Add handler term from plenary description

[bmeck]

No description provided.

[rkirsling]

I think this would need to be more specific than just "handler". 🤔 I think the most common use of "handler (function)" is as a synonym for "(event) callback".

[ljharb]

Suggested change

A parameter value that is a JavaScript object with properties looked up lazily. It is used to configure behavior of a function. Methods on the options bag object should be invoked with the handler as the default value for `this`. It is expected that this parameter has specific well known properties in their structure. Handlers are expected to potentially be extended over time.

A parameter value that is a JavaScript object with properties looked up lazily (used in `new Proxy`). It is used to configure behavior of a function. Methods on the options bag object should be invoked with the handler as the default value for `this`. It is expected that this parameter has specific well known properties in their structure. Handlers are expected to potentially be extended over time.

[bmeck]

this is not unique to proxy if we consider DOM. I also dont think we should carve out that it only applies to proxies. future APIs may adopt this pattern

[ljharb]

sure, my suggestion doesn't indicate it's unique to proxy, just that it is used in Proxy, as an example.

Where in the DOM is this pattern used?

[bmeck]

Various places in DOM, easy to see for .addEventListener

// never even returns a function to invoke
window.addEventListener('foo', {
  UNIQUE_NAME: true,
  get handleEvent() { console.log(this); }
});
window.dispatchEvent(new CustomEvent('foo'));

[ljharb]

wow, i have never seen any code pass anything but a function in that position of addEventListener.

[codehag]

Maybe we should add here that this is a specialized kind of options bag?