解决JS修改background-image时图片闪烁

stackoverflow上搜到的，记录一下

原文链接： How to prevent a background image flickering on change

解决方法：

// create a new Image object
var img_tag = new Image();

// when preload is complete, apply the image to the div
img_tag.onload = function() {

    document.querySelector('#div').style.backgroundImage = 'url(' + img + ')';
}

// setting 'src' actually starts the preload
img_tag.src = img;