js实现图片上传预览

使用FileReader实现图片上传预览

通过HTML5的FileReader API可以读取用户上传的图片文件并显示预览。需要创建文件输入元素和图片预览容器。

<input type="file" id="upload" accept="image/*">
<img id="preview" style="max-width: 300px; display: none;">

document.getElementById('upload').addEventListener('change', function(e) {
  const file = e.target.files[0];
  if (!file.type.match('image.*')) return;

  const reader = new FileReader();
  reader.onload = function(e) {
    const preview = document.getElementById('preview');
    preview.src = e.target.result;
    preview.style.display = 'block';
  };
  reader.readAsDataURL(file);
});

使用URL.createObjectURL实现预览

这种方法不需要等待文件完全读取，性能更好但需要手动释放内存。

document.getElementById('upload').addEventListener('change', function(e) {
  const file = e.target.files[0];
  if (!file) return;

  const preview = document.getElementById('preview');
  if (preview.src) URL.revokeObjectURL(preview.src);

  preview.src = URL.createObjectURL(file);
  preview.style.display = 'block';
});

多图片预览实现

对于需要同时预览多个图片的场景，可以使用以下方法：

<input type="file" id="multiUpload" accept="image/*" multiple>
<div id="previewContainer"></div>

document.getElementById('multiUpload').addEventListener('change', function(e) {
  const container = document.getElementById('previewContainer');
  container.innerHTML = '';

  Array.from(e.target.files).forEach(file => {
    if (!file.type.match('image.*')) return;

    const img = document.createElement('img');
    img.style.maxWidth = '200px';
    img.style.margin = '5px';
    img.src = URL.createObjectURL(file);
    container.appendChild(img);
  });
});

图片压缩和调整大小

上传前可以对图片进行压缩处理：

function compressImage(file, maxWidth, maxHeight, quality) {
  return new Promise((resolve) => {
    const img = new Image();
    img.src = URL.createObjectURL(file);

    img.onload = () => {
      const canvas = document.createElement('canvas');
      let width = img.width;
      let height = img.height;

      if (width > maxWidth) {
        height *= maxWidth / width;
        width = maxWidth;
      }
      if (height > maxHeight) {
        width *= maxHeight / height;
        height = maxHeight;
      }

      canvas.width = width;
      canvas.height = height;
      const ctx = canvas.getContext('2d');
      ctx.drawImage(img, 0, 0, width, height);

      canvas.toBlob(resolve, 'image/jpeg', quality);
    };
  });
}

// 使用示例
document.getElementById('upload').addEventListener('change', async (e) => {
  const file = e.target.files[0];
  const compressedFile = await compressImage(file, 800, 800, 0.7);
  // 使用compressedFile进行上传或预览
});

拖放上传预览实现

支持拖放操作的图片预览：

<div id="dropArea" style="border: 2px dashed #ccc; padding: 20px;">
  拖放图片到这里
</div>
<div id="dropPreview"></div>

const dropArea = document.getElementById('dropArea');

['dragenter', 'dragover', 'dragleave', 'drop'].forEach(eventName => {
  dropArea.addEventListener(eventName, preventDefaults, false);
});

function preventDefaults(e) {
  e.preventDefault();
  e.stopPropagation();
}

['dragenter', 'dragover'].forEach(eventName => {
  dropArea.addEventListener(eventName, highlight, false);
});

['dragleave', 'drop'].forEach(eventName => {
  dropArea.addEventListener(eventName, unhighlight, false);
});

function highlight() {
  dropArea.style.borderColor = '#666';
}

function unhighlight() {
  dropArea.style.borderColor = '#ccc';
}

dropArea.addEventListener('drop', handleDrop, false);

function handleDrop(e) {
  const dt = e.dataTransfer;
  const files = dt.files;
  const preview = document.getElementById('dropPreview');
  preview.innerHTML = '';

  Array.from(files).forEach(file => {
    if (!file.type.match('image.*')) return;

    const img = document.createElement('img');
    img.style.maxWidth = '200px';
    img.style.margin = '5px';
    img.src = URL.createObjectURL(file);
    preview.appendChild(img);
  });
}