> For the complete documentation index, see [llms.txt](https://lochiwei.gitbook.io/web/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://lochiwei.gitbook.io/web/browser/event/default-action/hide-or-not-to-hide.md).

# hide or not to hide?

[browser](/web/browser.md) ⟩ [event](/web/browser/event.md) ⟩ [default action](/web/browser/event/default-action.md) ⟩ example ⟩ hide or not to hide? 

{% hint style="info" %}

```javascript
function
```

{% endhint %}

{% tabs %}
{% tab title="💈範例" %}

* :floppy\_disk: [replit](https://replit.com/@pegasusroe/epreventDefault#script.js)

```javascript
const {log} = console;

// ⭐️ import
import {$} from './js/Node+ext.js';
// -------------------------------------------

// elements
const rabbit = $('#rabbit');
const button = $('button');

// scenario A
// -------------------------------------------------------------
// 1. click the button
//
// 2. confirm dialog                          ("rabbit.onhide")
//    (confirm dialog shows BEFORE the log message, why❓)
//
// 3. click "OK"
//
// 4. dispatching "hide" event on rabbit ...  ("button.onclick")
//    (this message shows AFTER the confirm dialog, why❓)
//
// 5. default action prevented ...            ("rabbit.onhide")
// 6. "hide" event on rabbit dispatched ...   ("button.onclick")
// 7. "hiding rabbit" cancelled ...           ("button.onclick")
//
// scenario A
// -------------------------------------------------------------
// 1. click the button
// 2. confirm dialog                          ("rabbit.onhide")
// 3. click "Cancel"
// 4. dispatching "hide" event on rabbit ...  ("button.onclick")
// 5. "hide" event on rabbit dispatched ...   ("button.onclick")

// button.onclick
button.onclick = () => {
    
    // ⭐️ cancelable custom "hide" event
    let hideEvent = new CustomEvent("hide", {
      cancelable: true  // ⭐️ preventDefault doesn't work without this
    });

    // start to dispatch hide event
    log(`dispatching "hide" event on rabbit ...`);

    // ⭐️ dispatch 'hide' on rabbit and see if the event handler
    // decides to do the default actions or not.
    let doDefaults = rabbit.dispatchEvent(hideEvent);
    
    log(`"hide" event on rabbit dispatched ...`);

    if (!doDefaults) {
      log('"hiding rabbit" cancelled ...');
      return;
    }
      
    // hide the rabbit
    rabbit.hidden = true;
};

// rabbit.onhide
rabbit.addEventListener('hide', (event) => {
    if (confirm("Cancel hidding?")) {

      // By calling event.preventDefault(), an event handler 
      // may send a signal to cancel default actions.

      // ⭐️ In this case, elem.dispatchEvent(event) returns false❗️
      //    "Don't do default actions."
      event.preventDefault();

      log('default action prevented ...');
    }
});
```

{% endtab %}

{% tab title="⭐️ 重點" %}
{% hint style="warning" %} <mark style="color:yellow;">**return value**</mark> of [EventTarget](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget) ⟩ [.dispatchEvent(event)](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/dispatchEvent)

* <mark style="color:red;">**false**</mark>&#x20;
  * if <mark style="color:blue;">`event`</mark> is <mark style="color:yellow;">**cancelable**</mark>, and&#x20;
  * <mark style="color:red;">**at least**</mark>**&#x20;**<mark style="color:yellow;">**one**</mark> of the event's handlers called [.preventDefault()](https://developer.mozilla.org/en-US/docs/Web/API/Event/preventDefault).&#x20;
* <mark style="color:green;">**true**</mark>, otherwise.
  {% endhint %}
  {% endtab %}

{% tab title="👥 相關" %}

* [event dispatching](/web/browser/event/dispatch.md)
* [capturing phase](/web/browser/event/propagation/capturing.md)
* the [return value](/web/browser/event/handler/return-value.md) of an [event handler](/web/browser/event/handler.md) is related to <mark style="color:yellow;">**canceling the default actions**</mark>.
  {% endtab %}

{% tab title="📗 參考" %}

* [ ] JS.info ⟩ [event.preventDefault()](https://javascript.info/dispatch-events#event-preventdefault)
* [ ] :green\_book: [Full Stack open](https://fullstackopen.com/en/part0/fundamentals_of_web_apps#single-page-app)
  {% endtab %}

{% tab title="📘 手冊" %}

* [EventTarget](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget) ⟩ [.dispatchEvent()](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/dispatchEvent)
* [Event](/web/browser/event.md) ⟩ [.preventDefault()](https://developer.mozilla.org/en-US/docs/Web/API/Event/preventDefault)
  {% endtab %}
  {% endtabs %}


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