js base64png 怎么转换成base64jpg

将base64png转换为base64jpg需要经过以下几个步骤：
1. 将base64png转换为二进制数据；
2. 将二进制数据转换为图像对象；
3. 将图像对象转换为canvas对象；
4. 将canvas对象转换为base64jpg。

具体的实现可以参考以下代码：

// 将base64png转换为二进制数据
const base64ToBlob = function(base64) {
  const bytes = window.atob(base64.split(','));
  const array = new Uint8Array(bytes.length);
  for (let i = 0; i < bytes.length; i++) {
    array[i] = bytes.charCodeAt(i);
  }
  return new Blob([array], {type: 'image/png'});
};

// 将二进制数据转换为图像对象
const blobToImage = function(blob) {
  return new Promise((resolve, reject) => {
    const img = new Image();
    img.onload = function() {
      resolve(img);
    };
    img.onerror = function() {
      reject();
    };
    img.src = URL.createObjectURL(blob);
  });
};

// 将图像对象转换为canvas对象
const imageToCanvas = function(image) {
  const canvas = document.createElement('canvas');
  canvas.width = image.width;
  canvas.height = image.height;
  const context = canvas.getContext('2d');
  context.drawImage(image, 0, 0, image.width, image.height);
  return canvas;
};

// 将canvas对象转换为base64jpg
const canvasToBase64 = function(canvas) {
  return canvas.toDataURL('image/jpeg', 1);
};

// 示例
const base64png = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAoAAAAKCAYAAACNMs+9AAAAFElEQVR4AWPw2v7Wfe0f+gYGBgYGBoYBQABJzAU4AAAAASUVORK5CYII=';
const blob = base64ToBlob(base64png);
blobToImage(blob)
  .then(imageToCanvas)
  .then(canvasToBase64)
  .then(function(base64jpg) {
    console.log(base64jpg);
  });