节流函数：是指能减少触发次数的函数，解决事件频繁触发导致性能下降的问题
实现：一般我们内置一个定时器
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>throttle</title>
</head>
<body>
<div style="height:5000px">
<div id="demo" style="position:fixed;"></div>
</div>
<script>
var COUNT = 0, demo = document.getElementById('demo');
function testFn() {demo.innerHTML += 'testFN 被调用了 ' + ++COUNT + '次<br>';}
var throttle = function (fn, delay, atleast) {
var timer = null;
var previous = null;
return function () {
var now = +new Date();
if ( !previous ) previous = now;
if ( atleast && now - previous > atleast ) {
fn();
// 重置上一次开始时间为本次结束时间
previous = now;
clearTimeout(timer);
} else {
clearTimeout(timer);
timer = setTimeout(function() {
fn();
previous = null;
}, delay);
}
}
};
window.onscroll = throttle(testFn, 200);
// window.onscroll = throttle(testFn, 500, 1000);
</script>
</body>
</html>