Add means to detect if a script is loading, has loaded, or has errored

[dead-claudia]

Something like a [SameValue] attribute Promise<void> loaded; on HTMLScriptElement would be nice, where it's resolved immediately with the load event itself when the script completes execution and rejected immediately with the error event itself if/when an error occurs.

Currently, there is no way to tell if a script is loaded or not, and that would be very useful in a lot of contexts, particularly when the script is in the HTML itself and might not yet be loaded.

This is possible to polyfill, but only for scripts you can control the loading of. Conceptually, it's as simple as this:

script.loaded = new Promise((resolve, reject) => {
	script.addEventListener("load", resolve, false)
	script.addEventListener("error", reject, false)
})

[rniwa]

Maybe we want script.readyState. A bunch of objects like XHR and FileReader have it already.

[domenic]

I'd love to do "loaded" or "ready" promises for elements as a more ergonomic, modern API than the onload/onerror/readyState trio. See also #127 / #1936.

[dead-claudia]

Amended my proposal to be script.loaded rather than script.ready. It's a little more self-descriptive.