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

There are two events for it:

• onload – successful load,
• onerror – an error occurred.

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";



…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.

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

For instance:

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"

// 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).

### 1.2. 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

script.onerror = function() {
};


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.

Important:

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.

## 2. Other resources

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

For example:

let img = document.createElement('img');
img.src = "https://js.cx/clipart/train.gif"; // (*)

alert(Image loaded, size ${img.width}x${img.height});
};

img.onerror = function() {
};


There are some notes though:

• Most resources start loading when they are added to the document. But <img> is an exception. It starts loading when it gets a src (*).
• For <iframe>, 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.

## 3. 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:

// 📁 error.js
noSuchFunction();


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

<script>
window.onerror = function(message, url, line, col, errorObj) {
alert(${message}\n${url}, ${line}:${col});
};
</script>


We can see a good error report, like this:

Uncaught ReferenceError: noSuchFunction is not defined


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

<script>
window.onerror = function(message, url, line, col, errorObj) {
alert(${message}\n${url}, ${line}:${col});
};
</script>


The report is different, like this:

Script error.
, 0:0


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 <script> 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.

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:

<script>
window.onerror = function(message, url, line, col, errorObj) {
alert(${message}\n${url}, ${line}:${col});
};
</script>


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

## 4. Summary

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

• load triggers on a successful load,
• error triggers on a failed load.

The only exception is <iframe>: 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.