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Add handler term from plenary description #87

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Add handler term from plenary description
bmeck Jul 22, 2020
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Update terminology.md
bmeck Jul 22, 2020
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Expand Up @@ -313,6 +313,27 @@ To make something a first-class concept in a language specification, such that i
#### References
- [Reification](https://en.wikipedia.org/wiki/Reification_(computer_science))

### Handler
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#### Definition

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.
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@ljharb ljharb Jul 22, 2020

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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.

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@bmeck bmeck Jul 22, 2020

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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

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@ljharb ljharb Jul 22, 2020

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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?

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@bmeck bmeck Jul 23, 2020

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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'));

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@ljharb ljharb Jul 23, 2020

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

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@codehag codehag Jan 4, 2022

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


The definition here is a bit fuzzy as some parameters are objects but not options bags.

#### Example

```js
const handler = {
getCount: 0,
get(target, key, receiver) {
const count = this.getCount++;
return count;
}
};
new Proxy(obj, handler);
```

TEMPLATE
### [NAME HERE]

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