"while a JavaScript event loop is running." is probably not intended

[jakearchibald]

https://w3ctag.github.io/design-principles/#js-rtc

Don’t modify data accessed via JavaScript APIs while a JavaScript event loop is running.

An event loop is running from the creation of an agent, and it doesn't stop until the agent is destroyed. By this definition, data can never really update.

This should probably be reworded in terms of running scripts and callback invoking.

It might be worth calling out that new Date(), Date.now(), performance.now() are accepted breakages of this rule.

In fact, it's interesting to compare performance.now() to document.timeline.currentTime, as the former breaks run-to-completion, whereas the latter is stable for the whole frame (beyond a single callback).

[torgo]

Hi Jake - thanks for sending us this! We discussed today and absolutely agree this needs to be fixed. Would it be possible for you to suggest a more concrete re-wording that we could consider?

[jakearchibald]

Data can update synchronously from the result of developer action. Eg, node.remove() changes the DOM synchronously and is immediately observable.

However, things that are not the result of developer action, or are asynchronously delivered, should not happen in the middle of other JavaScript.

Meaning, during a while (true), you shouldn't expect things like:

• The DOM to update as a result of the HTML parser loading network content
• img.width to change as a result of loading image data from the network
• Buttons of a gamepad to change state
• scrollTop to change, even if scrolling can visually occur

These things aren't updated by the currently running script, so they shouldn't change during the currently running script. Instead, a task should be queued, or they should update as part of the render steps.

There are deliberate violations of this rule, such as Date.now() and performance.now(), which can observe changes in time, and things like the proposed isInputPending(), which read from task queues.

[torgo]

We looked at this again today and we're going to work on making a PR in the coming weeks that adapt's the text Jake provided above.