dragstart、dragover和drop是實(shí)現(xiàn)拖放交互的關(guān)鍵事件。dragstart在用戶開始拖動元素時觸發(fā)，用于設(shè)置拖動數(shù)據(jù)及視覺反饋；dragover在拖動經(jīng)過目標(biāo)區(qū)域時持續(xù)觸發(fā)，必須調(diào)用e.preventDefault()才能允許放置，并可用于添加高亮提示；drop在釋放鼠標(biāo)鍵時觸發(fā)，用于獲取數(shù)據(jù)并執(zhí)行相應(yīng)邏輯，同時需移除臨時樣式。此外，要確保元素可拖動，必須設(shè)置draggable屬性為true。

When working with the Drag and Drop API in web development, there are several key events you need to understand in order to build smooth drag-and-drop interactions. The most important ones are dragstart, dragover, and drop. These events fire at different stages of the drag-and-drop process and each plays a specific role.

What happens during dragstart

This is the event that kicks everything off. It fires as soon as the user starts dragging an element — usually by clicking and holding the mouse button down on it.

• You typically set the data you want to transfer during this event using setData().
• This is also where you can customize the drag image (optional), or change the opacity to give visual feedback.
• Make sure to only call setData() when needed — no need to overdo it if you're not actually transferring any data.

Example:

element.addEventListener('dragstart', function(e) {
  e.dataTransfer.setData('text/plain', 'Some data here');
});

A common mistake is forgetting to set the draggable attribute to true on the element you want to drag — otherwise, the browser won’t recognize it as draggable.

How dragover affects dropping

The dragover event is fired continuously while an element is being dragged over a valid drop target. Without handling this event properly, the drop event won't fire at all.

• You must call e.preventDefault() inside dragover to allow the drop to happen.
• Use this event to show visual cues like highlighting a drop zone.
• Avoid doing heavy computations here since it fires often.

For example:

dropZone.addEventListener('dragover', function(e) {
  e.preventDefault(); // Required for drop to work
  dropZone.classList.add('highlight');
});

Also, don’t forget to clean up the visual effect elsewhere — maybe in dragleave or drop.

What happens during the drop event

This is the final step in the interaction — it fires when the user releases the mouse button over a valid drop target.

• Retrieve the dragged data using getData() here.
• Perform your logic: move the item, update the UI, send data to the server, etc.
• Don’t forget to remove any temporary styles added during dragover.

Basic usage:

dropZone.addEventListener('drop', function(e) {
  e.preventDefault();
  const data = e.dataTransfer.getData('text/plain');
  console.log('Dropped data:', data);
  dropZone.classList.remove('highlight');
});

One thing people often miss is calling preventDefault() in both dragover and drop — skipping either will break the flow.

基本上就這些。

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