函数防抖和节流

防抖

		let time= null;
		document.onmousemove = function (ev) {
			if (time) {
				clearTimeout(time)
			}
			time = setTimeout(() => {
				// fn(ev)
			}, 500)
		)
		};

节流

	      let previous= 0;
		document.onmousemove = function (ev) {
				let now = Date.now();
				if (now - previous > 200) {
					fn(ev);
					previous = now;
				}
		};