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Showing content from https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/beforetoggle_event below:

HTMLElement: beforetoggle event - Web APIs

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The beforetoggle event of the HTMLElement interface fires on a popover or <dialog> element just before it is shown or hidden.

• If the element is transitioning from hidden to showing, the event.oldState property will be set to closed and the event.newState property will be set to open.
• If the element is transitioning from showing to hidden, then event.oldState will be open and event.newState will be closed.

This event is cancelable when an element is toggled to open ("show") but not when the element is closing.

Among other things, this event can be used to:

• prevent an element from being shown.
• add or remove classes or properties from the element or associated elements, for example to control the animation behavior of a dialog as it is opened and closed.
• clear the state of the element before it is opened or after it is hidden, for example to reset a dialog form and return value to an empty state, or hide any nested manual popovers when reopening a popup.

Use the event name in methods like addEventListener(), or set an event handler property.

addEventListener("beforetoggle", (event) => { })

onbeforetoggle = (event) => { }

The examples below demonstrates how the beforetoggle event might be used for a popover or <dialog> element. The same examples would work similarly on the other element types.

This example shows how to listen for the beforetoggle event and log the result.

The HTML consists of a popover and a button for toggling it open and closed.

<button popovertarget="mypopover">Toggle the popover</button>
<div id="mypopover" popover>Popover content</div>

#log {
  height: 150px;
  overflow: scroll;
  padding: 0.5rem;
  border: 1px solid black;
}

const logElement = document.querySelector("#log");
function log(text) {
  logElement.innerText = `${logElement.innerText}${text}\n`;
  logElement.scrollTop = logElement.scrollHeight;
}

The code gets adds an event listener for the beforetoggle event and logs the state.

const popover = document.getElementById("mypopover");

popover.addEventListener("beforetoggle", (event) => {
  if (event.newState === "open") {
    log("Popover is about to be shown");
  } else {
    log("Popover is about to be hidden");
  }
});

The beforetoggle event is cancelable if fired when opening an element.

Below we show how a popover that might first check if it is allowed to open, and if not, call Event.preventDefault() to cancel the event. In this example we use a button to set whether the popover can open or not: in a more "full featured" example this might depend on the application state, or the data in the popover being ready to display.

The HTML consists of a popover, a button for toggling it open and closed, and a button for setting whether the button can be opened.

<button popovertarget="mypopover">Toggle the popover</button>
<button id="allow_button"></button>
<div id="mypopover" popover>Popover content</div>

#log {
  height: 150px;
  overflow: scroll;
  padding: 0.5rem;
  border: 1px solid black;
}

const logElement = document.querySelector("#log");
function log(text) {
  logElement.innerText = `${logElement.innerText}${text}\n`;
  logElement.scrollTop = logElement.scrollHeight;
}

First we set up the code to simulate a state where we don't want to allow the popover to open. This is represented by the variable allowOpen, which is toggled when the associated button is clicked.

const allowButton = document.getElementById("allow_button");

let allowOpen = true;

function toggleState() {
  allowOpen = !allowOpen;
  allowButton.innerText = allowOpen ? "Open Allowed" : "Open Prevented";
}

toggleState();

allowButton.addEventListener("click", (event) => {
  toggleState();
});

The code gets adds an event listener for the beforetoggle event. If allowOpen is false then preventDefault() is called, which stops the popup from opening.

const popover = document.getElementById("mypopover");

popover.addEventListener("beforetoggle", (event) => {
  if (event.newState === "open") {
    if (allowOpen) {
      log("Popover is about to be shown");
    } else {
      log("Popover opening prevented");
      event.preventDefault();
    }
  } else {
    log("Popover is about to be hidden");
  }
});

If multiple beforetoggle events are fired before the event loop has a chance to cycle, only a single event will be fired. This is referred to as "event coalescing".

For example:

popover.addEventListener("beforetoggle", () => {
  // …
});

popover.showPopover();
popover.hidePopover();
// `beforetoggle` only fires once

• Opening a modal dialog example in HTMLDialogElement

• popover HTML global attribute
• Popover API
• Related event: toggle

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