css

#drag {
    width: 300px;
    height: 200px;
    border: 1px dashed #ccc;
    border-radius: 4px;
    overflow: hidden;
}

#drag:hover {
    border-color: cyan;
}

#drag.dragEnter {
    border: 2px dashed cyan;
    background-color: #eee;
}

#drag #view {
    width: 100%;
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: center;
}

#drag #view p {
    color: #2f2f2f;
}

#drag #view img {
    width: 100%;
    height: 100%;
    object-fit: cover;
    display: block;
}
 

html

<div id="drag">
    <input type="file" id="upload" style="display: none">
    <div id="view">
        <p>將圖片拖拽到此處</p>
    </div>
</div>

javascript

  const FILE_TYPES = ['jpg', 'jpeg', 'png', 'gif']
  const drag = document.querySelector('#drag')
  const upload = drag.querySelector('#upload')
  const view = drag.querySelector('#view')
  const accept = FILE_TYPES.map(v => `image/${v}`).join(',')
  upload.setAttribute('accept', accept)
  drag.addEventListener('dragenter', () => {
    drag.classList.add('dragEnter')
  })
  drag.addEventListener('dragleave', () => {
    drag.classList.remove('dragEnter')
  })
  drag.addEventListener('dragover', (e) => {
    e.stopPropagation()
    e.preventDefault()
  })
  drag.addEventListener('click', (e) => {
    e.stopPropagation()
    upload.click()
  })
  drag.addEventListener('drop', (e) => {
    e.stopPropagation()
    e.preventDefault()
    drag.classList.remove('dragEnter')
    const [file] = e.dataTransfer.files
    if (!file) return
    const { type = '' } = file
    const [, fileType] = type.toLowerCase().split('/')
    if (!FILE_TYPES.includes(fileType)) {
      alert('unsupported file type')
      return false
    }
    handleFile(file, view)
  })
  upload.addEventListener('click', (e) => {
    e.stopPropagation()
  })
  upload.addEventListener('change', function (e) {
    const [file] = e.target.files
    if (!file) return upload.value = ''
    const { type = '' } = file
    const [, fileType] = type.toLowerCase().split('/')
    if (!FILE_TYPES.includes(fileType)) {
      upload.value = ''
      alert('unsupported file type')
      return false
    }
    handleFile(file, view)
  })

  function handleFile(file, container) {
    const blob = URL.createObjectURL(file)
    const image = new Image()
    image.src = blob
    container.innerHTML = ''
    container.appendChild(image)
  }