將圖片的加載寫成一個Promise，一旦加載完成，Promise的狀態就發生變化。

const preloadImage = function (path) {
  return new Promise(function (resolve, reject) {
    var image = new Image();
    image.onload  = resolve;
    image.onerror = reject;
    image.src = path;
  });
};

應用

// success
preloadImage("http://img2.37wanimg.com/2017/10/1219382866Qt8.jpg")
    .then( () => {
        console.log("success");
    })
    .catch( error => {
        console.log( 'oh no', error );
    });

success

// error
preloadImage("http://img2.37wanimg.com/2017/10/1219382866Qt.jpg")
    .then( () => {
        console.log("success");
    })
    .catch( error => {
        console.log( 'oh no', error );
    });

error