异步加载图片

使用Promise封装一个异步加载图片的方法

function loadImg(url) {
  return new Promise((resolve, reject) => {
    const img = new Image();
    img.onload = function() {
      resolve(img);
    };
    img.onerror = function() {
    	reject(new Error('Could not load image at' + url));
    };
    img.src = url;
  });