Best practices and rules for custom element constructors

What's safe to do in the constructor

In the constructor, it's safe to

• create the shadow root;
• create and append elements *;
• attach event listeners to those elements (scoped to your own ShadowDOM);
• try to read attributes, just be aware that these can only be present in the upgrade case (declarative usage)

What you cannot do in the constructor

In the constructor, you are not allowed (amongst other things)

• to create attributes...
• to access child elements (do not confuse these with elements in your shadowRoot!)...

...because those might not be present in the non-upgrade case, and definitely won't be present when you dynamically create your custom element using either document.createElement('my-custom-element') or new MyCustomElement().

What's unwise to do in the constructor

In the constructor, you probably don't want to

• attach event listeners to elements outside of the component's shadow DOM (like e.g. document, window), because these are the kind of listeners you should clean up in your component's disconnectedCallback (which will be also be called when e.g. your component is moved in the DOM, so after removing them in the disconnectedCallback they need to be re-added when re-connecting).

Attaching these listeners in the constructor and properly cleaning them up in the disconnectedCallback results in missing listeners once your component gets removed from (and later re-added), or moved in, the DOM.

*Pitfalls and things to be aware of

You need to be aware of the custom element lifecycle to not fall into otherwise obvious pitfalls, which include:

• If you add attributes in the constructor and have included those in your component's observedAttributes, remember this will immediately trigger the attributeChangedCallback for those attributes, even if your element is not yet connected (a.k.a. in the DOM).
• If you create and append other custom elements into your component's shadow DOM, remember this will trigger those components' connectedCallback.

In part, these best practices and rules follow https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-conformance, in other parts they deviate from recommendations done there in the spec.

Specifically I disagree on the following (given the scope for the listeners is outside the component), for the reasons I gave above.

In general, the constructor should be used to set up initial state and default values, and to set up event listeners and possibly a shadow root.