IntersectionObserver懒加载

        const io = new IntersectionObserver(callback);
	let imgs = document.querySelectorAll('[data-src]');
	function callback(entries){
		entries.forEach((item) => {
			if(item.isIntersecting){
				item.target.src = item.target.dataset.src 
				io.unobserve(item.target)	
			}
			
		})
	} 
	imgs.forEach((item)=>{
		io.observe(item)
	})