实现一个异步加载图片的方法

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