js实现图片等比例缩放

function resizeImage(file, maxWidth, maxHeight) {
    return new Promise((resolve, reject) => {
        const img = new Image();
        img.src = URL.createObjectURL(file);
        img.onload = () => {
            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;
                }
            }

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

            canvas.toBlob((blob) => {
                resolve(blob);
            }, file.type);
        };
        img.onerror = reject;
    });
}

// 使用示例
const input = document.querySelector('input[type="file"]');
input.addEventListener('change', (event) => {
    const file = event.target.files[0];
    resizeImage(file, 800, 600).then((resizedImage) => {
        console.log('Resized image:', resizedImage);
    }).catch((error) => {
        console.error('Error resizing image:', error);
    });
});