/* 
	固定导航，回到顶部
 */
var scrollTop = document.getElementById('scrollTop');
var navBar = document.getElementById('navBar');
var returnTop = document.getElementById('returnTop');
var mian = document.getElementById('mian');

window.onscroll = function() {
	console.log(scrollTop.offsetHeight);
	console.log(scroll().top);
	if (scroll().top > scrollTop.offsetHeight) {
		navBar.className = "objfixed";
		//navBar变为固定定位后 脱离标准流 不在占位置，后面的就跳上来了。
		//防止后面内容跳上来 给后面加pading margin
		mian.style.paddingTop = navBar.offsetHeight + "px";
	} else {
		navBar.className = "";
		mian.style.paddingTop = 0;
	}

	if (scroll().top > 1000) {
		returnTop.style.display = "block";
	} else {
		returnTop.style.display = "none";
	}
}

returnTop.onclick = function() {
	var timer = null;
	var step = Math.ceil(scroll().top / 60);
	var leader = scroll().top;
	clearInterval(timer); //防止重复设置定时器
	timer = setInterval(() => {
		var target = 0;
		if (leader > target) {
			leader = leader - step;
			document.body.scrollTop = document.documentElement.scrollTop = leader;
		} else {
			clearInterval(timer);
		}
	}, 15) //每秒钟60贞会形成比较细腻的动画效果
}

function scroll() {
	return {
		top: window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop || 0,
		left: window.pageXOffset || document.documentElement.scrollLeft || document.body.scrollLeft || 0
	}
}