img转base64

一个简单的img地址转base64格式，并可以设置压缩比例以及图片格式

// 参数说明：图片地址（本地或者线上url）、处理后返回的base64内容、指定图片类型
function imgToBase64(url, outputFormat) {
  return new Promise((resolve, reject) => {
    let canvas = document.createElement('canvas');
    const ctx = canvas.getContext('2d');
    const img = new Image;
    img.crossOrigin = 'Anonymous'; //解决Canvas.toDataURL 图片跨域问题
    img.onload = () => {
      let width = img.width;
      let height = img.height;

      // 压缩比例 -- 可以自己修改参数。500px宽度以下原尺寸，大于500px比例处理
      let bili = Math.round(width / 500) || 1;
      let rate = 1 / bili;
      canvas.width = width * rate;
      canvas.height = height * rate;
      ctx.drawImage(img, 0, 0, width, height, 0, 0, width * rate, height * rate);
      let dataURL = canvas.toDataURL(outputFormat || 'image/jpeg');
      // 去除标头 -- 传递给后台时一般去除头部
      // let reg = new RegExp('^data:image/[^;]+;base64,');
      // dataURL = dataURL.replace(reg, '');
      canvas = null;
      resolve(dataURL);
    }
    img.src = url;
  })
};

// 调用：
imgToBase64("https://img0.bdstatic.com/static/searchdetail/img/logo-2X_2dd9a28.png").then(res => console.log(res))