the crossorigin attribute, plus the remote server must provide special headers.

There are three levels of cross-origin access:

1. No crossorigin attribute – access prohibited.
2. crossorigin=»anonymous» – access allowed if the server responds with the header Access-Control-Allow-Origin with * or our origin. Browser does not send authorization information and cookies to remote server.
3. crossorigin=»use-credentials» – access allowed if the server sends back the header Access-Control-Allow-Origin with our origin and Access-Control-Allow-Credentials: true . Browser sends authorization information and cookies to remote server.

You can read more about cross-origin access in the chapter Fetch: Cross-Origin Requests. It describes the fetch method for network requests, but the policy is exactly the same.

Such thing as “cookies” is out of our current scope, but you can read about them in the chapter Cookies, document.cookie.

In our case, we didn’t have any crossorigin attribute. So the cross-origin access was prohibited. Let’s add it.

We can choose between «anonymous» (no cookies sent, one server-side header needed) and «use-credentials» (sends cookies too, two server-side headers needed).

If we don’t care about cookies, then «anonymous» is the way to go:

Now, assuming that the server provides an Access-Control-Allow-Origin header, everything’s fine. We have the full error report.

Summary

Images , external styles, scripts and other resources provide load and error events to track their loading:

The only exception is : for historical reasons it always triggers load , for any load completion, even if the page is not found.

The readystatechange event also works for resources, but is rarely used, because load/error events are simpler.

Tasks

Load images with a callback

Normally, images are loaded when they are created. So when we add to the page, the user does not see the picture immediately. The browser needs to load it first.

To show an image immediately, we can create it “in advance”, like this:

let img = document.createElement('img'); img.src = 'my.jpg';

The browser starts loading the image and remembers it in the cache. Later, when the same image appears in the document (no matter how), it shows up immediately.

Create a function preloadImages(sources, callback) that loads all images from the array sources and, when ready, runs callback .

For instance, this will show an alert after the images are loaded:

function loaded() < alert("Images loaded") >preloadImages(["1.jpg", "2.jpg", "3.jpg"], loaded);

In case of an error, the function should still assume the picture “loaded”.

In other words, the callback is executed when all images are either loaded or errored out.

The function is useful, for instance, when we plan to show a gallery with many scrollable images, and want to be sure that all images are loaded.

In the source document you can find links to test images, and also the code to check whether they are loaded or not. It should output 300 .

1. Make img for every source.
2. Add onload/onerror for every image.
3. Increase the counter when either onload or onerror triggers.
4. When the counter value equals to the sources count – we’re done: callback() .

Источник