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I recently came across a very sneaky bug in one of our projects using custom web components at work and I thought I’d document it for future google searches. The structure of the code is this: We have a shared base class called
ReactiveElement which is, as the name implies, a class handling reacting to property and attribute changes, via several different callbacks abstracting some of the functionality of custom web components. The project in questions has a lot of components extending this class, specifying which properties and attributes it wants to have callbacks fired for. This is the very core of several projects and it works well.
We also have a class called
VirtualList that does caching, scrolling and handling large lists of items with some optimizations. We both use this VirtualList stand-alone and we have some specific list classes extending
And update in the
ReactiveElement class broke this but only in a very specific way and only for certain use cases, namely the components extending
VirtualList. And the update was in
static get observedAttributes() which the browser calls on a class when it is registered as a custom element via the
customeElementRegistry.define() method, to avoid it running twice for the same class, due to some server rendering functionality. But I digress.
Time for some code:
Expected behaviour is that the class SpecificListElement should have both the property
VirtualListElement when it’s created. But it only ends up with the property
listItems. What gives?
Turns out the fix in
static get observedAttributes() now includes this code
which means that if the method has been called once for a class and its static
this._observedAttributes has been set, it won’t be set again. Remember this is a static method and
this in this context is the class itself, not an instance. This fact, combined with that
VirtualListElement was both used standalone and as superclass, creates trouble. Deep in the
VirtualList code, was this code:
So when that code was run first, the
static get observedAttributes() is called first, that class gets its attributes set first. When
SpecificListElement comes along, the browser will call its observedAttributes method (in reality on the super class
ReactiveElement), but as the
_observedAttributes_ property doesn’t exists on the class itself, it will look up the inheritance chain, find it on
VirtualListElement and then stop. The properties specified in
SpecificListElement will be ignored.
The fix is quite simple. Instead of
to register the element with a anonymous class expression instead of a named class. You can also create any other named class extending the class used both as stand-alone element and superclass if you wish. A simple fix for a very confusing bug.
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Knut Haugen [Knu:t Hæugen] is a Norwegian software developer with a soft spot for dynamic languages, DevOps and anything to with developer testing. Writes mostly typescript by day and create streaming video and audio players. I am an agile methodology geek with bias on Lean and Kanban. I ride mountain bikes and drink proper beer, but not at the same time.
I automate, therefore I am.
You can reach me via electric letter on the address knuthaug ætt gmail