MutationObserver attribute change record only gives us the oldValue, but not the newValue?

[trusktr]

I only see an oldValue when trying to observe attribute changes. Are we expected to access the attribute values on the element, inside the observer's callback?

For example, here's mine:

        const observer = new MutationObserver( records => {

            for (const record of records) {
                behavior.attributeChangedCallback(
                    this.ownerElement,
                    record.attributeName,
                    record.oldValue,
                    this.ownerElement.getAttribute( record.attributeName )
                )
            }

        } )

where in my case behavior is part of an implementation of element behaviors (the has="" attribute), implemented on to of custom-attributes, but I haven't published it yet.

The behavior.attributeChangedCallback signature is (element, attributeName, oldValue, newValue), so as you see I'm getting the new value from the element directly.

But this seems dirty. I might be calling some Custom Element's overriden or extended getAttribute method and causing unknown side-effects.

How does the MutationObserver implementation get the element's oldValue? It seems that the new value should be gotten the same way as the engine gets the old value (if not already) in order to be consistent.

I don't know exactly what "the same way" means though. Does the engine get the original value passed into HTMLElement.prototype.setAttribute (the native one, even is monkey patched)? Or what does it do?

Should I be looping on element.attributes instead, and using that as the source of truth, rather than getAttribute?

I'm wondering about what MutationObserver does for oldValue, but in general, considering that nothing will change for a while, I'm looking for advice on what's best to do. Thanks!

[annevk]

If you cannot trust getAttribute() you cannot trust MutationObserver either. The idea is that you'd use getAttribute().

[annevk]

Also, for usage questions, I encourage you to reach out to the stackoverflow.com community. This repository is for issues with the standard itself.