did-upsert

Modifier that is called when a DOM node is inserted and/or an argument is updated.

Additionally the callback function provided should expect an event shaped object rather than a reference to a DOM node itself, therefore the DOM node can be obtained by referencing target or currentTarget instead. This means that you can use the same callback function for both {{on 'click'}} modifiers and {{did-upsert}} modifiers. The one dowbside is that the signature of functions used for did-upsert are slightly different than when you use did-insert/did-update, but moving forwards we are more likely to make did-insert/did-update use a more standard based signature (or some other approach to make things consistent)

<div
  {{did-upsert this.callback @arrayThatMightGetAddedToTODetectAnUpdate.length}}
>
</div>

  callback(e) {
    console.log(e.currentTarget);
    console.log(e.target);
  }

Other than the above exception, the modifer works the same as the did-insert/did-update modifiers.