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Our experience upgrading web components from Shadow DOM/Custom Elements v0 to v1

With Google now shipping both Shadow DOM v1 and Custom Elements v1 in Chrome, and Apple shipping Shadow DOM v1 in Safari, we’ve been upgrading the Basic Web Components library from the original v0 specs to v1. Here’s what we learned, in case you’re facing a similar upgrade of your own components, or just want to understand some ramifications of the v1 changes.

Upgrading components to Shadow DOM v1: Easy!

Google developer Hayato Ito has a great summary of What’s New in Shadow DOM v1. Adapting our components to accommodate most of the changes on that list was trivial, often just a matter of Find and Replace. The v0 features that were dropped were ones we had never used (multiple shadow roots, shadow-piercing CSS combinators) or had avoided (<content select=”...”>), so their absence in v1 did not present a problem.

One v1 feature that we had heavily lobbied for was the addition of the slotchange event. The ability of an element to detect changes in its own distributed content is a critical addition to the spec. We are happy to replace our old, hacky method of detecting content changes with the new, official slotchange event. This allows us to easily write components that meet the Content Changes requirement on the Gold Standard checklist for web components.

Upgrading components to Custom Elements v1: Some challenges

The changes from Custom Elements v0 to v1 were more challenging, although some were easy:

One small obstacle we hit is that a v1 component now needs to declare which attributes it wants to monitor for changes. This performance optimization in Custom Elements v1 requires that your component declare an observedAttributes array to avoid getting attributeChangedCallback invocations for attributes you don’t care about. That sounds simple, but in our mixin-based approach to writing components, it was actually a bit of a pain. Each mixin had to not only declare the attributes it cared about, but it had to cooperatively construct the final observedAttributes array. We eventually hit on the idea of having the aforementioned AttributeMarshalling mixin programmatically inspect the component class for all custom properties, and automatically generate an appropriate array of attributes for observedAttributes. That seems to be working fine.

A more problematic change in v1 is that component initialization is now done in a class constructor instead of a createdCallback. The change itself is a desirable one, but we expected it would be tricky, and it was. The biggest problem we’ve encountered is that the list of Requirements for custom element constructors prohibits a new component from setting attributes in its constructor. The intention, as we understand it, is to mirror standard element behavior. Calling createElement('div') returns a clean div with no attributes, so calling createElement('my-custom-element') should return a clean element too, right?

That sounds good but turns out to be limiting. Custom elements can’t do everything that native elements can, and sometimes the only way to achieve a desired result is for a custom element to add an attibute to itself:

  1. A component wants to define default ARIA attributes for accessibility purposes. For example, our ListBox component needs to add role=”listbox” to itself. That helps a screen reader interpret the component correctly, without the person using the component having to know about or understand ARIA. That role attribute is a critical part of a ListBox element, and needs to be there by default.
  2. A component wants to reflect its state as CSS classes so that component users can provide state-dependent styling. For example, our CollapsiblePanel component wants to let designers style its open and closed appearances by adding CSS classes that reflect the open/closed state. This component reflects the current state of its closed property via CSS classes. It’s reasonable that a component would want to set the initial state of that closed property in a constructor. But setting that default value of that property in the constructor will trigger the update to the CSS class, which is not permitted in Custom Elements v1.

In these cases, it doesn’t seem like it would be hard to just set the attributes in the connectedCallback instead. In practice, it introduces complications because a web app author that instantiates a component would like to be able to immediately make changes to it before adding it to the document. In the first scenario above, the author might want to adjust the role attribute:

class ListBox extends HTMLElement {
  connectedCallback() {
    this.setAttribute('role', 'listbox');

let listBox = document.createElement('basic-list-box');
listBox.setAttribute('role', 'tabs'); // Set custom role
document.body.appendChild(listBox); // connectedCallback will overwrite role!

Because ListBox can’t apply a default role attribute at constructor time, its connectedCallback will have to take care to see if a role has already been set on the component before applying a default value of role=”listbox”. It’s easy for a developer to forget such a check. The result will likely be components that belatedly apply default attributes, stomping on top of attributes that were applied after the constructor and before the component is added to the document.

Another problem comes up in the second scenario above. The creator of the component would like to be able to write a property getter/setter that reflects its state as CSS classes:

let closedSymbol = Symbol('closed');

class CollapsiblePanel extends HTMLElement {

  constructor() {
    // Set defaults
    this.closed = true; // Sets the “class” attribute, so will throw!

  get closed() {
    return this[closedSymbol];
  set closed(value) {
    this[closedSymbol] = value;
    this.toggleClass('closed', value);
    this.toggleClass('opened', !value);


Since the above code won’t work, the developer has to take care to defer all attribute writes (including manipulations of the classList, which updates the class attribute) to the connectedCallback. To make that tolerable, we ended up creating safeAttributes, a set of helper functions that can defer premature calls to setAttribute() and toggleClass() to the connectedCallback.

That’s working for now, but it feels like the v1 restrictions on the constructor are overly limiting. The intention is to ensure that the component user gets a clean element from createElement() — but if the resulting element is just going to add attributes to itself in the connectedCallback, is that element really clean? As soon as the attribute-less element is added to the document, it will suddenly grow new attributes. In our opinion, that feels even more surprising than having createElement() return an element with default attributes.

The current state of Shadow DOM and Custom Elements v1

Overall, we’re excited that we’ve got our components and mixins working in production Chrome 54, which just shipped last week with support for both Shadow DOM v1 and Custom Elements v1. The Chrome implementation of the specs feels solid, and we haven’t hit any bugs.

Shadow DOM v1 is also coming together in Safari, including in Mobile Safari. At the moment, it feels more like a beta than a production feature — we’ve hit a number of critical bugs in WebKit that prevent most of our components from working. Apple’s working through those bugs, and we hope to see WebKit’s support for Shadow DOM improve soon.

In the meantime, Google has been doing the thankless, herculean task of upgrading the Shadow DOM and Custom Elements polyfills to the v1 specs. That’s great to see, because without an answer for older browsers, web components won’t see wide adoption. At the moment, the v1 polyfills also feel like a beta, but they’re coming along quickly. As soon as the polyfills are stable enough, we’re looking forward to making a full release of Basic Web Components based on the v1 specs.