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function compressImage(file, maxWidth = 800, maxHeight = 800, quality = 0.7) {
  return new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = function(event) {
      const img = new Image();
      img.src = event.target.result;
      img.onload = function() {
        const canvas = document.createElement('canvas');
        let width = img.width;
        let height = img.height;

        if (width > height) {
          if (width > maxWidth) {
            height *= maxWidth / width;
            width = maxWidth;
          }
        } else {
          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((blob) => {
          resolve(new File([blob], file.name, { type: file.type }));
        }, file.type, quality);
      };
    };
  });
}

// 使用示例
const input = document.querySelector('input[type="file"]');
input.addEventListener('change', async (event) => {
  const file = event.target.files[0];
  const compressedFile = await compressImage(file);
  // 现在可以发送compressedFile
});